CA1233581A - Self adjusting bias power supply for line powered telephones - Google Patents
Self adjusting bias power supply for line powered telephonesInfo
- Publication number
- CA1233581A CA1233581A CA000481686A CA481686A CA1233581A CA 1233581 A CA1233581 A CA 1233581A CA 000481686 A CA000481686 A CA 000481686A CA 481686 A CA481686 A CA 481686A CA 1233581 A CA1233581 A CA 1233581A
- Authority
- CA
- Canada
- Prior art keywords
- power supply
- voltage
- line
- transistor
- telephone
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
TITLE
SELF ADJUSTING BIAS POWER SUPPLY FOR
LINE POWERED TELEPHONES
ABSTRACT OF THE DISCLOSURE
A DC voltage regulator circuit with capaca-tive filtering for use with line powered telephone instruments or microprocessor based telephones having a requirement for a precision voltage supply to be derived from the telephone line as a power source.
Precise regulation also permits use of the present supply in connection with multiplexed liquid crystal displays.
SELF ADJUSTING BIAS POWER SUPPLY FOR
LINE POWERED TELEPHONES
ABSTRACT OF THE DISCLOSURE
A DC voltage regulator circuit with capaca-tive filtering for use with line powered telephone instruments or microprocessor based telephones having a requirement for a precision voltage supply to be derived from the telephone line as a power source.
Precise regulation also permits use of the present supply in connection with multiplexed liquid crystal displays.
Description
123~5~:~
SELF ADJUSTING BIAS POWER SUPPLY FOR
..... _ _ _ . . _ .
LINE POWERED TELEPHONES
sACKGROUND OF THE INVENTION
Field of the Invent on The present inven-tion relates -to subscriber line telephone instruments which derive their power source from the -telephone line and more particularly to a temperature~
dependent, precision, direct current, regulated voltage supply designed for use of such subscriber line telephone instruments.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a par-tial schema-tic diagram of a prior art tachnique utilizing a current regulator diode to power an unregula-ted voltage supply for an electronic telephone.
Fig. 2 is a partial schematic diagram of a prior art me-thod of powering a shun-t regulated voltage supply using an inductor to achieve high A.C. isolation.
Fig. 3 is a partial schematic diagram of a D.C.
vol-tage supply for subscriber line powered telephones in accordance wi-th the present invention.
_ckground Art A number of different methods have been daveloped to power subscriber line telephone instruments containing electronic circuitry. The available power sources include .~.
. ~ '''` ~.~ .
.
; .
' - .:
1~35~3~
the 110 volt alternat.ing current power line, bat-teries, and/or the subscriber's telephone line.
The "Feature Phone", made by GTE Communication Sys-tems Corporation, is an example of the -telephone powered from ei-ther -the A.C. power line or backup batteries. This telephone utilizes a transformer -to step the 110 volt A.C. power line down to 24 volts A.C. It is then converted to positive 12, negative ].2 and positive 5 volt D.C. utilizing a switching D.C. voltage regulator.
Because of this arrangement repertory memory is maintained during the absence of A.C. power by a rechargeable battery supply.
Some electronic telephones obtain the necessary power required by the dialer circuit from the ; 15 -telephone subscriber's line utilizing a current regulating diode. The circuit, shown in prior art Fig.
1, represen-ts this approach and was utilized in the "F'lip -lA-.~.. ,.,~, .
, . : . .
.'' , .' -.
3358~
phone"~ II manufactured by GTE Communication Systems corporation. In this application the constant current diode CR2 is used to supply D.C. current in the range of several hundred microamperes for the dialer circuit.
This method generates an unregulated, D.C. supply voltage which is dependent upon the input voltage supplied to the tip and ring inputs to the telephone loop. This technique is not suitable for either the current needs of microprocessor based telephones or the precise, regulated voltage required by multiplexed liquid crystal displays which have found their way into some o the more sophisticated telephone designs.
Recently~ a few microprocessor based telephones such as the "Hangtel" manufactured by American Tele-communications Corporation and the "Duophone 160"marketed by Radio Shack Inc. have utilized the subscriber line's tip and ring connections as a source of power.
The circuitry shown in prior art Fig. 2 represents the technique used to supply power to the MOS micro-processor; random access memory (R~M) and dialerintegrated circuitry.
