Common-Input Difference Amplifier With No Noninverting Input Attenuator Sign Switcher

Dual-Supply, Common-Input Difference Amplifier With No Noninverting Input Attenuator Sign Switcher In this circuit the switch configures the operational amplifier circuit as a voltage follower or as a unity gain inverting amplifier. Vin is the input voltage while Vctl is the control voltage. The resistors are: Rbbc - bias balance compensating resistor Rinv - resistor from the input to the inverting terminal of the operational amplfier Rf - feedback resistor When the control voltage is high the bias balancing resistor is connected to the input signal. The voltage at the noninverting terminal is equal to the input voltage since no current (ideally) flows through Rbbc. The voltage at the inverting terminal is equal to the voltage at the noninverting terminal so it is also equal to the input voltage. Because the voltages at the ends of Rinv are equal, the current through this resistor is zero. It follows that the current through the feedback resistor is also zero since no current (ideally) flows to the inverting terminal. The voltage drop through Rf is then zero and the output voltage is equal to the voltage at the inverting terminal which is equal to the input voltage. Superposition can also be applied to obtain the result. When the control voltage is low the bias balancing resistor is connected to ground. The circuit simply becomes a unity gain inverting amplifier.