US3026454A

US3026454A - Current driving circuit

Info

Publication number: US3026454A
Application number: US856894A
Authority: US
Inventors: Francis E Goodwin
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1959-12-01
Filing date: 1959-12-01
Publication date: 1962-03-20
Anticipated expiration: 1979-03-20
Also published as: GB900605A

Description

r R F F. E. GOODWIN CURRENT DRIVING CIRCUIT 2 Sheets-Sheet l llll R 1L g March 20, 1962 Filed Dec. 1, 1959 Q? 2E 3 -62: 23m

FRANCIS E GOODWIN,

INVENTOR BY ATTORNEY March 20, 1962- F. E. GOODWIN 3,026,454

CURRENT DRIVING CIRCUIT Filed Dec. 1, 1959 2 Sheets-Sheet 2 VOLTAGE VOLTAGE CURRENT w FRANCIS E. cooowm, INVENTOR s6 *54 *sz 50 CURRENT AMPERES ATTORNEY United States "neat 3,026,454 Patented Mar. 20, 1962 This invention relates to current driving circuits and particularly to a transistor current source for ferrite phase shifters and other inductive loads.

An inductive element when being driven with a step source of impedance R has a basic time constant L/R for changing current through the inductive element where L is the inductance of the element. For the time constant of changing current through the inductive element to be small, either the inductance L must be small or the effective resistance R must be large, or both. The value of the inductance L may be fixed by other considerations such as when utilizing a ferrite phase shifter where the L may be established at a fixed value.

One prior art means to drive ferrite phase shifters with a small time constant is to use high resistance vacuum tubes as current drivers. However, this type of driving arrangement requires a very large current and wastes a relatively large amount of power, especially if a large constant current is to be maintained through the inductance of the phase shifter for a long period of time. In circuit arrangements where a large number of phase shifters are utilized, the large power loss makes vacuum tube drivers impractical.

Another conventional driver for inductive loads such as phase shifters is a current source utilizing power transistors which will effectively hold a constant current indefinitely. However, the conventional transistor arrangement provides a source impedance of only a few ohms and the power source has a relatively small potential so that a very large time constant is present when changing the current through a phase shifter, for example. A conventional regulated current source utilizing transistors may include a switching transistor coupled in series with the inductance and a transistor utilized as a regulating amplifier coupled between a first and a second resistor that are connected between the current source and the inductance and the base of'the switching transistor. The voltage rating of the power source and the relative size of the two resistors determines the current passed through the inductance.

A current source utilizing transistors that would provide high voltages without destroying the transistors and high impedances to reduce the time constant so as to rapidly change from one current level through the inductance to another would be very valuable to the art.

It is thus an object of this invention to provide a current source for inductive loads utilizing transistors elements that is highly efiicient because of requiring a minimum amount of power.

It is a further object of this invention to provide an efficient current source for switching the current from one level to another through an inductive load with a minimum of time delay.

It is a still further object of this invention to provide a source of driving current for shifting the phase of a phase shifter element by rapidly changing the current to a plurality of predetermined levels.

It is another object of this invention to provide an efficient current source for a phase shifter that utilizes transistors and that utilizes switching transients for changing current through the inductive load of the phase shifter with a minimum of delay.

Briefly, this invention is a current source for inductive loads that includes a driving transistor having its load current path coupled in series with the inductive load. A regulating transistor has its load current path coupled across a power source through a protective diode for applying a potential to the base of the driving transistor during steady state conditions. The base of the regulating transistor is coupled through a programming signal generator to a point between a first and a second resistorconnected between the power supply and the inductive load. The programming signal generator responds to a source of signals to develop switching signals having predetermined volta-ge levels corresponding to currents required through the inductive load. These voltage levels control the regulating transistor to maintain the desired current through the inductive load during steady state conditions and respond to a negative feedback signal from the inductive load to maintain the regqilating transistor biased so that a constant current is maintained through the inductive load for each steady state or direct current voltage level of the switching signal. To provide large voltage transients when the switching signals change voltage level, a transient amplifier is coupled in series between the signal generator and the base of the driving transistor for applying switching transients signals to the base of the driving transistor and presenting a high impedance to the inductive load during switch ing transients of a first polarity. A diode is coupled in series in the load current paths of the driving transistor for providing a high impedance to the inductive load during switching transients of a second polarity. The switching transients of both polarities are applied to the inductive load to provide rapid changes of current therethrough. Thus, the current driving circuit of this invention maintains a regulated steady state current through the inductive load and during transient conditions, rapidly changes the current through the load to a new value.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, taken in conjunction with the accompanying drawings, in which like characters refer to like parts,

' and in which:

' FIG. 1;

FIG. 3 is a graph of voltage versus time showing a representative sequence of voltages developed by the programming signal generator of FIG. 1.

