Abstract:
An amplifier (FIGS. 1-3) is disclosed for amplifying an amplitude and frequency varying input signal. The amplifier includes a circuit (24, 100) responsive to the input signal to provide a digital representation thereof. In addition, a plurality of signal sources (30-36) are provided which each provide an associated signal. Another circuit (38-52) combines selected ones of the associated signals in accordance with the digital representation to provide a first combined signal having substantially the same waveform as, but of greater amplitude than the input signal. 
     In several embodiments, a circuit (74, 76-78) provides an error signal in accordance with the difference between the desired and actual forms of the first signal. This error signal is combined with the first combined signal to derive an error corrected signal.

Description:
This is a continuation, of application Ser. No. 098,749 filed Nov. 30, 1979, and now abandoned. 
    
    
     BACKGROUND AND FIELD OF THE INVENTION 
     The present invention relates to amplifiers, and more particularly to an amplifier employing digital switching of incremental signal sources for amplifying an amplitude and frequency varying input signal to high power levels. 
     In amplifier design, one important consideration is the efficiency at which the amplifier operates. Other important considerations include reliability, size, etc. It may be well appreciated that these considerations assume exaggerated importance in very high power amplifiers, such as those used in the modulating circuits of conventional broadcast transmitters. These audio amplifiers must amplify an input audio signal up to a power level ranging from several kilowatts to several tens of kilowatts. 
     An additional consideration in the selection of an amplifier design is the compatibility of that design with available solid state devices. Solid state devices, such as transistors, VFETs, SCR&#39;s, etc. are preferred over vacuum tube devices in view of their size, relative efficiency, and reliability. Unfortunately, such solid state devices as are readily available do not generally have sufficient power handling capabilities for use in standard very high power amplifier designs. 
     In the prior art, pulse duration modulation has been used effectively in increasing the efficiency of these high power amplifiers. Thus, as shown in the patent to Swanson, U.S. Pat. No. 3,506,920, high power amplification may be obtained by pulse width modulating a carrier signal in accordance with the changing level of an audio signal, amplifying the resulting bi-level pulse waveform, and then recovering an amplified audio signal by filtering the resulting amplified PDM signal. Even with these PDM amplifiers, however, no entirely satisfactory design has been found which employs high powered solid state devices in view of the existing limitations on power and speed of these devices. 
     One approach which has been taken in order to resolve this incompatibility is described in the patent to Swanson, U.S. Pat. No. 4,164,714, entitled &#34;Polyphase PDM Amplifier&#34;. The amplifier described in this patent receives a time varying input signal and converts it into a plurality of modulated pulse trains of like polarity and frequency (each of which may, for example, be a PDM signal) and which are phase displaced from one another by a known amount. The pulse trains are combined to form a composite signal of increased magnitude and of substantially the same waveform as the input signal. Unlike previous PDM amplifiers, the design of this Polyphase PDM amplifier has been found to be compatible with existing solid state devices. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an improved high power amplifier. 
     It is another object of the present invention to provide a high power amplifier whose design is compatible for use with conventional solid state elements. 
     It is yet another object of the present invention to provide an amplifier for providing power amplification with high efficiency. 
     It is still another object of the present invention to provide an amplifier which performs in accordance with the objects set forth above, and which has reduced output filtering requirement. 
     It is a further object of the present invention to provide an amplifier for use in an RF amplitude modulation system which achieves the foregoing objects. 
     In accordance with one aspect of the present invention, an amplifier is provided for amplifying an amplitude and frequency varying input signal. The amplifier includes means responsive to the input signal to provide a digital representation thereof. A plurality of signal sources are provided which each provide an associated signal. Combining means combines selected ones of these associated signals in accordance with the digital representation to provide a first combined signal having substantially the same waveform as, but of greater magnitude than, the input signal. 
     In accordance with another aspect of the present invention, means are further included for providing an error signal which varies in accordance with the difference between the desired and actual forms of the first combined signal. This error signal is combined with the first combined signal to form an error-corrected first combined signal. 
