JP2006173900A - Bias generating circuit, cascode differential amplifier comprising the same, and analogue/digital converter equipped with differential amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bias generation circuit, differential amplifier, and analogue/digital converter capable of applying optimum bias to a gate of cascode transistor. <P>SOLUTION: The bias generation circuit, differential amplifier, and analogue digital converter to apply a bias to the gate of a cascode transistor comprise a first current source connected to a power supply, a first transistor which is diode-connected between the first current source and the ground, a second transistor connected to the power supply, a second current source connected between the second transistor and the ground, a third current source connected to the power supply, and a third transistor which is diode-connected between the third current source and the second current source. A drain of the first transistor is connected to the gate of second transistor, and the gate of third transistor is connected to the gate of cascode transistor. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a bias generation circuit, a cascode differential amplifier having the circuit, and an analog / digital converter including the differential amplifier.

  Conventionally, an analog / digital converter (hereinafter referred to as “A / D converter”) generates a digital signal corresponding to an analog signal by comparing an input analog signal with a plurality of stages of reference voltages. It was.

  For this reason, the A / D converter is provided with a plurality of differential amplifiers for comparing a voltage obtained by sample-holding an analog signal at a predetermined timing with a reference voltage.

  The differential amplifier provided in the A / D converter is generally configured to obtain an output by connecting a load circuit.

  The load circuit is provided with a ground transistor in which a source functioning as a current source load is connected to the ground, and further provided with a cascode transistor connected in cascode to the ground transistor, thereby increasing the gain of the differential amplifier, In addition to improving the conversion accuracy when converting an analog signal to a digital signal, the high speed and wide bandwidth of the differential amplifier have been improved (see, for example, Patent Document 1).

  A circuit including such a ground transistor and a cascode transistor cascode-connected to the ground transistor normally includes a bias generation circuit, and an appropriate bias is applied to the gate of the cascode transistor by the bias generation circuit. Therefore, the ground transistor can operate in the saturation region.

  As shown in FIG. 4, the bias generation circuit 100 includes a current source I100 connected to the power source Vcc, a first transistor T101 and a second transistor diode-connected in series between the current source I100 and the ground GND. T102, and a third transistor T103 and a fourth transistor T104 connected in series between the power supply Vcc and the ground GND.

  And while connecting the gate of the second transistor T102 and the gate of the fourth transistor T104, the gate of the first transistor T101 is connected to the gate of the third transistor T103 and the gate of the ground transistor T105, The bias generating circuit 100 is configured by connecting the gate of the cascode transistor T106 cascode-connected to the ground transistor T105 and the source of the fourth transistor T104.

  In the bias generation circuit 100 configured as described above, the gate width dimension of each of the first transistor T101, the third transistor T103, the fourth transistor T104, the ground transistor T105, and the cascode transistor T106 is divided by the gate length dimension. The ratio of values (hereinafter referred to as “W / L size ratio”) are all designed to be equal, and the W / L size ratio of the second transistor T102 is set to be equal to that of all the other transistors T101, T103, T104, T105, and T106. By designing it to be 1/4 of the W / L size ratio, an optimum bias is applied to the gate of the cascode transistor T106 so that the ground transistor T105 can operate in the saturation region.

That is, in the bias generation circuit 100 in which the W / L size ratio of each transistor is designed as described above, when the gate-source voltage of the first transistor T101 is Vgs and the threshold voltage of the first transistor T101 is Vth, A voltage of 3Vgs-Vth is generated at the node a, whereby a voltage of 2Vgs-Vth can be generated at the node b. Then, by applying the voltage generated at the node b to the gate of the cascode transistor T106, a voltage Vgs-Vth is generated at the node c which is the source-drain voltage of the ground transistor T105, and the ground transistor T105 is It was possible to operate in the saturation region.
JP 2004-7134 A

  However, in the conventional bias generation circuit 100, in order to operate the ground transistor T105 in the saturation region, each of the first transistor T101, the third transistor T103, the fourth transistor T104, the ground transistor T105, and the cascode transistor T106. The transistor W / L size ratios are all designed to be equal, and the second transistor T102 W / L size ratio is set to 1 / W of all other transistors T101, T103, T104, T105, T106. The circuit design must be designed to be 4, so that the degree of freedom in circuit design is low and the circuit can be applied only to circuits having the same W / L size ratio between the ground transistor T105 and the cascode transistor T106.