FIG. 4 is a graph of voltage versus time showing the transient and steady state volt-ages developed by the transient amplifier of FIG. 1 in response to the voltages of FIG. 3;

FIG. 5 is a graph of current versus time for illustrating the rapid change of current through the phase shifter of FIG. 1.

Referring first to FIG. 1 which shows a schematic diagram of the current driving circuit of this invention, the arrangements of the elements therein will be described. A battery 10 having one end coupled to a ground lead 12 as a voltage reference and the other end coupled to a lead 14 is provided as a power source and may, for example, develop +6 volts. The lead 14 applies power through a series connected first resistor16 and second resistor 18 and through a lead 19 to one end of an inductive load or coil 20 of a phase shifter 22 that may be utilized with the circuit of this invention. The phase shifter 22 may include the coil 20 wound about a rectangular waveguide (not shown) with an axially disposed ferrite rod and an axial magnetic field provided by the solenoid. Suitable means are provided to prevent eddycurrent losses in the Waveguide itself. A phase shifter I of this general type is described on page 1510 of the November 195.7 issue of the IRE proceedings in an article by F.. Reggia and E. G. Spencer entitled A New Techniq-ue. in Ferrite Phase Shifting for Beam Scanning of Microwave Antennas.

The other end of the inductive load 2% passes current through the emitter to collector current path of a driver transistor 24 which is of the p-n-p type. The collector of the transistor 24 isv coupled through the anode to cathode path of a blocking diode 26 to the ground lead 12.

For steady state regulation of current through the inductive load 20, a regulating transistor 28 of the p-n-p type is provided. The emitter of the transistor 23 receives current from the lead 14 through the anode to cathode path of a zener diode 3,0 for providing a constant voltage drop so that the regulating transistor 28 operates in a desired amplification range. The collector of the transistor 28 is arranged to apply current to the ground lead 12 through the anode to cathode path of a protective diode 34 and through a lead 36 and a signal developing resistor 38, The potential developed by the resistor 38 is applied through a lead 40 to the base of the driving transistor 24 for controlling the current through the inductive load 2.0..

The base of the transistor 28.. is arranged to respond 'to the voltage developed by a programming signal genorator. 4.4 which includes a stepping switch 46 having an arm 48 movable to form a closed circuit with

contacts

59, 52, 54 and 56. A precision resistor 58 is provided with tapped points, connected to the

contacts

50, 52, 54 and 56 to provide different potential levels. One end of the resistor 5,8 isv coupled to the positive terminal of a battery 62. through a lead 64. and the other end of theresistor 5,8 is biased from the other end of the battery 62. The battery 62 may be selected to develop +2 volts. The lead. 64 is biased from a point between the resistors 16-, and 18 so that a change of current through the load effectively provides a negative feedback signal from the inductive load 20.

The arm 48 of the stepping. switch 46, is responsive through a mechanical connection 66 to thestate of rotation of a scan motor 63 for example. Thus, the arm 48 may sequentially move, for example, from the

contacts

50, 52, 54, 56v and back to the contact 50. The scan motor 68, may include an intermittent gear arrangement such as a rocket drive that allows. the movement to occur rapidly from one contact to another after a predetermined amount of rotation of the motor. It is to be noted that the continuous sequence; from a higher voltage to decreasing voltage levels then back to the higher voltage level is only one sequence that may be provided by the signal generator 64.. By connecting the contacts such as 50 to different tapping points of the resistor 58, any desired sequence may be developed.

The signal applied to the arm 48. is. in turn applied through a lead/70 to the base of the transistor 28 and to an amplifier 72 of a transient amplifier 74. The amplifier. 72 may be either an A.C. (alternating current) or a DC. (direct current) amplifier. The D.C. amplifier 72v appliesan amplified signal through a lead 76 to a high pass filter 78 also included in the transient amplifier 74. The high pass filter 78 applies only a transient signal to the lead 40 in response to. a change of voltage level of the switching signal developed by the signal generator 44, as will be explained subsequently.