     More specifically, there is disclosed hereinafter an amplifier including a converter which is responsive to the amplitude and frequency varying input signal to provide a digital word including plural bits which together indicate and change with the analog level of the amplitude and frequency varying input signal. Each of the bits of this word has an assigned value representing its contribution to the total signal. Means are further provided responsive to this digital word for reconstructing the input signal in amplified form. This means comprises a plurality of incremental voltage sources, each having a value corresponding to the value assigned to the corresponding one of the bit positions of the individual words. A plurality of switching means are also incorporated in the reconstructing means, each associated with a corresponding one of the incremental voltage sources. Each of the switching means has at least two selectable states. In the first state the associated incremental voltage source is interconnected in series with the other selected voltage sources across the output, whereas in the other state the associated incremental voltage source is disconnected from the output. The state of each of the switching means is selected by the bit in the corresponding bit position of the digital word. 
     In another of the described embodiments of the present invention, the amplifier as defined above includes a pulse duration modulation stage for adding an incremental pulse duration modulated (PDM) signal onto the reconstructed analog signal, wherein the width of the pulses in the PDM carrier signal are modulated in accordance with the difference between the reconstructed signal and the input signal. A sum signal is then provided corresponding to the additive sum of this PDM signal and the reconstructed analog signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects and advantages of the present invention will become more readily apparent from the following detailed description, as taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagramatic representation of one embodiment of an amplifier in accordance with the teachings of the present invention; 
     FIG. 2 is a block diagram of another embodiment of an amplifier in accordance with the teachings of the present invention, wherein a PDM stage has been added; 
     FIGS. 2A and is a block diagram of an alternative embodiment of a portion of the amplifier of FIG. 2; 
     FIG. 3 is a block diagram of a third embodiment of the present invention; 
     FIG. 4 is a circuit diagram of an incremental voltage source such as might be useful in the embodiments of FIGS. 1-3; 
     FIG. 5 is a schematic illustration of a switching circuit useful for switching the incremental power sources as required by the embodiments of FIGS. 1-3; and 
     FIG. 6 is an illustration of a reconstructed analog waveform produced by an amplifier in accordance with the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, the invention will largely be described with respect to an ampifier used in the RF modulation stage of a conventional AM transmitter. This specific embodiment is utilized for exemplary purposes only, however, and it should be appreciated that the amplifier described hereinafter will find extensive use in many applications wherein high power amplification of an amplitude and frequency varying input signal is required. 
     Referring now to FIG. 1, there is illustrated an AM transmitter incorporating an amplifier in accordance with the teachings of the present invention. In this figure, a transmitter system 10 includes an audio signal source 12 which generates the amplitude and frequency varying audio signal which is to be transmitted. This audio signal (V in ) is supplied to an amplifier 14 which amplifies it to a high power level and provides the resulting amplitude signal (V out ) at its output to filter stage 16. The filter 16 removes out-of-band artifacts of the amplification process from the amplified signal. The resulting amplified and filtered signal is then supplied to the audio input of a conventional RF power amplifier 18 where it amplitude modulates an RF carrier supplied by an RF oscillator 20. The resulting AM signal is then transmitted over a conventional antenna or antenna array 22. 
     In accordance with the present invention, the amplifier 14 includes a circuit 24 to which the input signal is applied. This circuit 24 converts this amplitude and frequency varying input signal into a digital word comprised of plural individual bits, each provided upon a corresponding output line 26 at the output of the circuit 24. These bits together indicate the instantaneous level of the input audio signal to within an incremental limit, and change with time to indicate the changing level of the audio signal. In FIG. 1, the circuit 24 comprises a conventional analog-to-digital (A/D) converter. The digital word provided along the output line 26 therefore comprises a binary representation of the input signal, with the four bits provided along the four output lines 26 of converter 24 have a weighting of 8-4-2-1. This digital word will vary between a lower limit of &#34;0000&#34;, to an upper limit of &#34;1111&#34; in accordance with changes in the instantaneous level of the audio signal V in , and will have an average or D.C. value of &#34;1000&#34;. For some A/D convertors, it may be necessary to D.C. bias the input signal at one half of the supply voltage to achieve this result. 