  Therefore, even in the load circuit connected to the differential amplifier that constitutes the A / D converter, the W / L size ratio between the ground transistor T105 and the cascode transistor T106 must be designed to be equal, and the parasitic capacitance of the cascode transistor T106 is reduced. It was difficult to reduce.

  Therefore, in the present invention according to claim 1, in the bias generation circuit for generating a bias to be applied to the gate of the cascode transistor cascode-connected to the transistor having the source connected to the ground, the first current source connected to the power source, A first transistor diode-connected between the first current source and ground; a second transistor connected to the power supply; a second current source connected between the second transistor and ground; A third current source connected to the power source, and a third transistor connected in diode connection between the third current source and the second current source are provided, and the drain of the first transistor and the second transistor The gate was connected, and the gate of the third transistor and the gate of the cascode transistor were connected.

  Further, in the present invention according to claim 2, in the bias generation circuit according to claim 1, each current density of current flowing through the second transistor and the transistor having the source connected to the ground is set to be the same first current density. By setting each current density of the current flowing through the third transistor and the cascode transistor to the same second current density, and by setting the current density of the current flowing through the first transistor to a predetermined current density, the ground transistor is saturated in the saturation region. An operable voltage is generated between the drain and source of the ground transistor.

  In the present invention according to claim 3, in the bias generation circuit according to claim 2, the predetermined current density is set to a current density that is substantially four times the first current density.

  Further, in the present invention according to claim 4, in the bias generation circuit according to claim 2 or 3, the predetermined current density is obtained by dividing the transistor size obtained by dividing the gate width dimension of the transistor by the gate length dimension. The transistor and the ground transistor are set to take the same first value, the third transistor and the cascode transistor are set to take the same second value, and the first transistor is set to the first value. The first current source and the third current source are set to flow the same amount of current, and the second current source is set to be approximately 1/4 of the value of 1. It is determined by setting the current to flow twice as much as the current that each of the first current source and the third current source flows.

  In the present invention according to claim 5, a load circuit having a current source transistor functioning as a current source load, a cascode transistor connected in cascode to the current source transistor, and a bias applied to the gate of the cascode transistor are generated. In the cascode differential amplifier having the bias generation circuit that performs the bias generation circuit, the bias generation circuit includes a first current source connected to a power supply, and a first transistor connected in a diode between the first current source and the ground. A second transistor connected to the power source, a second current source connected between the second transistor and the ground, a third current source connected to the power source, the third current source and the second current source A diode-connected third transistor is provided between the two current sources, and the drain of the first transistor and the gate of the second transistor are connected. The gate of the third transistor and the gate of the cascode transistor are connected, and each current density of the current flowing through the second transistor and the ground transistor is set to the same first current density, and the third transistor Each current density of the current flowing through the cascode transistor was set to the same second current density, and the current density of the current flowing through the first transistor was set to a current density that was approximately four times the first current density.

  In the present invention according to claim 6, a load circuit having a current source transistor functioning as a current source load, a cascode transistor connected in cascode to the current source transistor, and a bias applied to the gate of the cascode transistor are generated. In an analog / digital converter including a cascode differential amplifier having a bias generation circuit that performs a bias generation circuit, the bias generation circuit includes a first current source connected to a power source, and the first current source and a ground. A diode-connected first transistor, a second transistor connected to a power source, a second current source connected between the second transistor and ground, a third current source connected to the power source, A third transistor connected in a diode is provided between the third current source and the second current source, and the drain of the first transistor The gates of the second transistors are connected, the gates of the third transistors and the gates of the cascode transistors are connected, and the current densities of the currents flowing through the second transistor and the ground transistor are equal to each other. The current density of each of the currents flowing through the third transistor and the cascode transistor is the same second current density, and the current density of the current flowing through the first transistor is approximately four times the current density of the first current density. It was.

  In this invention, there exists an effect as described below.

  In the present invention according to claim 1, in the bias generation circuit for generating a bias to be applied to the gate of the cascode transistor cascode-connected to the transistor having the source connected to the ground, the first current source connected to the power source and the first current source A first transistor connected in a diode between the current source and ground, a second transistor connected to the power source, a second current source connected between the second transistor and ground, and a power source A third current source connected and a third transistor diode-connected between the third current source and the second current source; a drain of the first transistor; a gate of the second transistor; Since the gate of the third transistor and the gate of the cascode transistor are connected, the W / L size ratio of the ground transistor and the cascode transistor Without equal designing a W / L size ratio, it is possible to operate the grounded transistor in the saturation region, it is possible to increase the freedom of the circuit design.