The high pass filter 78 may be any conventional type of filter and may include a capacitor 80 having one end coupled to the lead 76 and the other end coupled to ground through a resistor 82 and to one end of a second capacitor 84, The other end of the second capacitor 34 is coupled to thelead 40 and to groundthrough a resistor 86, thus passing only transient signals of short duration.

Referring now to FIG. 2 showing a graph of phase versus current for the phase shifter 22, the operation of the circuit will be explained in further detail. A curve 88, which may be determined experimentally for any selected phase shifter 22 or any inductive load shows the required current through the inductance coil 20 for four phase conditions of a microwave signal applied through a waveguide (not shown) of the phase shifter 22. Currents i 1' 1' and i indicate the current required to be passed through, the phase shifter 22 to develop a phase shift of 0 degree, 120 degrees, 240 degrees and 360' degrees.

Referring now to FIG. 1 and to FIG. 3, the latter of which shows the step voltage developed by the signal generator 44,, to FIG. 4 which shows the transient signals developed by the transient amplifier 74 and to FIG. 5 which shows waveforms of the currents passed through the phase shifter 22, the operation of the circuit will be further explained. The sequence of step or switching voltages of a waveform in FIG. 3 is determined by the amount of phase shift desired of the phase shifter 22. The voltages e e e and e of the waveform 90 are developed. at. respective times t through A, by the contact arm 48, respectively, moving in response to the scan motor 68 to

contacts

50, 52, 54 and 56.

Before explaining the transient signals. developed when the switching voltages of FIG. 3 change voltage level, the steady state regulating operation of the, circuit will be explained. Each, switching voltage level of the waveform 90 such as the voltage 6 is applied to the base of the transistor 28 to bias the transistor 28 to pass a predetermined current from its emitterto collector and through the diode, 34 and the signal forming resistor 38. In response to each, voltage level of the: waveform 90, a predetermined voltage is developed on the lead 40 to establish a desired current through the emitter to the col lectoIT of the driver transistor 24. Thus,v in. response to each higher Voltage level e e e or 6 of the waveform 90,, the impedance through the'driver transistor 24 is increased and the, current through theinductive load 20 is decreased to the respective currents 1' i 1' or 1' as indicated by the waveform 88 of FIG. 2.

In order to maintain a desired current level such as through the inductive load 20, to overcome variations of the Source. of potential such as the battery 10, and variations of. the characteristics of the elements in the circuit such as. by temperature changes, a negative feed: back arrangement is also. provided. The size of the resistors 16 and 18 is designed so that for each voltage level of thewaveform 90 a predeterminedvoltage is developed on the lead 64, so that the desired current is supplied to the inductive load 20. In order to maintain this current, a change of current through the resistors 16 and 18 is reflected as a change of. voltage. through the resistor 58 and on the base, of the transistor 28-. Thus, the potential" on the lead 40-is changed to correct the impedance of the transistor 24 so as to maintain the: desired current through the inductive load 20. For example, a decrease of current through the inductive loadv 20' causes an increase of potential onthe lead 64 and a resultant increase of potential on the base of the. transistor 28.

Thus, the current through the resistor 38. decreases with Referring now primarily to FIGS. 1, 3 and 5, the

transient operation of the circuit in accordance with this invention will be explained. When the voltage level of the switching signal of the waveform 90 changes such as at times t to L, as a result of the contact arm 48 moving to a difierent contact 50 through 56, a transient signal is applied through the lead 70 to the amplifier 72 of the transient amplifier circuit 74. This transient signal is amplified in the amplifier 72 to form signals similar to

transient signals

92, 94, 9'6, 98 and 100' of a waveform 102 of FIG. 4. The amplifier transient signals similar to the signal 94 are applied to the high pass filter 78 and passed therethrough as current signals to the lead 40. The high pass filter 78 only passes current signals therethrough during the time of occurrence of the transient signals such as 94, acting to block all signalsduring steady state conditions. The current associated with the transient signals such as 94 passes through the lead 4%) and the resistor 38 to develop the transient voltage signals 92, M, 96, 98 or 100 on the lead v40. It is to be again noted that the transient signals such as 94 have a relatively short time of duration.