I'ypically the voltage regulator function is performed by diode CR6, a 5.1 volt clamping zener diode. Alternating current isolation between the low A.C. impedance capacitor C3 and the telephone subscriber line is provided by inductor Ll, assuming that transistors Ql and Q2 are both switched lnto the "on" stat~. Unfortunately, this simple technique causes a portion of the D. C. loop to be shunted dir-ectly to circuit ground. Furthermore, this currentincreases in proportion to the input voltage across the tip and ring inputs. Since this is also a function of the length of the telephone line to the central office, this technique has a direct effect on the . ' .~ . ' , - ' .
3358~
telephone's acoustic characteristics and the related method for loop length compensation. In addition, such telephones have difficulty meeting the input voltage requirement on long subscriber loops. Accord-ing to the Electronic Industry Association (EIA) speci-fication RS470, the input voltage for a loop current of 20 milliamperes in the off-hook state must not exceed 6 volts. The preferred value for the industry is 5 volts. As a result telephones utilizing this circuitry may either fail to function or no longer be operating from a regulated voltage supply. If a lower voltage zener diode is used for diode CR6, then this transformation from a zener with an abrupt breakdown voltage characteristic to one with a gradual breakdown voltage characterîstic will significantly degrade the quality of the voltage regulator. It will also tend to discharge the filter/storage capacitor C3 during dial pulse address signaling by the telephone, such that the controlling micoprocessor may cease to function. Finally, the supply voltage VDD character-iskics are not consistent with the precise voltage re~uirements of multiplexed liquid crystal displays.
Ideally, liquid crystal display (LCD) drivers in this application would enjoy a supply voltage in the vicinity of 3.2 volts D.C. at 25 centlgrade with a temperature coefficient of approximately minus 7 millivolts per degree centigrade. This circuit would require the user to frequently adjust a contrast adjustment rheostat which would create an adjustable voltage drop between the voltage supply and the integrated circuit LCD driver.
A further refinement, including temperature compensation, would be the series addition of a posi-tive temperature compensating thermister; for example, the "Tempistor" made by Midwest Components Inc.
~2335~3~
The prior art does not appear to include a regulated D. C. voltage supply which i~ both powered by the telephone subscriber line and temperature comp-ensated for the operation of liquid crystal displays.
~ccordingly the object of the present invention is the development o a ~emperature compensated, low cost, low power, direct current, regulated voltage supply ~or a line powered microprocessor-based telephone which may incorporate multiplexed, liquid crystal dispiays~
SUMM~RY OF THE INVENTION
, The present invention consists of circuitry combining two transistors, a voltage regulating circuit which encompasses a third transistor, three diodes, three filter capacitors, three resistors, and a load circuitry represented by a fourth resistor. The input for the first resistor is coupled to the telephone subscriber's line posîtive input voltage via the usual hook switch and diode bridge rectifier. The other slde of this first, high valued, resistor is coupled to a high valued, first filter capacitor which, in turn, is tied to circuit ground and coupled back to the telephone subscriber's line. ~ string o~ three forward biased diodes is are connected in parallel with the first capacitor. The parallel combination o~ a second ~ 25 resistor and capacitor is coupled between the base - o~ an emitter follower NPN transistor and the node formed between the aforementioned first resistor and capacitor. The emitter circuit is represented by a third resistor to circuit ground. The collector of this NP~, first transistor is in turn connected to the base of a PNP, or second transistor with the PNP
transistor emitter being directly connected to the ; hook switch and the first resistor. The collector of the NPN transistor is tied to a third ~ilter capac-itor, the input ~or a voltage regulating circuit and . . - -' ~
. . , ~ , . ~
-~335~
the VDD voltage supply~ The supply's load circuit, coupled between the PWP transistor's collector and cir~uit ground, i~ in par~ represented here by a fourth resis~or.
The output of the voltage regula~ing circuit represented by the collector of a NP~ common emi~ter, third tran~is-tor is connected to the base of the first, NP~ transistor.
The invention, as described, consists of a direct current, regulated voltage supply powered by the t~lephone ~ubscriber's line. It is compo~ed of a direct curr~nt ~ource controlled by a voltage regu-lating circuit. The D.C. current source is comprised of the aforementioned, PNP second transistor which is, in turn, bias~d by the first NPN translstor which also act~ as a voltage controlled D.C. current source.
