JP2501311B2 - Active low-pass filter - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to an active low-pass filter in which all elements are integrated on the same semiconductor substrate, and particularly suppresses high-frequency components outside the band of a signal to be handled and integrates them. It is used to improve characteristics such as S / N at the output of the circuit itself.

[Technical background of the invention and its problems]

FIG. 1 shows a conventional active low-pass filter (2
Next) is an example. The components are resistors R _A and R _B , capacitors C _A and C _B , and an operational amplifier (1) with a gain of + K. V _IN1 is a signal input terminal, V _IN
OUT1 is a signal output terminal. The transfer function is However Normally, the allowable deviation of R and C is large, so K1 is used.
At this time, the operational amplifier is used as a voltage follower.

FIG. 2 shows an example of a second-order low-pass filter constructed by a conventional switched capacitor filter. The components are analog switches 11 ₁ , 12 _{1 to} 11 ₆ , 12 ₁₆ and capacitors C ₀₁ , C.
_{11, C 12, C 21,} C 1, C 2, an operational amplifier (op amp) 12, 13, is an analog ground 14. The transfer function is However _S is the sampling frequency.

Generally, when making a low-pass filter, an inductance L, a capacitance C, and a resistance R are used. This requires the use of a coil as L.
Since L cannot be used when making IC, an active low-pass filter was devised in consideration of removing L for this coil. An active low-pass filter requires an external power supply, but it can be made up of R, C and operational amplifiers. An operational amplifier essentially functions in this active low-pass filter, and positive feedback is applied through the capacitor C _{A in} FIG. 1 to realize the characteristic of the equation (1). In the active low-pass filter, various construction methods are possible by using the amplifying element, and there are many degrees of freedom in defining the circuit. Active low-pass filters are usually difficult to make monolithic and are often implemented in hybrid ICs (integrated circuits).

The switched capacitor filter is a technology that has begun to be put to practical use in recent years, and is composed of a combination of a MOS capacitor, an analog switch, and an operational amplifier. In switched capacitor filters, analog switches 11 ₁ , 12 ₁ to 1
1 ₆ and 12 ₆ are switched by a clock that does not overlap to work as a resistor. The band of the switched capacitor filter is determined only by the capacitance ratio and the clock frequency. The components of the switched-capacitor filter can be realized using conventional MOSLSI technology, and can be made monolithic.

However, the active low-pass filter shown in Fig. 1 has a limited operational frequency range of the operational amplifier.
The operating frequency range of the active low-pass filter configured using this is also limited.

Switched-capacitor filters are superior to active lowpass filters in that they are monolithic and provide bandwidth accuracy, but again, like active lowpass filters, they have a limited operational frequency range. However, it is quite difficult to construct a low-pass filter when handling a signal in the video band, for example.

Further, both of them use an operational amplifier, which makes the circuit complicated and increases the chip size. Further, in the active low-pass filter, the cut-off frequency changes depending on the output impedance of the preceding stage of the circuit to which it is applied, which causes a problem.

[Object of the Invention]

The present invention has been made in view of the above circumstances, and an object thereof is to provide an active low-pass filter that can operate in a relatively high frequency band such as a cutoff frequency of 100 kHz or more. In addition, for example, in signal processing ICs for various purposes such as handling analog signals in the video band, various high frequency components outside the band are suppressed, and output signals with good linearity, S / N, frequency characteristics, and phase characteristics are output outside the IC. The present invention provides an active low-pass filter that can be incorporated in the IC with a smaller area so that it can be taken out.

[Outline of Invention]

In the present invention, in order to make it an active low-pass filter that can be used in a relatively high frequency band such as a video band, an operational amplifier is conventionally used as an amplifier instead of an FET.
It was realized by the source follower by or the configuration with the same function. The source follower can have a band of several tens of MHz or more and a gain of approximately 0.9 times or more. The resistance component and the capacitance component can be realized by using the same MOSLSI technology as the current technology for forming the FET. Further, the input of the active low-pass filter is performed through a source follower or a structure having the same function as this, and this source follower is always a constituent element of the active low-pass filter. This allows the insulated gate to have a high impedance in the input section, so that the active impedance can be set regardless of the output impedance of the circuit system placed before the active low-pass filter.
The band of the low-pass filter can always be set to be constant. Further, if an active low-pass filter using a source follower that can be operated in a high frequency region is configured, all the elements can be formed on the same semiconductor substrate and the characteristics of each element can be well matched.