The elfect of a positive transient signal such as 94, 96 or 98 resulting from an increase in voltage level of the switching voltage of the waveform 947 of FIG. will now be explained. The positive transient signal such as 94 applied to the lead 40 rapidly biases the transistor 24 into a nonconductive state to uncouple the inductive load 29 from the ground lead 12. Further, the positive potential of the switching transient such as 94 is applied tothe emitter of the transistor 24 and to the end of the inductive load 20 coupled thereto. This positive transient signal 94 rapidly decreases the current through the inductive load 20 by effectively presenting a high impedance thereto so that the current level decreases in a short period of time to a current level of i Because of the short time interval of the transient signal such as 94, the current only decreases to the level i rather than to a lower current level and then the transient signal is terminated allowing the normal regulating action of the circuit to maintain the current level i Thus, only transient signals of short duration are effectively passed through the high pass filter 78.

Without the transient amplifier arrangement in accordance with this invention, the characteristic delay in changing current through an inductive element would cause the current to change to the new level as indicated by a dotted waveform 6. Thus, the current driver of this invention allows the phase developed by the phase shifter 22 to be rapidly changed without an extensive time delay. It is to be noted that without the transient amplifier circuit 74 or" this invention, a current driver is limited to a voltage for effectively causing the change of current no greater than the power source or battery 10. The driver transistor 24 is not damaged by exceeding its power limitations because it is biased out of conduction during the presence of the positive transient signal. It is to be noted that the driver transistor 24 must be selected so that the voltage of the transient signal does not exceed its characteristic voltage limit. At the same time, the positive transient signal such as 94 is impressed on the cathode of the diode 34 biasing it out of conduction. Thus, the regulating transistor 28 is protected from current passing in a direction opposite to its emitter to collector path and exceeding the power rating of the transistor 28.

The operation of the circuit in response to a negative switching transient signal such as 100 will now be explained. A negative transient signal such as 100 biases the driver transistor 24 in a conductive state so as to act as a closed circuit. However, the negative transient signal is impressed on the anode of the diode 26 biasing it out of conduction and de-coupling the inductive load 20 from the ground lead 12. At the same time, the negative transient signal such as 100 is impressed through the transistor 24 to the end of the inductive load 20 connected to the emitter of the driver transistor 24. Thus, current is rapidly increased through the inductive load 20 from the current level i to the new current level i The high impedance of the diode 26 and the negative transient signal applied to the inductive load 20 develops a large effective impedance so that the time constant is very small for the required current change. Similar to the positive transient signal, the negative transient signal has a short time duration so that the current is only changed to the desired. higher current level i The negative switching transient appears equally on the emitter, base and collector of the driver transistor 24 so that the voltage rating is not exceeded. Also, because the diode 26 is biased out of conduction current is prevented from passing through the driver transistor 24 to exceed its power rating. Although the negative transient signal such as 100 causes a relatively large current flow through the transistor 2% for a short period of time, the current is limited by the resistor 38 so that the power rating of the transistor 28 is not exceeded. Thus, the transistor elements in the circuit are protected from overload and damage.

It is to be noted that regardless of the sequence of voltages developed by the signal generator 44, the transients act to rapidly change the current through the inductive load 2 to different current levels and the regulating portion of the circuit maintains the steady state current therethroug-h further corrected by the negative feedback arrangement. Because of the short time duration and large amplitude of the transient signals, large current changes may he made in large inductive loads at a very rapid rate. It has been found that the circuit in accordance with this invention has reduced the rise time of phase shifter current from 300 microseconds to 10 microseconds utilizing a maximum transient voltage of 60 volts. If the transistors are able to withstand higher voltage and power ratings or if means are utilized to protect the transistors, transient voltages as high as 500 volts may be utilized, thus providing current changes in a period as short as 2 microseconds.

Although the programming signal generator 44 responding to the scan motor 68 has been shown as providing the switching signals, it is to be understood that other means may be utilized for developing the switching signals of the waveform 90. For example, digital techniques may be utilized with a rotating memory drum containing binary numbers stored thereon in a sequence indicative of the desired sequence of voltage levels of the waveform 90. Read heads and amplifiers may develop digital signals as the memory drum rotates that are then passed through a digital to analogue converter to convert the binary nun hers to volt-age levels such as indicated by the waveform of FIG. 3. Binary storage arrangements of this type are well known in the computer art and will not be explained in further detail.