The c~llector o~ a NP~, third transistor, representing the output of a voltage regulating circuit, is used to control the operation of these two interdependent current source~. The regulating circuit, which might ; be like that in my co-pending Canadian ap~lication No.
481,683 ~iled May 16, 1985, causes its output to con-duct current from the base of the NPN, first transistor to circuit ground when the VDD voltage supply begins to exceed the pr~set threshold which ~efines the regulated output voltage. As a result the D . C . current flowing rom the subscriber line to the VDD voltage supply is reduced in a voltage regulating manner.
In operation~ the input voltage from the telephone sub~criber line is coupled to a low pas~
ilter .~ormed by the aforementioned flrst resistor and capaeitor. The filter output is clamped in the vicinity of 1~3 volts D.C. by the string of three ' : . :
.
. ~ ,' , , :
~3;35~31 diodes to create a fixed bias voltage supply. This technique is used to make the filter's output voltage insensitive to either alternating current alerting ring signals or lightning surges which may appear at the telephone subscriber's line tip and ring inputs.
The filter's output is coupled to the base of the NPN first transistor by the parallel combination of a second resistor and capacitor. The second resistor serves as the primary source of blas current to the NPN transistor under normal, steady state conditions.
The second capacitor, in this case, serves in conjunction with the first capacitor as an A.C. short circuit between the base of the NPN first transistor and circuit ground. As a result, the logic switching transients, appearing on the VDD voltage supply, have a minimal effect on the operation of the aforementioned voltage dependent current source such that the noise emitted to the telephone subscriber line meets the requirements oE the FCC Rules and Regulations, Parts 15 and 68.
Similarly the high A.C. impedance exhibited by the D.C. current source decouples the voice band slgnals appearing on the telephone line from both the swltching transients and the low A. C. impedance associated with the VDD voltage supply.
Prior to achieving steady state e~uillbrium, however, the aforementioned second capacitor must be charged to the steady state voltage drop appearing across the second resistor. As a result, the base current is significantly increased during the initial period following a telephone on-hook to off-hook transition. The emitter of this first NPN transistor is connected to the circuit ground via a third voltage dropping resistor. This resistor, in turn, acts in conjunction wlth the voltage applied to the base of . .
~;~335i~3~
the first NPN transistor to efEect a voltage controlled sink of collector current~ The collector, of khis transistor, is connected to the base of the aforemen-tioned PNP, second transistor whose emitter is coupled to the positive side of the subscriber's line. The collector of this PNP transistor is connected to the VDD voltage supply and a third filter capacitorO
Since both of these transistors are designed to operate within their active regions, the combination maintains its function as a D.C. voltage controlled current source for the VDD voltage supply~ The in rush of charging current through the aforementioned, second capacitor, via a hook switch transition is reflected by a temporary increase in the current source's output such that the VDD voltage supply is ramped up by the aforementioned voltage controlled current source until it exceeds the threshold (approximately 3.2 volts D.C. at 25 centigrade) of the voltage regulating circuit. It should be noted that a suitable voltage regulating circuit is shown in the aforementioned co-pending application. At this point the output of the voltage comparator is used to negatively bias the base of the first NPN transistor. This reduces the voltage on the base of this transistor which, in turn, reduces the bias current to the base of the PMP
transistor. As a result this device reduces the current which it sources to the VDD voltage supply causing the circuit to exhibit a voltage regulating action.
~he temperature coefficient for the regulated VDD voltage supply is deEined by the input threshold voltage of the voltage regulating circuit. The circuit, described by this embodiment, can be used to effect a temperature coefficient in the vicinity of minus .
' 358~
7 millivolts per degree centigrade. This characteris-tic can b0 used in li~uid crystal display applica-tions to minimize the contrast variations exhibited as a function of temperature.
DESCRIPTION OF T~IE PREFERRED EMBODIMENT
Referring now to Fig. 3, PNP transistor Q1 acts as the gating element of a D.C. aurrent source coupled between the V voltage supply and the telephone DD
subscriber's line. Transistor Q1 is, in -turn, biased and controlled by transistor Q2. Transistor Q2, an NPN -type transistor, functions in conjunation with its emitter resistor R3 as a current source under the control of a voltage applied to i-ts base. The A.C. response of this current source is minimized by filter capacitors C1 and C2. Under steady state conditions, these two capacitors provide a low A.C. impedance pa-th to circuit ground so that high frequency variations in the regulating, negative bias current from NPN regulating -transistor Q3 have virtually no effect on the current source operation.