     The A/D convertor 24 may be either clocked (i.e., updates the value of the output digital word only upon a &#34;convert&#34; command) or unclocked (i.e., the output digital word continually reflects the value of the input analog signal). If the former, the conversion rate should be great enough that the output word essentially changes concurrently with any changes in the analog input signal. 
     The digital word provided along the line 26 at the output of A/D 24 is supplied to a circuit generally indicated at 28 whose purpose is to reconstruct the analog audio signal therefrom in amplified form. This recontruction circuit 28 includes a plurality of incremental D.C. voltage sources 30, 32, 34, and 36. Each of the incremental voltage sources includes a switching circuit comprised of a corresponding switch 38, 40, 42 and 44, and a corresponding bypass diode 46, 48, 50 and 52. (For convenience of description, the combination of an incremental voltage source and its associated switching circuit and bypass diode will be referred to occasionally hereinafter as an amplifier &#34;cell&#34;.) The level of D.C. voltage supplied by the various incremental voltage sources will be selected to correspond to the weighting of the individual bits provided at the output of the converter 24. Thus, in the embodiment illustrated in FIG. 1, the four incremental voltage sources 30, 32, 34 and 36 will have a weighting of 8-4-2-1, since the digital word is coded in binary form. Moreover, each source will &#34;float&#34; with respect to the other sources. Consequently, different output voltages can be formed by simply connecting the necessary voltage sources together in series. 
     The switching circuits associated with each of the incremental voltage sources are controlled by the bit having the corresponding value. At any given time, this bit can assume either of two possible values, a binary &#34;0&#34; or a &#34;1&#34;. When the bit has one value, then the incremental voltage source will be connected in series with other selected incremental voltage sources across the load, in this case comprised of the filter circuit 16 and the RF power amplifier 18. When the bit has the second binary state, however, the switching circuit will instead disconnect the incremental voltage source from the output, so that the incremental voltage of that source provides no contribution to the composite signal appearing across the load. 
     Although, in FIG. 1, the switches 38-44 are illustrated as convention SPST mechanical switches, in a practical embodiment solid state switches, such as described hereinafter, will instead be used. 
     To better understand the operation of the reconstruction circuit 28, consider the amplifier cell including incremental voltage source 36 and its associated switching elements 44 and 52, which are exemplary of the remaining cells in the reconstruction circuit. In this amplifier cell, as in all others, the incremental voltage source 36 is connected in series with its associated switch 44, and in parallel with the bypassing diode 52. Furthermore, this cell is connected in series with all of the other cells in the reconstruction circuit 28. When the switch 44 is &#34;open&#34;, then the voltage source 36 is disconnected and thus does not contribute to the total signal supplied at the output of the amplifier. The disconnection of the voltage source does not, however, interfere with the operation of the remaining incremental voltage sources, since the diode 52 provides a path for current to bypass the incremental voltage source 36, and thereby flow into the load connected to the output of the amplifier. When the switch 44 is &#34;closed&#34;, however, then the voltage source 36 will effectively reverse bias the associated bypassing diode 52, and will instead connect the incremental voltage source 36 into a series combination with the remaining cells in the amplifier. Each of the incremental voltage sources 30-36 may thus be connected in series with any selected combination of the remaining incremental voltage sources. 