Thus, there has been described a current driving circuit that includes a regulating means having a negative feedback arrangement for providing a preselected steady state current to a load. During the presence of switching transients when changing the current through the inductance, a transient amplifier arrangement is provided to develop transient signals for efficiently increasing the resistance in the circuit during a very short period. Thus, current is rapidly changed through the inductance. This circuit is very useful, for example, for driving one or a plurality of ferrite phase shifters such as may be utilized in radar scanning.

What is claimed is:

1. A circuit for driving current through an inductive load comprising a power source, driver means coupled in series with said inductive load, regulating means coupled across said power source and to said driver means for controlling the current therethrough, a source of switching signals of changing voltage levels coupled to said regulating means for maintaining a selected steady state current through said inductive'load, and transient amplifier pled in series with said load and said power supply, said first variable impedance means having a control terminal, second variable impedance means coupled across said power supply and having a control terminal, a signal form-- ing resistor coupled in series with said second variable impedance means and to the control terminal of said first variable impedance means for developing signals to control the current through said load, programming means for developing switching signals having desired voltage levels indicative of desired current levels through said I load and coupled to the control terminal of said second variable impedance means, and transient amplifier means coupled between said programming means and said signal forming resistor to develop transient signals to rapidly change the current levels through said load.

3. A current driving circuit for an inductance comprising a power source coupled across said inductance, a driver transistor having a control electrode and a load pathcoupled in series with said inductance, a regulating transistor having a control electrode and a load path acoupled across said power source, a signal forming resistor coupled in Series with the load current path of said regulating transistor, said load current path coupled to the control electrode of said driver transistor, signal generator means for developing switching signals of predetermined different voltage levels, said generator means coupled to the control electrode of said regulating transi'stor for maintaining a current through said inductance in response to the steady state portions of the switching signals, and transient amplifier means coupled to said generator means and to the control electrode of said driver transistor for rapidly changing current through said inductance in response to the transient portions of said switching signals.

4. A current driving circuit comprising a first source of potential having a positive and a negative terminal, an inductive load, a first and a second resistor coupled in seriesv between the positive terminal of said first source of potential and said inductive load, a driver transistor having a control electrode and having a load path coupled between said inductive load and said negative terminal of said first source of potential, a diode coupled in the load current path of said driver transistorbetween said driver transistor and the negative terminal of said first source of potential, a regulating transistor having a control terminal and having a load path coupled between the positive and, negative terminals of said first source of potential, asignal forming resistor coupled in the load path of said regulating transistor between said regulating transistor and the negative terminal of said first source of potential, said signal forming resistor also coupled to the control terminal of said driver transistor, a second source of potential having a positive and negative terminal, a tapped resistor coupled between the positive and negative terminal of said second source of potential and with the positive'terminal of said second source of potential coupled to a point between said first and second resistors, a stepping switch including a plurality of contacts coupled to said tapped resistor and a movable arm for contacting said contacts coupled to the control electrode of said reguilating transistor, means for moving said contact arm of. said stepping switch, an amplifier coupled to said movable arm of'said stepping switch, and a high pass filter coupled between said amplifier and said signal forming resistor..

5. A circuit for supplying steady state current through an inductive load at selected current levels and for rapidly changing the current. levels from one to another through the load comprising. a source of potential coupled across said load, variable impedance means coupled in series with said load and having a control terminal, a diode coupled between said variable impedance means and said source of potential, regulating means and signal forming means coupled in series across said source of potential, said regulating means having a control terminal, said signal forming means coupled to the control terminal of said variable impedance means for maintaining said selected current levels through said load, programming means for developing switching signals having a sequence of voltages at selected levels indicative of said selected current levels and coupled to the control terminal of said signal forming means, and transient amplifier means coupled between said programming means and said signal forming means for developing and applying transient signals to said load at the time when the selected voltage levels of said switching signals change so as to change the current levels through saidload in a short period of time.