Resistor R~ is used to represent a portion af the load across the V voltage supply.
DD
. -', . ' : ., ': ~,' ' '. '' :' :'.' ' ' . ' ' ' ~335~3~
Resistor Rl acts in conjunction with capacitor C2 to form a low pass filter~ The output has clamped at approximately 1.3 volts D.C. by reference diodes;
CRl, CR2 and CR3.
Resistor R2 is used to couple bias current from this output to the base of NPN transistor Q2. This is the primary source of positive base bias current under steady state conditions. During turn on conditions, however, capacitor Cl acts as an additional source of base current as it is charged up to the steady state voltage which is eventually exhibited across resistor R2. This causes a temporary surge in the current flowing in both transistor Ql and Q2. As a result the filter/storage capacitor C3 experiences a fast charging rate as the VDD voltage supply ramps up to the threshold voltage o the regulating circuitry.
Both of these transistors maintain their current source functions and operate within the active region during this turn on transient. This is due to the fact that the emitter current in transistor Q2 will increase only until resistor R3 "swamps" the transistor by limiting any further increase in the controlling of the VDD potential of transistor Q2. The maximum voltage on the base is limited to approximately 1.3 volts by the string oE three diodes. This causes the oper-ation of ~he above current source to be relatlvely insensitive to either A.C. alerting ring signals or lightning surge voltages.
When the VDD voltage supply exceeds the threshold voltage o, for example, 3.2 volts D.C., the regulating circuit's NPN output transistor Q3 or its equivalent, acts as a regulating, negative source of bias current to the base of NPN transistor Ql.
:
.
.
~23~S8~
The resulating circuit, described, here is made up of a temperature-dependent voltage reference which is connected to the positive input (or its functional equivalent) o a voltage comparator whose output ls represented by the collector of transistor Q3. As the V~D voltage supply increases above the input thres-hold voltage, transistor Q3 turns on "harder" drawing more current away from the base o transisor Q2.
Transistor Q2 then reduces drive current from the base of transistor Ql. Subsequently transistor Ql reduces its current flow, thereby lowering the supply voltage back toward the threshold point of the comparator.
On the other hand, if the supply voltage drops trans-istor Q3 reduces its current drain on the base of transistor Q2. Therefore transistor Q2 increases its current drive for transistor Ql allowing more collector current to Elow through transistor Ql so îhat the VDD supply voltage is increased back up to a point slightly above the comparator's threshold.
The regulating action is, therefore established at a stable point determined by the circuit's component values and the temperature coefficients oE the regu-lating circuit. The temperature coefficient exhibited by the VDD voltage supply oE this embodiment with the referenced regulating circult is in the vicinity of minus 7 millivolts per degree centigrade. This condition o a preset voltage level and temperature coefficient for the VDD voltage supply can be used in those applications involving the use of liquid crystal displays so that the contrast oE the display is maintained regardless of the temperature. In a typical application, this might involve a Hughes Solid State Products part no. HLCD0515, CMOS LCD driver and a triplex display exhibiting threshold and satura-tion voltages in the vicinity of 1.3 volts A.C. and1.8 volts A.C. respectively.
, ~ 233~
It will be obvious to those skilled in the art that numerous modifications may be made without departing from the spirit of the present invention which shall be limited only by the scope of the claims appended hereto.
, .
' ' ' . . ~' ' .
SELF ADJUSTING BIAS POWER SUPPLY FOR
..... _ _ _ . . _ .
LINE POWERED TELEPHONES
sACKGROUND OF THE INVENTION
Field of the Invent on The present inven-tion relates -to subscriber line telephone instruments which derive their power source from the -telephone line and more particularly to a temperature~
dependent, precision, direct current, regulated voltage supply designed for use of such subscriber line telephone instruments.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a par-tial schema-tic diagram of a prior art tachnique utilizing a current regulator diode to power an unregula-ted voltage supply for an electronic telephone.