Example of Invention

An embodiment of the present invention will be described below with reference to the drawings. In the second-order active low-pass filter shown in FIG. 3, the drain is connected to the power supply V _DD , the gate is connected to the signal input end 21, and the FET 22 is provided. One end is connected to the power supply V _SS . A circuit system 23 for performing a constant current operation is provided, the other end of which is connected to the source of the FET 22. _A resistor R'A whose one end is connected between the source of the FET 22 and the other end of the circuit system 23.
The provided, resistor R having one end connected to the other end of the resistor R _'A
B is provided, and a capacitor C _B connected between the other end of the resistor R _B and the power supply V _SS is provided. A constant current operation in which a connection point between the resistor R _B and the capacitance C _B is connected to the gate, and a drain is connected to the power supply V _DD , and a FET 24 is provided and one end is connected to the power supply V _SS A circuit system 25 for performing the above is provided, the connection point of the source of the FET 24 and the circuit system 25 is connected to the signal output terminal 26, and the capacitance C _A
Is connected to the output terminal 26, and the other end of the capacitor C _A is connected between the resistors R ′ _A and R _B. The elements shown in FIG. 3 are integrated on the same semiconductor substrate.

The third-order active low-pass filter shown in FIG. 4 is obtained by adding a resistor R _C and a capacitor C _C to the configuration of FIG. 3 and integrating them on the same semiconductor substrate. That is, a resistor R _C is inserted between the source of the FET 22, the circuit system 23, and the resistor R _A , and the resistors R _C and R _A
A capacitor C _C is connected between the power supply V _SS and the power supply.

The transfer function of the configuration of FIG. 3 is approximately expressed by equation (1). That is However, R _A = R ′ _A + Z _O , where Z _O is the output impedance of the first-stage source follower.

 The transfer function of the configuration of FIG.

However Although R = R _C + Z _O FET22 are depletion type, FET 24 may be a depletion in enhancement type. These two
By using a depletion type FET for at least the first-stage source follower that receives V _IN3 among the two source followers, the operating points at the input and output of the active low-pass filter can be set to be almost equal. According to this, there is an advantage that the active low-pass filter of the present invention can be inserted at an arbitrary position in the middle of signal processing. Furthermore, even if a plurality of active low-pass filters of the present invention are connected in series, the operating points at input and output can be set to be substantially the same, which is convenient.

The circuit systems 23 and 25 that perform the constant current operation can have various configurations using single or a plurality of enhancement type and depletion type MOSFETs. Resistance components are polysilicon resistance, diffusion resistance, ion implantation resistance, enhancement type MOSFET, depletion type MOSFET
Etc. and combinations thereof. If the capacitance component is, for example, a double-polysilicon process, the capacitance component is constituted by the capacitance of the first-layer polysilicon and the second-layer polysilicon, the gate capacitance of the MOSFET, the capacitance of the polysilicon with respect to the substrate, and the like, and combinations thereof. be able to.

The method of determining each constant of the active low-pass filter may be as follows, for example. The value of each C and R is decided based on the method of setting the constant of the active low pass filter which is usually done. After that, considering the patterns on these integrated circuits and looking at the parasitic capacitance (mirror capacitance, wiring capacitance, etc.), a final configuration is determined by performing computer simulation in consideration of these values.

The main causes of variations in the cutoff frequency of the active low-pass filter are as follows.

(A) Threshold value of each MOSFET (b) Oxide film thickness that forms each capacitance (c) Pattern finished dimensions of each component.

Also, when analog signal processing is performed using this active low-pass filter, if good phase characteristics (delay characteristics) are required, it is necessary to design a Butterworth type when setting the constants. The output impedance Z ₁ of the source follower formed by the FET 24 and the circuit system 25 of this active low-pass filter is required to be low. However, if you can only lower it to a certain extent due to the balance of total power,
The error can be further reduced by taking that point into consideration by the simulation.