     At any given time, the ones of the voltage sources 30, 32, 34 and 36 connected in series across the output of the amplifier will be dependent strictly upon the binary word provided at the output of the A/D converter 24. If this binary word were, for example, &#34;1011&#34;, then the voltage appearing across the load circuit would be 8 V 1  plus 2 V 1  plus V 1 , or 11 V 1 . Since each switching circuit has two possible states, there are a total of 2 4 , or sixteen different combinations of the incremental voltage sources possible. Consequently, there are sixteen different analog voltage levels which may be provided at the output, under the control of the digital signals provided by the A/D converter 24. Since the A/D converter 24 is controlled by the analog signal provided by audio signal source 12, and since the coding of the digital word is the same as the weighting of the incremental voltage sources, this output signal will mirror the input signal in a stepwise fashion. Because the size of the steps (equal to the smallest incremental voltage V 1 ), is dependent only on the magnitude of the voltages provided by the voltage sources, this output signal may be of much greater magnitude by appropriate scaling of these voltages. 
     FIG. 6 illustrates an analog input signal V in  and the output signal V out  which the amplifier of FIG. 1 will provide in response thereto. Although, for convenience of illustration, V in  and V out  are shown as having approximately the same magnitude, the steps in V out  will generally be very much larger, leading to significant voltage gain through amplifier 14. 
     As is apparent from FIG. 6, the reconstructed analog signal includes step transitions from each signal level to the next succeeding level, thereby introducing unwanted high frequency components to the reconstructed waveform. The filter 16, illustrated in FIG. 1 as a conventional T network comprised of inductors 54 and 56 and a capacitor 58, will be designed to filter out the high frequency components introduced by the step transitions, thereby smoothing the reconstructed waveform. 
     FIG. 2 illustrates an alternative embodiment of the amplifier of FIG. 1, incorporating a pulse width modulation (PWM) amplifier cell to provide improved resolution of the reconstructed waveform. In this circuit, the analog signal V in  will again be provided by an analog signal source such as audio source 12 of FIG. 1. The amplifier will of course amplify this signal to provide an amplified signal to a load such as filter 16 and transmitter 18. 
     The amplifier 60 of FIG. 2 includes a first portion 14 which may be substantially identical to the amplifier 14 of FIG. 1. As stated previously, the reconstructed waveform provided at the output of the amplifier 14 will have stepwise transitions between the sixteen different levels thereof. A certain degree of inaccuracy of amplification arises from the finite number of steps in the output signal. Greater resolution of amplification could be provided by including an A/D converter 24 having greater resolution, such as 8, 10, or 12 bit resolutions, together with a correspondingly greater number of weighted amplifier cells. Presuming that the A/D converter provides a binary coded output and again presuming that the incremental voltage sources are weighted in corresponding fashion, the inclusion of each subsequent cell of amplification multiplies the total possible number of amplifier levels by a factor of 2. The use of an eight-bit A/D converter in conjunction with an eight-cell amplifier would therefore provide 256 possible analog levels at the output. At some point, however, a minimum step size will be found such that it will be impractical to provide cells including smaller incremental voltages. The amplifier 60 of FIG. 2 utilizes a PWM amplifier stage 62 to increase the resolution even beyond this limit. 
     The amplifier 62 includes an additional amplifier cell 64 which, as with the previous cells, incorporates an incremental voltage source 66, a switch 68, and a bypassing diode 70. In the embodiment illustrated in FIG. 2, the incremental voltage source 66 associated with the cell 64 has an output voltage which is equal to the difference in amplitude between adjacent voltage levels obtainable by the amplifier 14. Thus, if the amplifier 14 is constructed as the amplifier 14 of FIG. 1, then this voltage source 66 will provide the same incremental voltage as the voltage source 36. The switching of the incremental voltage source 66 into and out of the circuit is again accomplished by means of the solid state switch 68. Switch 68, however, is in this embodiment controlled by a pulse width modulator 72, which may take any conventional form (as disclosed, for example, in the aforementioned patent of Swanson, U.S. Pat. No. 4,164,714). 