Fig. 2 is a partial schematic diagram of a prior art me-thod of powering a shun-t regulated voltage supply using an inductor to achieve high A.C. isolation.
Fig. 3 is a partial schematic diagram of a D.C.
vol-tage supply for subscriber line powered telephones in accordance wi-th the present invention.
_ckground Art A number of different methods have been daveloped to power subscriber line telephone instruments containing electronic circuitry. The available power sources include .~.
. ~ '''` ~.~ .
.
; .
' - .:
1~35~3~
the 110 volt alternat.ing current power line, bat-teries, and/or the subscriber's telephone line.
The "Feature Phone", made by GTE Communication Sys-tems Corporation, is an example of the -telephone powered from ei-ther -the A.C. power line or backup batteries. This telephone utilizes a transformer -to step the 110 volt A.C. power line down to 24 volts A.C. It is then converted to positive 12, negative ].2 and positive 5 volt D.C. utilizing a switching D.C. voltage regulator.
Because of this arrangement repertory memory is maintained during the absence of A.C. power by a rechargeable battery supply.
Some electronic telephones obtain the necessary power required by the dialer circuit from the ; 15 -telephone subscriber's line utilizing a current regulating diode. The circuit, shown in prior art Fig.
1, represen-ts this approach and was utilized in the "F'lip -lA-.~.. ,.,~, .
, . : . .
.'' , .' -.
3358~
phone"~ II manufactured by GTE Communication Systems corporation. In this application the constant current diode CR2 is used to supply D.C. current in the range of several hundred microamperes for the dialer circuit.
This method generates an unregulated, D.C. supply voltage which is dependent upon the input voltage supplied to the tip and ring inputs to the telephone loop. This technique is not suitable for either the current needs of microprocessor based telephones or the precise, regulated voltage required by multiplexed liquid crystal displays which have found their way into some o the more sophisticated telephone designs.
Recently~ a few microprocessor based telephones such as the "Hangtel" manufactured by American Tele-communications Corporation and the "Duophone 160"marketed by Radio Shack Inc. have utilized the subscriber line's tip and ring connections as a source of power.
The circuitry shown in prior art Fig. 2 represents the technique used to supply power to the MOS micro-processor; random access memory (R~M) and dialerintegrated circuitry.
I'ypically the voltage regulator function is performed by diode CR6, a 5.1 volt clamping zener diode. Alternating current isolation between the low A.C. impedance capacitor C3 and the telephone subscriber line is provided by inductor Ll, assuming that transistors Ql and Q2 are both switched lnto the "on" stat~. Unfortunately, this simple technique causes a portion of the D. C. loop to be shunted dir-ectly to circuit ground. Furthermore, this currentincreases in proportion to the input voltage across the tip and ring inputs. Since this is also a function of the length of the telephone line to the central office, this technique has a direct effect on the . ' .~ . ' , - ' .
3358~
telephone's acoustic characteristics and the related method for loop length compensation. In addition, such telephones have difficulty meeting the input voltage requirement on long subscriber loops. Accord-ing to the Electronic Industry Association (EIA) speci-fication RS470, the input voltage for a loop current of 20 milliamperes in the off-hook state must not exceed 6 volts. The preferred value for the industry is 5 volts. As a result telephones utilizing this circuitry may either fail to function or no longer be operating from a regulated voltage supply. If a lower voltage zener diode is used for diode CR6, then this transformation from a zener with an abrupt breakdown voltage characteristic to one with a gradual breakdown voltage characterîstic will significantly degrade the quality of the voltage regulator. It will also tend to discharge the filter/storage capacitor C3 during dial pulse address signaling by the telephone, such that the controlling micoprocessor may cease to function. Finally, the supply voltage VDD character-iskics are not consistent with the precise voltage re~uirements of multiplexed liquid crystal displays.
Ideally, liquid crystal display (LCD) drivers in this application would enjoy a supply voltage in the vicinity of 3.2 volts D.C. at 25 centlgrade with a temperature coefficient of approximately minus 7 millivolts per degree centigrade. This circuit would require the user to frequently adjust a contrast adjustment rheostat which would create an adjustable voltage drop between the voltage supply and the integrated circuit LCD driver.
A further refinement, including temperature compensation, would be the series addition of a posi-tive temperature compensating thermister; for example, the "Tempistor" made by Midwest Components Inc.