  According to a second aspect of the present invention, in the bias generation circuit according to the first aspect, the current densities of the currents flowing through the second transistor and the transistor whose source is connected to the ground are equal to each other. By setting each current density of the current flowing through the third transistor and the cascode transistor to the same second current density, and by setting the current density of the current flowing through the first transistor to a predetermined current density, the ground transistor is saturated in the saturation region. Since an operable voltage is generated between the drain and source of the ground transistor, the ground transistor is saturated even if a cascode transistor with any desired W / L size ratio is provided regardless of the W / L size ratio of the ground transistor. When designing electrical circuits with cascode transistors that can be operated in the region It is possible to increase the freedom of the circuit design.

  Further, in the present invention according to claim 3, in the bias generation circuit according to claim 2, the predetermined current density is substantially four times the first current density, so that it is always optimal for the gate of the cascode transistor. A bias generation circuit capable of applying a gate voltage can be provided.

  Further, in the present invention according to claim 4, in the bias generation circuit according to claim 2 or 3, the predetermined current density is obtained by dividing the transistor size obtained by dividing the gate width dimension of the transistor by the gate length dimension. The transistor and the ground transistor are set to take the same first value, the third transistor and the cascode transistor are set to take the same second value, and the first transistor is set to the first value. The first current source and the third current source are set to flow the same amount of current, and the second current source is set to be approximately 1/4 of the value of 1. Because it is decided to set the current to flow approximately twice as much as the current that each of the first current source and the third current source flows, an optimum gate voltage is applied to the cascode transistor as a theoretical value. It is possible Thus, it is possible to operate the ground transistor always saturation region.

  In the present invention according to claim 5, a load circuit having a current source transistor functioning as a current source load, a cascode transistor connected in cascode to the current source transistor, and a bias applied to the gate of the cascode transistor are generated. In the cascode differential amplifier having the bias generation circuit that performs the bias generation circuit, the bias generation circuit includes a first current source connected to a power supply, and a first transistor connected in a diode between the first current source and the ground. A second transistor connected to the power source, a second current source connected between the second transistor and the ground, a third current source connected to the power source, the third current source and the second current source A diode-connected third transistor is provided between the two current sources, and the drain of the first transistor and the gate of the second transistor are connected. The gate of the third transistor and the gate of the cascode transistor are connected, and each current density of the current flowing through the second transistor and the ground transistor is set to the same first current density, and the third transistor Since each current density of the current flowing to the cascode transistor is set to the same second current density, and the current density of the current flowing to the first transistor is set to a current density that is approximately four times the first current density, the cascode transistor in the load circuit The parasitic capacitance of the cascode transistor can be reduced by making the W / L size ratio of the cascode transistor larger than the W / L size ratio of the current source transistor and reducing the W of the cascode transistor. It is possible to provide a differential amplifier that can be operated at the same time.

  In the present invention according to claim 6, a load circuit having a current source transistor functioning as a current source load, a cascode transistor connected in cascode to the current source transistor, and a bias applied to the gate of the cascode transistor are generated. In an analog / digital converter including a cascode differential amplifier having a bias generation circuit that performs a bias generation circuit, the bias generation circuit includes a first current source connected to a power source, and the first current source and a ground. A diode-connected first transistor, a second transistor connected to a power source, a second current source connected between the second transistor and ground, a third current source connected to the power source, A third transistor connected in a diode is provided between the third current source and the second current source, and the drain of the first transistor The gates of the second transistors are connected, the gates of the third transistors and the gates of the cascode transistors are connected, and the current densities of the currents flowing through the second transistor and the ground transistor are equal to each other. The current density of each of the currents flowing through the third transistor and the cascode transistor is the same second current density, and the current density of the current flowing through the first transistor is approximately four times the current density of the first current density. Therefore, the parasitic capacitance of the cascode transistor can be reduced by making the W / L size ratio of the cascode transistor larger than the W / L size ratio of the current source transistor and making the W of the cascode transistor smaller. As well as reducing the power consumption of the A / D converter, the differential amplifier can maintain sufficient gain. Thus, an A / D converter with improved conversion accuracy when converting an analog signal to a digital signal can be provided.

(First embodiment)
In the first embodiment, a bias generation circuit will be described in which an optimum gate voltage is applied to the gate of a cascode transistor that is cascode-connected to a ground transistor whose source is connected to the ground, so that the ground transistor can operate in a saturation region. .