     The pulse width modulator 72 has for its input an analog signal representing the difference between the reconstructed waveform generated by amplifier 14 (divided down to be of equal amplitude to the input signal) and the input signal V in . To scale the reconstructed waveform down to be of the same gain as the input signal V in , a resistive divider is provided comprised of a fixed resistor 74 in series with a potentiometer 76. The tap on the potentiometer 76 will be adjusted until the voltage appearing at the tap is of substantially the same scale as the analog signal provided to the input of the amplifier. A difference amplifier 78 will then subtract one from the other and will provide a signal at its output representative of the difference between the desired waveform (represented by the input signal V in ) and the actual reconstructed waveform, as derived from the tap on the potentiometer 76. The output of PWM 72 will be a pulse train, the pulses of which occur at an appropriate frequency (for example 100 kHz) and with a duty cycle ranging from 0% to 100% in direct dependence upon the amplitude of the signal provided by difference amplifier 78. 
     In operation, the voltage provided by the differential amplifier 78 will be very small or zero when the reconstructed waveform coincides exactly with the ideal waveform. In that case, the output signal provided by the pulse width modulator 72 will have a very small, or perhaps zero duty cycle, thus the switch 68 will remain open and the reconstructed waveform will pass through the amplifier 62 substantially without change. As the difference between the reconstructed waveform and the desired waveform becomes greater and greater, however, the duty cycle of the cyclical signal at the output of the pulse width modulator 72 will increase, thus switching the incremental voltage source at a corresponding duty cycle, and adding a PWM component to the output signal V out . Just prior to amplifier 14 switching to the next successive amplitude level, the PWM waveform will have a nearly 100% duty cycle. Upon the switching of the amplifier 14 to the next successive level, of course, the difference between the desired amplitude and the actual amplitude will drop to substantially zero, causing the duty cycle of the pulse width modulator to similarly drop to zero. Since the cell 64 is connected essentially in series with the cells of the amplifier 14, the PWM signal is added into the reconstructed waveform in the same fashion as are the incremental voltages provided by the other cells in amplifier 14. 
     The insert I in FIG. 6 illustrates, on an expanded scale, the appearance of the output waveform V out  for the amplifier of FIG. 2 when an input signal V in  is applied thereto. 
     Although illustrated in FIG. 2 with the amplifier 62 interposed between the amplifier 14 and filter 16, the circuit could have as easily have been constructed with the amplifier 62 located between the last cell in amplifier 14 and ground. In this event, however, the scaled signal provided to the differential amplifier 76 would have to be derived in a different manner. For example, as shown in FIG. 2A the outputs 26 of A/D converter 24 of FIG. 1 could each be connected into a single, common input of a current amplifier 80 through corresponding resistors 82 so as to develop a current signal into the current amplifier which varies with the digital word provided on the output 26. By weighting the resistors 82 by factors of 1, 1/2, 1/4 and 1/8, as illustrated, the total current going into the input of the amplifier can be made to correspond linearly with the magnitude of the reconstructed waveform appearing across the amplifier cells controlled by these outputs. The voltage output thereof would then correspond to the current input from the four lines 26, and hence would reflect the contribution to the reconstructed waveform provided by the amplifier 14. This voltage signal would then be provided to the input to differential amplifier 76 in place of the connection to the tap of potentiometer 74 in the embodiment of FIG. 2. The resulting operation will be essentially identical to the operation provided by the amplifier of FIG. 2. 
     In the embodiments illustrated in FIGS. 1 and 2, one of the incremental voltage sources will always have an incremental voltage equal to substantially half of the total voltage range over which the output signal may change. In FIG. 1, for example, the incremental voltage source 30 has a magnitude of 8 V 1 , when the total voltage range is only 16 V 1 . Although this will not represent a problem for many applications, it does represent a significant constraint in those applications requiring output voltages extending into the kilovolt range, and beyond. This is essentially because the solid state elements utilized to switch the incremental voltage sources are limited in the voltages which they can handle. Consequently, if the amplifier designs of FIGS. 1 and 2 were strictly followed, the total voltage range of the output signal will be constrained to twice this limiting voltage. Practical considerations will even further limit this, since the highest power switching elements are quite expensive and may not be cost effective in many applications. 