6. A circuit for supplying current through an inductive load at a plurality of selected current levels and for rapidly changing from one current level to another comprising a power source having first and second terminals, variable impedance means having a control terminal and having a load path coupled between said load andthe first terminal of said power source, unilateral means coupled in said load path between saidvariable im--' pedance means and the first terminal of said powersource, current regulating means having a control terminal and having a load path coupled between the first and second terminals of said power source, signal forming means coupled in the load path of said regulating means and to the control terminal ofsaid variable impedance means, programming means for developing switching signals having a plurality of predetermined levels of voltages and coupled to said control terminal of said regulating means, said regulating means controlling the current through said load in response to steady state portion of said' switching signals, and transient amplifier means coupled between said programming means and said signal form-ing means for applying transient signals to said lead for rapidly changing the current levels therethrough in response to the changes in voltage levels of said switching signals.

7. A circuit for driving current through an inductance comprising a power source having a positive and nega-' tive terminal, a first transistor having base, an emitter and collector with the emitter coupled to a first end of said inductance, a first diode having an anode coupled to the collector of said first transistor and a cathode coupled to the negative terminal of said power source, a

first resistor having a first" end coupled to a second endof said inductance, a second resistor having a first end coupled to a second'end of said first resistor and having a second end coupled to the positive terminal of saidpower source, a second transistor having a base, an emitter and a collector, a zener diode coupled between the emitter of said second transistor and said positive terminal of said power source, a second diode having an anode and a cathode with the anode coupled to the collector of said second transistor, a third resistor having a; first end coupled to the cathode of said second diode and to the base of said first transistor, and having a second end coupled to the negative terminal of said power source, a

second end of said first resistor, a plurality of tapping terminals coupled to said. fourth resistor, a movable arm for contacting; only one: of said tapping terminals at atime and coupled to the base of said second transistor, means coupled to said movable arm for controlling said arm to contact a desired sequence of said tapping terminals, an amplifier coupled to said movable arm, and a high pass filter coupled to said amplifier and to the first end of said third resistor.

8. A current driving circuit for an inductance comprising a power source having a positive and negative terminal, a first transistor having base, an emitter and collector with the emitter coupled to a first end of said inductance, a first diode having an anode coupled to the collector of said first transistor and a cathode coupled to the negative terminal of said power source, a first resistor having a first end coupled to a second end of said inductance, a second resistor having a first end coupled to a second end of said first resistor and having a second end coupled to the positive terminal of said power source, a second transistor having a base, an emitter and a collector, a zener diode coupled between the emitter of said second transistor and said positive terminal of said power source, a second diode having an anode and a cathode with the anode coupled-t the collector of said second transistor, a third resistor having a first end coupled to the cathode of said second diode and to the base of said first transistor, and having a second end coupled to the negative terminal of said power source, a source of biasing potential having a positiveland a negative terminal, programming means coupled to the second end of said first resistor and to the base of said second transistor for developing switching signals having selected voltage levels indicative of a desired current level through said load, said switching signals including direct current and transient portions and a transient amplifier coupled between said programming means and said third resistor, whereby said second transistor is controlled to develop signals to control said first transistor during the direct current portions of said switching signals and said transient amplifier is controlled to develop signals to control said first transistor and to apply potentials to said inductance so that said current levels through said inductance change from one to another in a short period of time.

9. A circuit for driving current through a load comprising a power source, a first transistor having a load path and a control terminal with said load path coupled be tween a first end of said load and the negative terminal of said power source, a first diode coupled in series in the load current path of said first transistor between said first transistor and the negative terminal of said power source, current sensitive means coupled between a second end of said load and the positive terminal of said power source, a second transistor having a control terminal and a first and a second load terminal, a potential source coupled to the first load terminal of said second transistor, a second diode coupled to the second load terminal of said second transistor, impedance means coupled between said second diode and the negative terminal of said power source to form a current path, said base of said first transistor coupled in said current path between said second diode and said impedance means, programming means coupled to said current sensitive means for responding to current changes therethrough, said programming means developing a switching signal having a plurality of desired voltage levels each indicative of a desired current level through said load, said switching signals having steady state and transient portions, said programming means coupled to the control terminal of said second transistor for controlling current therethrough for applying signals to said control terminal of said first transistor to maintain said desired current levels through said load as determined by the steady state portions of said switching signal, an amplifier coupled to said programming means, and a high pass filter coupled to said amplifier and to said current path between said impedance means and said second diode for passing current therethrough in response to the transient portions of said switching signal to develop transient signals for changing the current level through said load in a short period of time.

Vinding Apr. 5, 1960 Lippincott June 14, 1960