  As shown in FIG. 1, the bias generation circuit 1 includes a first current source I1 connected to the power supply Vcc, a first transistor T1 diode-connected between the first current source I1 and the ground GND, The second transistor T2 connected to the power supply Vcc, the second current source I2 connected between the second transistor T2 and the ground GND, the third current source I3 connected to the power supply Vcc, The third current source I3 and the second current source I2 are configured by a third transistor T3 diode-connected.

  The drain of the first transistor T1 and the gate of the second transistor T2 are connected, and the gate of the third transistor T3 and the gate of the cascode transistor Ta are connected.

  In the bias generation circuit 1 of the first embodiment, in order to clarify the reference current amount I flowing through the ground transistor Tb, a fourth current source I4 that flows the reference current amount I is connected to the power source Vcc. A fourth transistor T4 is connected between the fourth current source I4 and the ground GND, and the gate of the fourth transistor T4 and the gate of the ground transistor Tb are connected.

  In the bias generating circuit 1, the current densities of the currents flowing through the fourth transistor T4, the second transistor T2, and the ground transistor Tb are set to the same first current density, and the third transistor T3 The current density of the current flowing through the cascode transistor Ta is set to be equal to the second current density, and the current density of the current flowing through the first transistor T1 is theoretically four times the first current density. It is formed to have a predetermined current density.

  Specifically, the value obtained by dividing the gate width (W) dimension of the transistor by the gate length (L) dimension (hereinafter referred to as “transistor size”) is the fourth transistor T4 and the second transistor T2. And the ground transistor Tb are set to take the same first value, and the third transistor T3 and the cascode transistor Ta are set to take the same second value, respectively, and the first transistor T1 Is set so that the transistor size is a theoretical value that is 1/4 of the first value.

  That is, the transistor size ratios of the fourth transistor T4, the second transistor T2, and the ground transistor Tb (hereinafter referred to as “W / L size ratio”) are made equal to each other, and the third transistor T3 and the cascode The W / L size ratio with the transistor Ta is made equal, and the W / L size ratio between the fourth transistor T4 and the first transistor T1 is 4 to 1.

  Further, the fourth current source I4, the first current source I1, and the third current source I3 are set to flow the current of the reference current amount I equally, and the second current source I2 is set to the reference current amount. The current is set to flow 1/2 of I.

Since the bias generation circuit 1 is formed in this way, if the gate voltage of the fourth transistor T4 is Vgs1, the threshold voltage of the fourth transistor T4 is Vth, and the gate voltage of the third transistor T3 is Vgs2,
The node d, which is the gate voltage of the second transistor T2, is
Node d = 2Vgs1-Vth
The node a which is the source voltage of the second transistor T2 is
Node a = Vgs1-Vth
The node b which is the gate voltage of the third transistor T3 is
Node b = Vgs2 + Vgs1-Vth.

Therefore, the node c which is the drain voltage of the ground transistor Tb is
Node c = Vgs2 + Vgs1-Vth-Vgs2 = Vgs1-Vth, and the source-drain voltage of the ground transistor Tb is Vgs1-Vth.

  That is, as described above, by setting the W / L size ratio of each transistor T1, T2, T3, T4, Ta, Tb, and setting the amount of current that each current source I1, I2, I3, I4 flows, Even when the W / L size ratio between the cascode transistor Ta and the ground transistor Tb is not designed to be equal, the ground transistor Tb can be operated in the saturation region. The degree of freedom can be increased.

In the present embodiment, in order to simplify the explanation, the explanation is made assuming that there is no variation in the substrate bias effect and the characteristics of each element, so the W / L size ratio of the first transistor T1 is the theoretical value. The value is formed to be 1/4 of the first value as described above, but the present invention is not limited to this, and when there is variation in the substrate bias effect and the characteristics of each element, the first value By adjusting the W / L size ratio of the transistor T1 and the amount of current flowing through the first current source I1, the node a is set so that a voltage equal to the voltage Vgs1-Vth desired to be generated at the node c is generated at the node a. By adjusting the voltage of d, a voltage that enables the ground transistor Tb to operate in the saturation region can be generated at the node c.
(Second embodiment)
In the second embodiment, a cascode differential having a load circuit having a ground transistor (hereinafter referred to as a “current source transistor”) that functions as a current source load and a cascode transistor that is cascode-connected to the current source transistor. A case where the bias generation circuit according to the present invention is applied to an analog / digital converter having a built-in amplifier will be described as an example.

  In the bias generation circuit of the second embodiment, the same components as those of the bias generation circuit 1 of the first embodiment will be described with the same reference numerals.

