A Type TL081 opamp and some passive components are sufficient to construct a small amplifier whose amplification can be varied between +1 to -1 with a potentiometer – see the diagram.
The input signal is applied to both inputs of the opamp: to the inverting input via C1 and R1, and to the non-inverting input via C1 and P1. The amplification of the amplifier is R2: R1=1. However, the level of the signal at the +input is determined by the position of the wiper of P1. When the wiper is at the center of its travel, the two input signals cancel each other, so that there is no output.when the wiper is at ‘high-end of the potentiometer, the signal at the +input is larger than available, amplified by 1, at the output. When the wiper is at earth potential, the opamp functions as a normal inverting amplifier with unity gain.
The input impedance of the circuit is about 50 k(ohm). With a value of C1 as shown, the amplifier can handle frequencies from 30 Hz upwards.
The circuit requires a power supply of (+,-) 5 -15 and draw a current of only a few mA. If such a supply is not available, it may be produced from a single 10-30 V supply as shown in the diagram.