~2335~3~
The prior art does not appear to include a regulated D. C. voltage supply which i~ both powered by the telephone subscriber line and temperature comp-ensated for the operation of liquid crystal displays.
~ccordingly the object of the present invention is the development o a ~emperature compensated, low cost, low power, direct current, regulated voltage supply ~or a line powered microprocessor-based telephone which may incorporate multiplexed, liquid crystal dispiays~
SUMM~RY OF THE INVENTION
, The present invention consists of circuitry combining two transistors, a voltage regulating circuit which encompasses a third transistor, three diodes, three filter capacitors, three resistors, and a load circuitry represented by a fourth resistor. The input for the first resistor is coupled to the telephone subscriber's line posîtive input voltage via the usual hook switch and diode bridge rectifier. The other slde of this first, high valued, resistor is coupled to a high valued, first filter capacitor which, in turn, is tied to circuit ground and coupled back to the telephone subscriber's line. ~ string o~ three forward biased diodes is are connected in parallel with the first capacitor. The parallel combination o~ a second ~ 25 resistor and capacitor is coupled between the base - o~ an emitter follower NPN transistor and the node formed between the aforementioned first resistor and capacitor. The emitter circuit is represented by a third resistor to circuit ground. The collector of this NP~, first transistor is in turn connected to the base of a PNP, or second transistor with the PNP
transistor emitter being directly connected to the ; hook switch and the first resistor. The collector of the NPN transistor is tied to a third ~ilter capac-itor, the input ~or a voltage regulating circuit and . . - -' ~
. . , ~ , . ~
-~335~
the VDD voltage supply~ The supply's load circuit, coupled between the PWP transistor's collector and cir~uit ground, i~ in par~ represented here by a fourth resis~or.
The output of the voltage regula~ing circuit represented by the collector of a NP~ common emi~ter, third tran~is-tor is connected to the base of the first, NP~ transistor.
The invention, as described, consists of a direct current, regulated voltage supply powered by the t~lephone ~ubscriber's line. It is compo~ed of a direct curr~nt ~ource controlled by a voltage regu-lating circuit. The D.C. current source is comprised of the aforementioned, PNP second transistor which is, in turn, bias~d by the first NPN translstor which also act~ as a voltage controlled D.C. current source.
The c~llector o~ a NP~, third transistor, representing the output of a voltage regulating circuit, is used to control the operation of these two interdependent current source~. The regulating circuit, which might ; be like that in my co-pending Canadian ap~lication No.
481,683 ~iled May 16, 1985, causes its output to con-duct current from the base of the NPN, first transistor to circuit ground when the VDD voltage supply begins to exceed the pr~set threshold which ~efines the regulated output voltage. As a result the D . C . current flowing rom the subscriber line to the VDD voltage supply is reduced in a voltage regulating manner.
In operation~ the input voltage from the telephone sub~criber line is coupled to a low pas~
ilter .~ormed by the aforementioned flrst resistor and capaeitor. The filter output is clamped in the vicinity of 1~3 volts D.C. by the string of three ' : . :
.
. ~ ,' , , :
~3;35~31 diodes to create a fixed bias voltage supply. This technique is used to make the filter's output voltage insensitive to either alternating current alerting ring signals or lightning surges which may appear at the telephone subscriber's line tip and ring inputs.
The filter's output is coupled to the base of the NPN first transistor by the parallel combination of a second resistor and capacitor. The second resistor serves as the primary source of blas current to the NPN transistor under normal, steady state conditions.
The second capacitor, in this case, serves in conjunction with the first capacitor as an A.C. short circuit between the base of the NPN first transistor and circuit ground. As a result, the logic switching transients, appearing on the VDD voltage supply, have a minimal effect on the operation of the aforementioned voltage dependent current source such that the noise emitted to the telephone subscriber line meets the requirements oE the FCC Rules and Regulations, Parts 15 and 68.
Similarly the high A.C. impedance exhibited by the D.C. current source decouples the voice band slgnals appearing on the telephone line from both the swltching transients and the low A. C. impedance associated with the VDD voltage supply.
Prior to achieving steady state e~uillbrium, however, the aforementioned second capacitor must be charged to the steady state voltage drop appearing across the second resistor. As a result, the base current is significantly increased during the initial period following a telephone on-hook to off-hook transition. The emitter of this first NPN transistor is connected to the circuit ground via a third voltage dropping resistor. This resistor, in turn, acts in conjunction wlth the voltage applied to the base of . .