  As shown in FIG. 2, an analog / digital converter (hereinafter referred to as “A / D converter A”) includes a sample hold circuit 2 that samples and holds an analog signal, and a reference voltage generator that generates a plurality of different reference voltages. A circuit 3, a comparison circuit 4 that compares the voltage of the analog signal and a plurality of different reference voltages, and a logic processing circuit 5 that outputs a digital signal corresponding to the analog signal by logically processing the output of the comparison circuit 4; Consists of.

Sample-and-hold circuit 2 is to output the hold signal line 6 the voltage of the applied analog signal to the input terminal T _in to a predetermined period of time maintained at a predetermined timing.

The reference voltage generation circuit 3 includes a high-potential-side reference power supply terminal T _rt that serves as a high-potential-side reference potential (power-supply potential) and a low-potential-side reference power-supply terminal _Trb that serves as a low-potential-side reference potential (ground potential). By connecting 16 resistors R1 to R16 having the same resistance value in series and dividing the voltage between the reference potential on the high potential side and the reference potential on the low potential side by 16 resistors R1 to R16 A plurality of reference voltages are generated, and a predetermined reference voltage is output from the upper bit side reference voltage signal lines 7 and 8 or the lower bit side reference voltage signal lines 9 and 10.

  When the reference voltage generation circuit 3 converts the analog signal into a digital signal on the upper bit side, all the switches SW1 to SW8 are disconnected and the reference voltage signal lines 7 and 8 are connected to the reference voltage. On the other hand, when an analog signal is converted into a digital signal on the lower bit side, only one pair of switches SW1 to SW8 is connected based on the conversion result on the upper bit side, and the lower bit side reference voltage signal A reference voltage is output from the lines 9 and 10.

  The comparison circuit 4 includes an upper bit side comparison circuit 11 that compares the voltage of the analog signal and the reference voltage on the upper bit side, and a lower bit side comparison circuit 12 that compares the voltage of the analog signal and the reference voltage on the lower bit side. Consists of. Here, since the upper bit side comparison circuit 11 and the lower bit side comparison circuit 12 have the same configuration, the upper bit side comparison circuit 11 will be described below.

  The upper bit side comparison circuit 11 comprises an amplification circuit 13 for amplifying the difference between the voltage of the analog signal and the reference voltage, and a comparison holding circuit 14 for comparing and holding the output of the amplification means 13.

  The amplifying means 13 includes two two-stage amplifiers 17 in which a differential amplifier 15 at the front stage and a differential amplifier 16 at the rear stage are connected in series, and differential amplifiers 15 and 15 at the front stage of adjacent two-stage amplifiers 17 and 17. The complementary amplifier 18 is connected to differentially amplify the outputs of the differential amplifiers 15 and 15 at the preceding stage. Note that the two-stage amplifier 17 is not limited to the case where the front-stage differential amplifier 15 and the rear-stage differential amplifier 16 are connected in series, and can be configured to have three or more differential amplifiers connected in series.

  Each of the two-stage amplifiers 17 has a variable gain differential amplifier 16 connected in series after the constant gain differential amplifier 15.

  As shown in FIG. 3, the subsequent differential amplifier 16 has a load circuit 22 connected to the differential amplifier circuit 21, and the load circuit 22 is configured to connect the entire load circuit 22 to the load of the differential amplifier circuit 21. The gain of the differential amplifier circuit 21 can be increased / decreased by switching to the whole load and the partial load that uses a part of the load circuit 22 as the load of the differential amplifier circuit 21.

  Each two-stage amplifier 17 has an offset compression function that apparently compresses the offset voltage of the front-stage differential amplifier 15 by increasing or decreasing the gain of the rear-stage differential amplifier 16 using load switching means. Yes.

  Hereinafter, a specific structure of the differential amplifier 16 in the subsequent stage will be described with reference to FIG.

  The differential amplifier 16 at the subsequent stage has a power source Vcc connected to the drain of an N-channel type transistor T31 serving as a current source, and N-channel type transistors T32 and T33 forming a differential pair are connected to the source of the transistor T31. The differential amplifier circuit 21 is configured, and the output terminal of the differential amplifier 15 of the preceding stage is connected to the gates of the transistors T32 and T33 of the differential amplifier circuit 21, while the differential of the latter stage is connected to the sources of the transistors T32 and T33. The non-inverting output terminal 25 and the normal output terminal 26 of the amplifier 16 are connected to take out the output.

  Capacitance cutting may be performed by connecting a capacitor between the gates of the transistors T32 and T33 and the differential amplifier 15 in the previous stage. In this case, it is necessary to apply a voltage at a predetermined DC operating point to the gates of the transistors T32 and T33 at a predetermined timing.

  In the differential amplifier 16 at the subsequent stage, the load circuit 22 is connected to the sources of the transistors T32 and T33 of the differential amplifier circuit 21.

  The load circuit 22 includes a first current source transistor Tb1 serving as a current source load between the source of the transistor T32 constituting the differential amplifier circuit 21 and the ground GND, and a cascode to the first current source transistor Tb1. The connected first cascode transistor Ta1 is connected in series, while the second current source transistor Ta2 serving as a current source load is connected between the source of the transistor T33 and the ground GND, which also form the differential amplifier circuit 21, and A second cascode transistor Tb2 cascode-connected to the second current source transistor Ta2 is connected in series.

  In the load circuit 22, the first switching transistor Tc1 is provided between the drain of the first cascode transistor Ta1 and the gate of the first current source transistor Tb1, and the drain of the second cascode transistor Ta2 and the second A second switching transistor Tc2 is connected between the gates of the two current source transistors Tb2, and a clock signal CLK is applied to the gates of the first and second switching transistors Tc1 and Tc2.

  Further, a first capacitor C1 for holding voltage is connected between the gate of the first current source transistor Tb1 constituting the load circuit 22 and the gate of the transistor T32 which is the input terminal of the differential amplifier 16 at the subsequent stage. Similarly, a second capacitor C2 for holding voltage is provided between the gate of the second current source transistor Tb2 constituting the load circuit 22 and the gate of the transistor T33 serving as the input terminal of the differential amplifier 16 at the subsequent stage. Is connected.

  In the subsequent differential amplifier 16 having the load circuit 22 configured as described above, when the first and second switching transistors Tc1 and Tc2 are disconnected, the entire load circuit 22 becomes a load (overall load). In this case, the first and second current source transistors Tb1 and Tb2 serve as current source loads to increase the output impedance, thereby increasing the gain of the differential amplifier 16 at the subsequent stage.

  On the other hand, when the first and second switching transistors Tc1, Tc2 are connected, a part of the load circuit 22 becomes a load (partial load). In this case, the first and second current source transistors Tb1 , Tb2 serves as a diode load, and the output impedance is reduced, whereby the gain of the differential amplifier 16 at the subsequent stage is reduced. At this time, since the voltage is held in the first and second capacitors C1 and C2, a DC potential is held.

  Moreover, since the input signal of the differential amplifier 16 (differential amplifier circuit 21) is applied to the gates of the first and second current source transistors Tb1 and Tb2 via the capacitors C1 and C2, the load circuit 22 The input signal of the differential amplifier circuit 21 is amplified by the first and second current source transistors Tb1 and Tb2 during the entire load with the first and second current source transistors Tb1 and Tb2 as current source loads. Yes.

  Therefore, the differential amplifier 16 in the subsequent stage can increase the gain at the total load when the first and second switching transistors Tc1, Tc2 are in the disconnected state, and accordingly, the differential at the full load is increased. The gain of the amplifier circuit 21 can be increased.

  In this way, by increasing or decreasing the gain of the differential amplifier 16 at the subsequent stage, the two-stage amplifier 17 apparently compresses the offset voltage of the differential amplifier 15 at the previous stage.

That is, the offset voltage of the differential amplifier 15 in the previous stage is Vos, the gain in the reset mode (diode load) is Gr, the gain in the comparison mode (current source load) is Gc, the output voltage is Vout, and the input during comparison When the voltage is Vin, the output voltage Vout in the reset mode is
Vout = Gr ・ Vos
On the other hand, the output voltage Vout at the time of comparison is
Vout = Gc ・ Vin
Because
Gr ・ Vos = Gc ・ Vin
And
Vin = Vos · Gr / Gc
It becomes.

  That is, in the two-stage amplifier 17 using the differential amplifier 16 having the above configuration, the offset voltage is compressed Gr / Gc times, and the input conversion offset can be expressed as Vos · Gr / Gc.

  In other words, this differential amplifier 16 can realize a high-precision comparator that has almost no offset by reducing the gain Gr in the reset mode and increasing the gain Gc in the comparison mode. it can.

  In addition, the load circuit 22 provided in the subsequent differential amplifier 16 includes a first cascode connected to the first and second current source transistors Tb1 and Tb2 in order to increase the gain Gc in the comparison mode. First and second cascode transistors Ta1 and Ta2 are provided.