FIG. 5 shows an output signal processing system of a charge transfer device (CTD) incorporating the active low-pass filter (third-order Butterworth type) of the present invention. The CTD is originally a sampling element, and the output signal sampled by the transfer clock is taken out to the floating diffusion 31 of the floating diffusion output system. Similar signals are output in the floating gate method. This signal is level-shifted and impedance-converted by the FETs 32 and 33, sampled and held by the sampling pulse applied to the gate terminal 35, the FET 34, and the capacitor 37,
Further, signal amplification is performed by the inverter amplifier 36. Even at this point, in many cases, the clock component has not been completely removed yet, and if the signal is processed and output as it is, it is necessary to attach a low-pass filter externally to take the clock component. However, as shown in FIG.
By incorporating the low-pass filter 38 in the IC, the clock component can be suppressed to a practically practical level. In this active low-pass filter, the resistance value is a depletion type MOSFET and the capacitance value is a gate capacitance. In this process, by using this combination as compared with the case of using other resistors and capacitors, the most accurate and small area can be realized. For example CTD
When this circuit is applied to the delay line for television signals, the bandwidth is about 4MHz, the transfer clock of CTD is 3 _SC = 10.74MHz.
Then, a low-pass signal with a cutoff frequency of about 4MHz
Design the filter. In this circuit, if the group delay due to the phase fluctuation occurs, it adversely affects the color of the television, so the active low-pass filter must be a Butterworth type. In addition, when the power supply voltage drops, the source follower used in the active low-pass filter 38 has two stages (the FETs 22 and 23 and FE
DC level drop due to T24,25) is a problem, so by using a depletion type FET22,
Substantially equal DC levels are obtained at circuit points 39 and 26. That is, in the source follower including the depletion type FET 22 and the circuit system 23 that operates in a constant current manner, the constant current flowing in the circuit system 23 is I ₀ , and the current amplification factor determined by the gate length and the gate width of the FET 22 is β. , If the gate-source voltage is V _GS and the threshold is V _TH , this source follower needs to amplify the input linearly, so FET22 needs to operate in the saturation region. The formula holds.

Here, if the gate voltage of FET22 is V _in and the output voltage of the source follower (source voltage of FET22) is V _out ,
_GS = V _in −V _out , and by substituting this into the above equation (5), the following equation (6) is obtained.

Then, by solving the above equation (6) for V _out , the following equation (7) is obtained.

In the above formula (7), if the FET 22 is the enhancement type, V _TH > 0, and if it is the expression type, V _TH <0. Therefore, the value of V _{out represented by} the above formula (7) Is clearly larger when the FET 22 is of the depletion type than when it is of the enhancement type.

Here, _in order to set V _out = V _in , it is necessary to satisfy the following expression (8).

The above formula (8) is satisfied only when V _TH <0. Therefore, in order to set V _out = V _in , the FET 22 needs to be a depletion type.

In FIG. 5, no DC voltage drop occurs in the path from the output of the source follower composed of the FET 22 and the circuit system 23 to the gate of the FET 24 in the final source follower composed of the FET 24 and the circuit system 25. Therefore, if an enhancement type FET is used as the FET24 of the source follower at the final stage, a voltage drop will occur at the source follower at the final stage, but as described above, the DC levels at the circuit points 39 and 26 are substantially equal. Is obtained. Further, if the expression type FET 24 is used as the final source follower FET 24, the circuit points 39 and 26 can have the same potential.
Of course, the linearity of the signal must be improved, so the dimensions are set so that the saturation operation of both source followers is guaranteed. The final source follower (FET2
4,25)) output impedance is about 300-1k
By setting Ω, the output impedance corresponding to the amplifier of the active low-pass filter is lowered, and the output end 26 can be taken out to the outside. The resistors are also configured to be FETs 41, 42 and 43, 44 to reduce the nonlinearity of the MOSFET.

6 and 7 show gain characteristics and delay characteristics as a result of simulation at the time of design. With the configuration of the present invention, the measured values of the gain and delay characteristics of the active low-pass filter formed on the same substrate were similar to those shown in FIGS. 6 and 7.