~;~335i~3~
the first NPN transistor to efEect a voltage controlled sink of collector current~ The collector, of khis transistor, is connected to the base of the aforemen-tioned PNP, second transistor whose emitter is coupled to the positive side of the subscriber's line. The collector of this PNP transistor is connected to the VDD voltage supply and a third filter capacitorO
Since both of these transistors are designed to operate within their active regions, the combination maintains its function as a D.C. voltage controlled current source for the VDD voltage supply~ The in rush of charging current through the aforementioned, second capacitor, via a hook switch transition is reflected by a temporary increase in the current source's output such that the VDD voltage supply is ramped up by the aforementioned voltage controlled current source until it exceeds the threshold (approximately 3.2 volts D.C. at 25 centigrade) of the voltage regulating circuit. It should be noted that a suitable voltage regulating circuit is shown in the aforementioned co-pending application. At this point the output of the voltage comparator is used to negatively bias the base of the first NPN transistor. This reduces the voltage on the base of this transistor which, in turn, reduces the bias current to the base of the PMP
transistor. As a result this device reduces the current which it sources to the VDD voltage supply causing the circuit to exhibit a voltage regulating action.
~he temperature coefficient for the regulated VDD voltage supply is deEined by the input threshold voltage of the voltage regulating circuit. The circuit, described by this embodiment, can be used to effect a temperature coefficient in the vicinity of minus .
' 358~
7 millivolts per degree centigrade. This characteris-tic can b0 used in li~uid crystal display applica-tions to minimize the contrast variations exhibited as a function of temperature.
DESCRIPTION OF T~IE PREFERRED EMBODIMENT
Referring now to Fig. 3, PNP transistor Q1 acts as the gating element of a D.C. aurrent source coupled between the V voltage supply and the telephone DD
subscriber's line. Transistor Q1 is, in -turn, biased and controlled by transistor Q2. Transistor Q2, an NPN -type transistor, functions in conjunation with its emitter resistor R3 as a current source under the control of a voltage applied to i-ts base. The A.C. response of this current source is minimized by filter capacitors C1 and C2. Under steady state conditions, these two capacitors provide a low A.C. impedance pa-th to circuit ground so that high frequency variations in the regulating, negative bias current from NPN regulating -transistor Q3 have virtually no effect on the current source operation.
Resistor R~ is used to represent a portion af the load across the V voltage supply.
DD
. -', . ' : ., ': ~,' ' '. '' :' :'.' ' ' . ' ' ' ~335~3~
Resistor Rl acts in conjunction with capacitor C2 to form a low pass filter~ The output has clamped at approximately 1.3 volts D.C. by reference diodes;
CRl, CR2 and CR3.
Resistor R2 is used to couple bias current from this output to the base of NPN transistor Q2. This is the primary source of positive base bias current under steady state conditions. During turn on conditions, however, capacitor Cl acts as an additional source of base current as it is charged up to the steady state voltage which is eventually exhibited across resistor R2. This causes a temporary surge in the current flowing in both transistor Ql and Q2. As a result the filter/storage capacitor C3 experiences a fast charging rate as the VDD voltage supply ramps up to the threshold voltage o the regulating circuitry.
Both of these transistors maintain their current source functions and operate within the active region during this turn on transient. This is due to the fact that the emitter current in transistor Q2 will increase only until resistor R3 "swamps" the transistor by limiting any further increase in the controlling of the VDD potential of transistor Q2. The maximum voltage on the base is limited to approximately 1.3 volts by the string oE three diodes. This causes the oper-ation of ~he above current source to be relatlvely insensitive to either A.C. alerting ring signals or lightning surge voltages.
When the VDD voltage supply exceeds the threshold voltage o, for example, 3.2 volts D.C., the regulating circuit's NPN output transistor Q3 or its equivalent, acts as a regulating, negative source of bias current to the base of NPN transistor Ql.
:
.
.
~23~S8~
The resulating circuit, described, here is made up of a temperature-dependent voltage reference which is connected to the positive input (or its functional equivalent) o a voltage comparator whose output ls represented by the collector of transistor Q3. As the V~D voltage supply increases above the input thres-hold voltage, transistor Q3 turns on "harder" drawing more current away from the base o transisor Q2.