  Further, in the load circuit 22, the gates of the first and second cascode transistors Ta1 and Ta2 are provided so that the load circuit 22 provided with the first and second cascode transistors Ta1 and Ta2 can operate with lower power. A bias generation circuit 23 for applying an appropriate bias is provided so that the first and second current source transistors Tb1 and Tb2 can operate in a saturation region with a minimum source-drain voltage.

  As shown in FIG. 3, the bias generation circuit 23 includes a first current source I1 connected to the power source Vcc, and a first transistor T1 diode-connected between the first current source I1 and the ground GND. A second transistor T2 connected to the power source Vcc, a second current source I2 connected between the second transistor T2 and the ground GND, a third current source I3 connected to the power source Vcc, A third transistor T3, which is diode-connected between the third current source I3 and the second current source I2, is provided.

  The drain of the first transistor T1 and the gate of the second transistor T2 are connected, and the gate of the third transistor T3 and the gates of the first and second cascode transistors Ta1 and Ta2 are connected.

  Further, the second transistor T2 and the first and second current source transistors Tb1 and Tb2 are formed so that the W / L size ratios thereof all take the same first value, and the third transistor T3 and the second transistor T3 The first and second cascode transistors Ta1 and Ta2 are formed so that the W / L size ratios of the first and second cascode transistors are all equal to each other. It is formed to take a value that is 1/4 of the value of 1.

  In addition, the first current source I1 and the third current source I3 are formed so as to flow the same amount of the reference current amount I, and the second current source I2 includes the current sources I1, It is configured to pass a current twice as much as the current that I2 flows.

  In addition, here, a current of the reference current amount I flows through the first and second current source transistors Tb1 and Tb2.

  Since the bias generation circuit 23 is formed in this manner, a gate voltage (node) for generating a voltage at the node a that is equal to a voltage at which the first and second current source transistors Tb1 and Tb2 can operate in the saturation region. d) is applied to the gate of the first transistor T1, the voltage at the node d is shifted to the node b, and the voltage at the node b is applied to the gates of the first and second cascode transistors Ta1 and Ta2. As a result, a voltage equal to the voltage generated at the node a can be generated at the node c.

  This eliminates the need to design the W / L size ratio between the first and second current source transistors Tb1 and Tb2 and the first and second cascode transistors Ta1 and Ta2 to be equal. Design freedom can be increased.

  As a result, the W / L size ratio of the first and second cascode transistors Ta1, Ta2 constituting the load circuit 22 is made larger than the W / L size ratio of the first and second current source transistors Tb1, Tb2, In addition, the W (gate width) of the first and second cascode transistors Ta1 and Ta2 can be further reduced, and the parasitic capacitance of the first and second cascode transistors Ta1 and Ta2 can be reduced. The amplifiers 15 and 16 can be operated with lower power, and an A / D converter with reduced power consumption can be provided.

  The bias generation circuit 23 is described as having no variation in the substrate bias effect and the characteristics of each element as in the first embodiment, but there is a variation in the substrate bias effect and the characteristics of each element. The first transistor T1 so as to generate a voltage at the node d that causes the first and second current source transistors Tb1, Tb2 to generate a voltage equal to a voltage at which the first and second current source transistors Tb1, Tb2 are operable in the saturation region. By adjusting the W / L size ratio and the amount of current flowing through the first current source I1, a voltage equal to the voltage generated at the node a can be generated at the node c. The two current source transistors Tb1 and Tb2 can be operated in the saturation region.

  Specifically, the gate voltage of the second transistor T2, which is the voltage at the node d, is higher than the sum of the gate-source voltage of the second transistor T2 and the source voltage of the second transistor T2. In addition, by appropriately changing and setting the W / L size ratio of the first transistor T1 and the amount of current flowing through the first current source I1, it is optimal for the gates of the first and second cascode transistors Ta1 and Ta2. A bias is applied to operate the first and second current source transistors Tb1 and Tb2 in the active region.

  In the second embodiment, a 4-bit sub-ranging A / D converter that performs conversion in two steps of 2 bits is described as an example. However, the present invention is not limited to this, and a flash type (parallel type) A A / D converter may be used, and not only a single input type but also a differential input type. Also, the specific circuit is not limited to the one with only the positive power supply, and may use a positive / negative power supply or only a negative power supply, and the specific elements constituting the circuit are appropriately selected. It's okay.

It is a circuit diagram which shows the bias generation circuit based on this invention. It is explanatory drawing which shows the A / D converter which concerns on this invention. 1 is a circuit diagram showing a differential amplifier including a bias generation circuit according to the present invention. FIG. It is a circuit diagram which shows the conventional bias generation circuit.