If a higher-order low-pass filter is required, it can be cascaded by appropriately combining the second- and third-order active low-pass filters shown in FIGS. 3 and 4.

〔The invention's effect〕

As described above, the active low-pass filter of the present invention occupies a small area on the substrate even when incorporated in various signal processing ICs (monolithic), can realize a high cutoff frequency, and has a good delay characteristic. Will be things. Also, since the gate of the MOSFET serves as an input, a low-pass filter with a constant cutoff frequency can be realized regardless of the output impedance of the previous stage. Further, by cascade-connecting the low-pass filters of the present invention, it is possible to realize a higher-order active low-pass filter.

[Brief description of drawings]

FIG. 1 is a conventional second-order active low-pass filter circuit diagram, FIG. 2 is a second-order switched capacitor filter circuit diagram, and FIG. 3 is a circuit configuration diagram of an embodiment of the present invention.
FIG. 5 is a circuit configuration diagram of another embodiment of the present invention, FIG. 5 is a circuit diagram of an application example of the present invention, and FIGS. 6 and 7 are gain and delay characteristic diagrams of the same circuit, respectively. 21 ... Input end, 22, 24 ... FET, 23, 25 ... Circuit system, 26 ... Output end, R _A , R _B , R _C ... Resistance component, C _A , C _B , C _C ... Capacitance component, V _DD , V
_SS ... power supply.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-56-44214 (JP, A) real-opening Sho-53-30840 (JP, U) real-opening Sho-54-35145 (JP, U) real-opening Sho-56- 169625 (JP, U)

Claims (2)

(57) [Claims] 1. A first depletion type device having a drain connected to a first power supply potential supply terminal and a gate connected to a signal input terminal.
Of MOS field effect transistors and one or more MOS field effect transistors, one end of which is connected to the second power supply potential supply end and the other end of which is connected to the source of the first MOS field effect transistor. A first circuit system that performs a constant current operation, and a first end whose one end is connected between the source of the first MOS field effect transistor and the other end of the first circuit system.
Resistance component, a second resistance component having one end connected to the other end of the first resistance component, and a second resistance component connected between the other end of the second resistance component and the second power supply potential supply end. Second MOS type field effect in which the first capacitance component and the connection point between the second resistance component and the first capacitance component are connected to the gate and the drain is connected to the first power supply potential supply terminal. A transistor,
One or more MOS field effect transistors, one end of which is connected to the second power supply potential supply end and the other end of which is the second MO.
A second circuit system connected to the source of the S-type field effect transistor and performing a constant current operation is connected to the connection point of the source of the second MOS type field effect transistor and the other end of the second circuit system. And the second capacitance component having one end connected to the signal output terminal and the other end connected to the connection point of the first and second resistance components are integrated on the same semiconductor substrate. Active low-pass filter characterized by: 2. A depletion type first device having a drain connected to a first power supply potential supply terminal and a gate connected to a signal input terminal.
Of MOS field effect transistors and one or more MOS field effect transistors, one end of which is connected to the second power supply potential supply end and the other end of which is connected to the source of the first MOS field effect transistor. A first circuit system that performs a constant current operation, and a first end whose one end is connected between the source of the first MOS field effect transistor and the other end of the first circuit system.
A second resistance component having one end connected to the other end of the first resistance component, a third resistance component having one end connected to the other end of the second resistance component, A first capacitance component connected between a connection point of first and second resistance components and the second power supply potential supply terminal, the other end of the third resistance component, and the second power supply potential Second connected to the supply end
And a second MOS type field effect transistor having a connection point between the third resistance component and the second capacitance component connected to the gate and a drain connected to the first power supply potential supply terminal, A plurality of MOS field effect transistors, one end of which is connected to the second power supply potential supply end and the other end of which is the second power supply potential supply end.
A second circuit system connected to the source of the MOS field effect transistor for constant current operation, and a connection point between the source of the second MOS field effect transistor and the other end of the second circuit system. The connected signal output terminal and the third capacitance component having one end connected to the signal output terminal and the other end connected to the connection point of the second and third resistance components are integrated on the same semiconductor substrate. An active low-pass filter characterized by: