Abstract:
A differential variable gain transconductance amplifier suitable for but not limited to applications requiring operation with a power supply of less than 1.0V. The circuit is suitable for fabrication by integrated circuit technology and is composed of three major blocks: an input stage, a current summing gain control stage, and an output stage. The differential input stage is comprised of two identical voltage-to-current converters, with one used as the positive input and the other as the negative input. The current summing stage uses the principal of summation of opposite but equal currents canceling to produce attenuated gain. The output stage converts current into voltage and produces a single ended output.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to variable gain amplifiers, and in particular to a differential variable gain transconductance amplifier suitable for use in voice frequency circuits and especially adapted for operation with very low power supply voltage. 
     Voice frequency amplifiers are widely used in telephone circuitry and variable gain amplifiers are desired in some applications. A variety of amplifier circuits have been utilized for varying gain, and two such circuits are shown in U.S Pat. Nos. 3,786,200 and 4,536,888. Other circuits of this type are shown in the following integrated circuit catalog sheets: Fairchild A706, A733, and A757, Signetics NE/SE592, Motorola Linear Integrated Circuit Data Book, Dec. 1971 MC 1590G; the National Semiconductor publication Linear Applications, Feb. 1973, pages AN15-2, AN31-9, AN31-15, AN32-5 and LB1-2; and RCA Application Note ICAN-6668 pages 411-413. 
     One important characteristic of all amplifiers is the magnitude of the power supply voltage required for operating the circuits. It is an object of the present invention to provide a new and improved variable gain amplifier suitable for use with voice frequency circuits which can operate with a voltage source of less than one volt. The specific circuit described herein is designed for operation with a voltage supply 1.3 volts, readily available from a 1.5 volt battery. However the circuitry of the invention can be operated with a supply voltage as low as 0.9 v at 25° C. As junction temperature decreases, the lower operation supply voltage limit rises ≈2.2 mv/°C. due to increasing base-emitter voltages. 
     Other objects, advantages, features and results will more fully appear in the course of the following description. 
     SUMMARY OF THE INVENTION 
     The amplifier of the present invention is a differential variable gain transconductance amplifier for amplification of voice frequency signals for providing an output within a controlled range with an input which may vary widely. The circuit provides a continuously variable gain which is controlled as a function of a gain control signal from a differential current source. 
     The presently preferred embodiment of the variable gain amplifier of the invnetion includes a differential input stage having two voltage-to-current converters, with one used for the positive input and the other for the negative input, for producing two current outputs, means for connecting a differential voltage to the inputs of the voltage-to-current converters, a current summing stage having a first pair of current inputs and a second pair of current inputs and including means for summation of opposite but equal currents canceling to produce attenuated gain at a current output, means for connecting the output currents of the input stage to the first pair of summing stage current inputs, another means for connecting a differential current source to the second pair of summing stage inputs as complimentary current sources, and an output stage having the summing stage current output as an input for converting current into voltage and producing the desired voltage output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE of the drawing is an electrical schematic of the presently preferred embodiment of the variable gain amplifier of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The differential variable gain transconductance amplifier of the drawing has an input stage 11, a current summing gain control stage 12, and an output stage 13. A voice frequency input signal VIN is connected at terminals 14, 15, and the output signal appears at terminal 16 and circuit ground. A source of differential currents is connected at terminals 19, 20, and functions as the gain control signal provided as an input to Q128 and Q129. A number of constant current sources 23a through 23f are provided for some of the transistors, with a control current Ic connected at terminal 24. A DC supply is connected at terminal 25 to provide VCC for the transistors, typically 1.3 volts DC. 
     INPUT V/I CONVERTER CIRCUITS 11 
     The input stage 11 provides voltage-to-current conversion of the input signal VIN, and includes two opposite polarity sections. One section connected to input terminal 14 via C5 is composed of PNP current sources 23a, 23b, and current mirror Q16, Q17. Q120 forms a current feedback path as well as providing an output current source. Constant current source 23a provides a constant current to the collector-base connection of Q16 and to the base of Q17. The Q17 collector is also provided a current that is the same value as that going to Q16. The Q17 emitter size is physically ten times larger than that of Q16. Since Q16 and Q17 bases are connected in common and Q17 emitter is larger, Q17 is forced to operate at nearly ten times less current density than Q16. The differences in Vbe can be calculated by: 
     