Explanation of symbols

1 Bias generation circuit
A Analog / digital converter
2 Sample hold circuit
3 Reference voltage generator
4 Comparison circuit
5 Logic processing circuit
6 Hold signal line
7,8 Upper bit side reference voltage signal line
9,10 Lower bit reference voltage signal line
11 Upper bit side comparison means
12 Lower bit side comparison means
13 Amplification means
14 Comparison holding circuit
15 Previous stage differential amplifier
16 Subsequent differential amplifier
17 2-stage amplifier
21 Differential amplifier circuit
22 Load circuit
23 Bias generator
Ta cascode transistor
Ta1 First cascode transistor
Ta2 Second cascode transistor
Tb Grounded transistor
Tb1 first current source transistor
Tb2 Second current source transistor
Tc1 first switching transistor
Tc2 Second switching transistor
T1 first transistor
T2 second transistor
T3 Third transistor
T4 4th transistor
I1 First current source
I2 Second current source
I3 Third current source
I4 4th current source

Claims (6)

In a bias generation circuit for generating a bias to be applied to the gate of a cascode transistor having a cascode connection to a transistor having a source connected to ground,
A first current source connected to a power source, a first transistor diode connected between the first current source and ground;
A second transistor connected to the power source, a second current source connected between the second transistor and ground;
A third current source connected to the power source, and a third transistor diode-connected between the third current source and the second current source,
Connecting the drain of the first transistor and the gate of the second transistor;
A bias generation circuit, wherein a gate of the third transistor and a gate of the cascode transistor are connected.   The current densities of the currents flowing through the second transistor and the transistor having the source connected to the ground are set to be equal first current densities, and the current densities of the currents flowing through the third transistor and the cascode transistor are respectively set. By setting the second current density to be equal and the current density of the current flowing through the first transistor to be a predetermined current density, a voltage at which the ground transistor can operate in a saturation region is generated between the drain and source of the ground transistor. 2. The bias generation circuit according to claim 1, wherein:   3. The bias generation circuit according to claim 2, wherein the predetermined current density is a current density that is substantially four times the first current density. The predetermined current density is set so that the transistor size obtained by dividing the gate width dimension of the transistor by the gate length dimension is such that the second transistor and the ground transistor each have the same first value,
The third transistor and the cascode transistor are each set to take an equal second value,
The first transistor is set to be approximately 1/4 of the first value,
In addition, the first current source and the third current source are set so that the same amount of current flows,
4. The second current source is determined by setting to flow a current that is twice as large as a current that each of the first current source and the third current source flows. The bias generation circuit described in 1. A cascode-type difference having a load circuit having a current source transistor functioning as a current source load, a cascode transistor cascode-connected to the current source transistor, and a bias generation circuit for generating a bias applied to the gate of the cascode transistor In the dynamic amplifier,
The bias generation circuit includes a first current source connected to a power source, a first transistor diode-connected between the first current source and the ground,
A second transistor connected to the power source, a second current source connected between the second transistor and ground;
A third current source connected to the power source, and a third transistor diode-connected between the third current source and the second current source,
Connecting the drain of the first transistor and the gate of the second transistor;
Connecting the gate of the third transistor and the gate of the cascode transistor;
Further, each current density of currents flowing through the second transistor and the ground transistor is set to be equal first current density, and each current density of currents flowing through the third transistor and the cascode transistor is set equal to each other. A cascode operational amplifier, characterized in that the current density of the current flowing through the first transistor is approximately four times the first current density. A cascode-type difference having a load circuit having a current source transistor functioning as a current source load, a cascode transistor cascode-connected to the current source transistor, and a bias generation circuit for generating a bias applied to the gate of the cascode transistor In an analog / digital converter with a dynamic amplifier,
The bias generation circuit includes a first current source connected to a power source, a first transistor diode-connected between the first current source and the ground,
A second transistor connected to the power source, a second current source connected between the second transistor and ground;
A third current source connected to the power source, and a third transistor diode-connected between the third current source and the second current source,
Connecting the drain of the first transistor and the gate of the second transistor;
Connecting the gate of the third transistor and the gate of the cascode transistor;
Further, each current density of currents flowing through the second transistor and the ground transistor is set to be equal first current density, and each current density of currents flowing through the third transistor and the cascode transistor is set equal to each other. An analog / digital converter characterized in that the current density of the current flowing through the first transistor is approximately four times the first current density.

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