     FIG. 3 illustrates another embodiment of the invention, not limited in the amplitude of voltage which may be provided at the output thereof. In this embodiment, as in the embodiment of FIGS. 1 and 2, an amplifier 98 is provided including a circuit 100 for converting the incoming analog signal into a multibit digital representation thereof. In this embodiment, the circuit 100 will comprise any of the conventionally available &#34;level detector&#34; circuits such as the Texas Instruments TL490 or others. This level detector, contrary to the A/D convertor of FIG. 1, does not include outputs which are differently weighted. Instead, each of the outputs represents an equivalent voltage increment, with the total voltage range representable by this circuit equaling this voltage increment times the number of outputs. When the instantaneous level of the input signal is at the lowest level representable by the level detector (i.e., negative full scale), all of the output lines of the level detector will be low. If the input signal is then steadily increased, the number of high output lines will also steadily increase until the input signal reaches the highest level representable by the level detector (i.e., positive full scale), when all of the outputs will be high. Of course, the D.C. level of the input signal will normally be biased halfway between positive and negative full scale. 
     As previously, the output of the circuit 100 is directed to a reconstruction circuit having a plurality of cells 102, with each cell including an incremental voltage source 104, the solid state switch 106, and a bypassing diode 108. In this embodiment, contrary to previous embodiments, each of the incremental voltages 104 provides an output voltage which is substantially equal to the output voltages provided by each of the other incremental voltage sources. This is possible because the outputs of the level detector 100 are equally weighted, rather than being coded in binary, BCD, or other fashion, as with the embodiments of FIGS. 1 and 2. Each of the incremental voltages 104 will thus have a value equal to the smallest incremental change required. The incremental voltages will therefore not be large enough to introduce difficulties associated with limiting operating voltages of the solid state switches used therewith. 
     The number of incremental voltage steps, however, will of course be much greater than that in the embodiments illustrated in FIGS. 1 and 2, since one incremental voltage source must be provided for each of the possible voltage steps. In other words, if 256 different levels of output signals are desired, then 256 different amplifier cells must be provided. The complexity of the circuit may be reduced by reducing the number of cells, however this reduces the number of possible voltage steps available and limits the resolution of the reconstructed waveform. 
     In the embodiment of FIG. 3, the competing constraints of signal resolution and system complexity are resolved by providing an additional amplifier stage 110 is series with the amplifier cells 102. This amplifier stage 110 may be substantially identical to either the amplifier 14 illustrated in FIG. 1, the amplifier 62 illustrated in FIG. 2, or the amplifier 60 of FIG. 2, representing the series combination of amplifiers 14 and 62. By using the amplifier 14 of FIG. 1 in combination with the previously described amplifier cells 102, the number of incremental voltage sources 104 required for any given degree of signal resolution may be diminished by a factor of 16. This is because, between each sequential setting of the cells 102, 16 different voltage steps will be provided by various combinations of the four differently weighted cells in the amplifier 14. Even greater resolution will be achieved by utilizing the amplifier 60 in series with the amplifier 98. Whichever amplifier is used for stage 110, the various incremental voltage sources 104 associated with the amplifier 98 may be selected to have voltages approaching the practical and economic limits of the switching elements associated therewith, and resolution between these various incremental voltage steps will be accomplished by the amplifiers 16 and/or 62 of FIG. 2. 
     When an amplifier 110 is used in series with the amplifier 98, the voltage signal provided at the input of that auxiliary amplifier 110 must, again, represent the difference between the stepwise signal provided across the amplifier 98 and the desired waveform. In FIG. 3, this difference signal is again derived by dividing down the signal appearing across the amplifier 98 with a fixed resistor 111 and a potentiometer 112. A tap on the potentiometer 112 provides a signal which is subtracted from the input signal V n  by a differential amplifier 114. Once more, as with the embodiment of FIG. 2, the amplifier stages 98 and 110 may be interchanged so that the amplifier 110 is located between the amplifier 98 and ground, and in this case the control signal to amplifier stage 110 will be derived in a different manner, such as the manner illustrated in FIG. 2A. If the technique of FIG. 2A is utilized, however, the resistors feeding the input to the current amplifier will of course have equal values, since the outputs of the level detector 100 are equally weighted. 