Transistor Q2 then reduces drive current from the base of transistor Ql. Subsequently transistor Ql reduces its current flow, thereby lowering the supply voltage back toward the threshold point of the comparator.
On the other hand, if the supply voltage drops trans-istor Q3 reduces its current drain on the base of transistor Q2. Therefore transistor Q2 increases its current drive for transistor Ql allowing more collector current to Elow through transistor Ql so îhat the VDD supply voltage is increased back up to a point slightly above the comparator's threshold.
The regulating action is, therefore established at a stable point determined by the circuit's component values and the temperature coefficients oE the regu-lating circuit. The temperature coefficient exhibited by the VDD voltage supply oE this embodiment with the referenced regulating circult is in the vicinity of minus 7 millivolts per degree centigrade. This condition o a preset voltage level and temperature coefficient for the VDD voltage supply can be used in those applications involving the use of liquid crystal displays so that the contrast oE the display is maintained regardless of the temperature. In a typical application, this might involve a Hughes Solid State Products part no. HLCD0515, CMOS LCD driver and a triplex display exhibiting threshold and satura-tion voltages in the vicinity of 1.3 volts A.C. and1.8 volts A.C. respectively.
, ~ 233~
It will be obvious to those skilled in the art that numerous modifications may be made without departing from the spirit of the present invention which shall be limited only by the scope of the claims appended hereto.
, .
' ' ' . . ~' ' .
Claims (11)
1. A DC power supply for line powered tele-phones comprising: a telephone line connected to a source of DC power; voltage output means adapted for connection to a telephone; gating means connected between said telephone line and said output means;
positive bias voltage means connected across said telephone line; control means connected between said positive bias voltage means and said gating means, operated to render said gating means conductive to conduct DC voltage from said telephone line to said output means; and a regulator circuit including an input connected to said output means and a circuit connection to said control means, operated in response to variations in said DC voltage at said output means to extend a variable negative bias voltage to said control means to further operate said control means and regulate the conductivity of said gating means.
positive bias voltage means connected across said telephone line; control means connected between said positive bias voltage means and said gating means, operated to render said gating means conductive to conduct DC voltage from said telephone line to said output means; and a regulator circuit including an input connected to said output means and a circuit connection to said control means, operated in response to variations in said DC voltage at said output means to extend a variable negative bias voltage to said control means to further operate said control means and regulate the conductivity of said gating means.
2. A DC power supply for line powered tele-phones as claimed in Claim 1 wherein: said gating means comprise a transistor.
3. A DC power supply for line powered tele-phones as claimed in Claim 2 wherein: said transistor is of the PNP type.
4. A DC power supply as claimed in Claim 1 wherein: said positive bias voltage means comprise serially connected resistance means and unidirectional conducting means.
5. A DC power supply for line powered tele-phones as claimed in Claim 4 wherein: said unidirec-tional conducting means comprise a plurality of simi-larly poled serially connected diodes.
6. A DC power supply for line powered tele-phones as claimed in Claim 1 wherein: said control means comprise a transistor.
7. A DC power supply for line powered tele-phones as claimed in Claim 6 wherein: said transistor is of the NPN type.
8. A DC power supply for line powered tele-phones as claimed in Claim 1 wherein: there is further included a telephone hookswitch connected between said telephone line and said gating means.
9. A DC power supply for line powered tele-phones as claimed in Claim 1 wherein: there is further included a voltage polarity guard connected between said telephone line and said gating means.
10. A DC power supply for line powered telephones as claimed in Claim 1 wherein: there is further included filter means connected to said output means.
11. A DC power supply for line powered telephones as claimed in Claim 10 wherein: said fil-ter means comprise a capacitor.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US67050984A | 1984-11-13 | 1984-11-13 | |
| US670,509 | 1984-11-13 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1233581A true CA1233581A (en) | 1988-03-01 |
Family
ID=24690684
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000481686A Expired CA1233581A (en) | 1984-11-13 | 1985-05-16 | Self adjusting bias power supply for line powered telephones |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1233581A (en) |
-
1985
- 1985-05-16 CA CA000481686A patent/CA1233581A/en not_active Expired
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