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Electronic Design

# Simple gain stage swings gain positive or negative

This idea is a simplification of a previously published design idea (see “Single Pot Swings Amplifier Gain Positive Or Negative,” Electronic Design, Jan. 20, p. 153). By using another previous design idea (see “One Transistor Differential Amp,” Electronic Design, March 21, 1994, p. 104), the circuit can be built with just one transistor¾no op amps are required. Thus, a gain which swings from positive to negative can be achieved with just one transistor.

This circuit (see the figure, b) operates exactly the same as the more complex (two op amp) circuit (see the figure, a). The simpler circuit (see the figure, b) is analyzed as follows:

By superposition, the output voltage of the circuit takes the form: Vout = KA × VA + KB × VB, where KA and KB are constants determined by resistor ratios. Note that VA and VB only differ by the scale factor introduced via RP. That is, VA = KP × Vin and VB = Vin.

The gain experienced by VA to Vout is that of a common-emitter stage (inverting gain) and gives: KA = −R3/ (R4||R5). The gain experienced by VB to Vout is that of a common base stage (non-inverting) and gives: KB = (R3/R5).

The potentiometer (RP) introduces the scale factor KP, which varies between 0 (wiper at bottom) to 1 (wiper at top) and scales the VA contribution to Vout. The wiper at the bottom setting results in a gain of +10, while the wiper at the top results in a gain of −10. Combining the above relations yields: Vout/Vin = (KP × KA) + KB.

Selecting resistor values to produce a gain that varies from -10 to +10 gives: KA = 20, KB = 10. As a result, R3= 27k, R4 = 2.7k, and R5 = 2.7k. The value for RP is non-critical, but should be somewhat smaller than the bias network (R1 and R2) to reduce the loading effect of the base circuit. The overall gain expression is now: Vout/Vin = −20 × KP + 10. R1 and R2 are selected to set the desired bias current in Q1.

Any other arbitrary gain range can be implemented by properly choosing the various resistors and ratios to set KA and KB as desired. R4 may be ac-bypassed with an appropriate resistor if the biasing and ac gain requirements necessitate doing so.

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