     FIGS. 4 and 5, respectively, illustrate specific forms which the incremental power sources and solid state switching elements of the embodiments of FIGS. 1, 2 and 3 could take. Of course, any number of alternative embodiments are possible, and these embodiments are only set forth as examples. 
     Referring now to the circuitry of FIG. 4, an incremental power source is shown which consists essentially of a three-phase power transformer 120, six diodes 122 connected together to a conventional full wave three-phase rectifying circuit, and a filter capacitor 124. If desired, regulating elements may be included to further stabilize the voltage provided at the output of the incremental voltage source, rather than directly taking the output of the filter capacitor 124. It will be noted that, due to the isolation provided by the transformer 120, the voltage appearing across the filter capacitor 124 will float with respect to earth ground and also with respect to the other incremental voltage sources. 
     FIG. 5 illustrates one form which the switching circuits could take. In FIG. 5, a high power transistor 126 is used for switching the incremental voltage source, with the remainder of the circuitry illustrated in FIG. 5 being utilized to drive this transistor 126. When connected in with an amplifier cell as shown in any of the previous figures, the collector connection P will be connected to the positive terminal on the associated incremental voltage source, whereas the emitter connection N will be connected to the cathode of the bypassing diode of that cell. 
     In FIG. 5, the driver circuit 128 is isolated from the control lines by means of a light emitting diode 130 and an optically coupled photo diode 132. When a high logic signal is applied to the input terminal 134, this voltage signal will be converted to a current by a resistor 136, with the current then passing through the diode 130. The diode 130 will emit light to the photo diode 132 which will respond by switching from a high impedence state to a low impedance state. 
     A fiber optic link may be provided between the LED 130 and the photo diode 132. A fiber optic link of this nature would be quite advantageous where the coding circuitry which produces the control signals is located at a physically remote location from the switching circuitry and incremental voltage sources. Thus, fiber optic links are not susceptible to electromagnetic noise, nor would they introduce reactance in the manner of conventional multi-conductor cables. 
     In any event, the application of a logic 1 signal to terminal 134 will cause the diode 132 to drop to a low impedance state, thereby permitting the current to flow through a resistor 135 and into the base of transistor 137. This, in turn, will turn on transistors 138 and 140, thereby switching &#34;on&#34; the output transistor 126. The amount of current drive provided to the output transistor 126 by transistor 140 is regulated by a &#34;baker clamp&#34; consisting of a diode 142. As transistor 126 drops toward saturation, a point is eventually reached at which the diode 142 becomes forward biased, thereby shunting base drive away from the transistor 140. If transistor 126 rises out of saturation, however, then diode 142 will become reversed biased, thereby increasing base drive to transistor 140 and again dropping the output transistor 126 into saturation. 
     When the voltage applied to terminal 134 drops to zero, the diode 132 will return to a high impedance state, thus switching off transistors 137, 138 and 140. Due to carrier storage in these transistors, however, the output transistor 126 might not turn off immediately were it not for the inclusion of an additional transistor 143. This transistor couples the base of the output transistor 126 to ground when transistor 138 switches off, thereby insuring that rapid, positive disablement of the output transistor 126 occurs. 
     The switching circuitry 128 thus described will be powered by positive and negative voltage supplies 144 and 146 which will be referenced to the terminal N in the fashion illustrated in FIG. 5. To permit this referencing, the power supplies 144 and 146 will be isolated from the AC power line by means of transformer coupling (not shown), as with the incremental power source of FIG. 4. 
     Although the invention has been described with respect to preferred embodiments, it will be appreciated that various rearrangements and alterations of parts may be made without departing from the spirit and scope of the invention, as defined in the appended claims. Thus, although voltage sources have been used as the incremental power sources in the specifically described embodiments, it will be appreciated that any other appropriate power sources (e.g., current sources) could instead be employed.