         Vbe16 -Vbe17=KT/q (1n 10)≈60mV 
    
     There will be a small error in the calculated value due to the added base current of Q120. The emitter of Q17 forms a current summing node with R45, R46 and one of the collectors of Q120. The value of R46 is chosen so that approximately two times the current available from PNP current source 23b is required to maintain the 60 mV bias. The extra current is initially the base current into Q120. It is then multiplied by beta/2 (due to the split collectors of PNP Q120) and fed back to the emitter of Q17. After the emitter of Q17 rises to the required 60 mV stability point, the voltage at the emitter of Q17 is held constant by the current feedback loop of Q120. 
     In addition to holding the emitter voltage of Q17 constant, any current offset caused by current through R45 (such as from an input signal) is cancelled by an equal and opposite current from Q120. Thus, Q17, coupled with Q120, acts as a high gain inverting current amplifier that opposes any current change through R46. The current from the other collector of Q120 is, to a first approximation, identical to the fed back current to the summing node and serves as the output current of the V/I converter on line 31. 
     The opposite polarity input amplifier (Q18, Q121, R50, R49 along with source 23d, source 23c, Q19) is identical to the above described circuit and receives the complimentary signal from input terminal 15 and C6 and provides an output on line 32. 
     CURRENT SUMMING CIRCUITRY 12 
     Q24, Q25 act as current mirror outputs that replicate the output current of Q120. Q21, Q22 serve the same purpose for Q121. Q20 and Q23 are diode connected and receive their currents from Q121 and Q120, respectively. 
     The differential currents at terminals 19, 20 are applied as input to Q128, Q129, which in turn serve as complimentary current sources whose outputs drive currents into R54, R55 and R57, R58. The emitters of Q24 and Q25 form current summing nodes with R57, R58 and the complimentary current sources. The emitter current summing nodes of Q21 and Q22 are functionally the same as Q24, Q25. 
     The collector currents of Q24 and Q25 are determined by the following transcendental equations. 
     Summing the voltages around the loop formed by Q23, Q24, R57, R56 and neglecting base currents, yields: 
     
         Vbe Q24+(IC128+IE24)R57-IE23R56-Vbe Q23=0 
    
     Rearranging the equation and solving in terms of delta Vbe differences yields: 
     
         KT/q(1n IE24/IE23)=IE23R56-(IC128+IE24)R57 
    
     and similarly: 
     
         Vbe Q25+(IC129+IE25)R58-IE23R56-Vbe Q23=0 
    
     
         KT/q(1n IE25/IE23)=IE23R56-(IE23+IC129)R58 
    
     And of course the same applied for the loops formed by Q20, Q21 and Q22. 
     Since the emitters of Q24 and Q25 are connected to complimentary outputs of the differential current sources, their collector currents are also complimentary and range from almost equal to Q120 output current to nearly zero. Any AC change in collector current of Q23 gets mirrored in a non-linear manner by Q24 and Q25 due to the logarithm term in the above expression. The amount of non-linearity will be determined by the current differences between Q23 and Q24 or Q25. This current difference is controlled by Q128 and Q129. 
     The collectors of Q22 and Q24 are connected together to form a current summing node at the base of diode connected transistor Q126. Transistors Q25 and Q21 are similarly connected and form a complimentary summing node at the base of Q127. This summation of currents becomes the bias current for Q126 and Q127 current mirrors. The current biasing Q127 is mirrored to diode connected Q1, then mirrored back up by Q2. 
     The collector current of Q2 forms a summing node with the mirrored collector of Q126. The summation current at the collectors of Q126 and Q2 contain the differential input signal plus the products produced by the non-linear term of the above transcendental equations. The collector current of Q2 contains equal but opposite DC current as that of Q126. After these currents combine, the net DC current remaining to drive the base of Q3 is nearly zero. These currents also contain differential products caused by the non-linear term. These differential currents act to cancel differential offset currents that exist at the collectors of Q128 and Q129. The DC and distortion components also combine in opposition and cancel out, resulting in a clean replication on line 33 of the input signal at all gain settings. 
     OUTPUT I/V CONVERTER CIRCUIT 13 
     Q3 acts as an inverting amplifier. Q4 serves as an emitter follower output driver. Feedback is accomplished by R5 and R5A. Current source 23f serves as the collector load for Q3 and provides base current to Q4. The output signal is taken across output load resistor R7. Furthermore, the current that may be driven into or out of the current summing node at the base of Q3 by Q2 or Q126 will be compensated for by an opposing current through R5 (caused by a change in output voltage) in order to maintain Q3 base at the required base emitter voltage level. Therefore, the output voltage waveform will be a replica of the current unbalance of Q2 and Q126 at any instant of time. It will also be the same as the differential input waveform VIN modified by variable gain. 
     In the input stage 11, the differential input voltage signal is handled in two parallel circuits which perform voltage-to-current conversion of the voice frequency signal, providing complimentary outputs on lines 31, 32. In the current summing gain control stage 12, the differential current sources at terminals 19, 20 control the complimentary current sources Q128, Q129, providing currents to the summing nodes at the emitters of Q24, Q25 and Q21, Q22. The collectors of Q24, Q22 are connected to Q126 summing node and Q25, Q21 are similarly connected to Q127 in a cross connection pattern to achieve the desired current control. With this arrangement, differential gain control is achieved at the summing node at Q2 collector, providing a current output on line 33. The output stage 13 functions as a current-to-voltage converter providing the desired voltage output at terminal 16 which replicates the voltage input signal at terminals 14, 15, with the magnitude of the output within desired limits as a function of the gain control signal at terminals 19, 20. 
     It is preferred that the variable gain amplifier of the invention be produced as an integrated circuit on a single substrate, preferably by use of monolithic technology, with matched components throughout. 
     By way of example, the presently preferred values for the resistances and capacitances of the specific circuit of the drawing are as follows: 
     
         ______________________________________R5 64.2K        R50 27KR5A 78.4K       R53 30K      C4 5 pfR7 10K          R54 30K      C5 10 mfR13 6.9K        R55 30K      C6 10 mfR45 27K         R56 30KR46 2.68K       R57 30KR49 2.68K       R58 30K______________________________________ 
    
     With these values the current from each of the sources 23 is of a value so that the current through Q17 and current fed back from one of the collectors of Q120 are approximately equal. This balanced condition maximizes the circuits ability to handle large input voltages before saturation occurs. Of course, the same conditions apply for Q18 and Q121.