Figure 1.  Circuit Diagram for an Inverting AC Amplifier

This circuit produces an inverted, amplified copy of the AC input voltage. The active component of this circuit is an operational amplifier, which is configured as an inverting amplifier.  To ensure that the output can swing in the positive and negative directions equally, a DC voltage equal to Vcc/2 is inserted at the non-inverting input through the voltage divider formed by the 100K resistors.

When Vin is an AC signal within the circuit's bandwidth, the gain G of the amplifier is given by G = -(RF/R1). The op amp output Vo' is the sum of the DC (Vcc/2) and AC output voltages. Output capacitor Co removes the DC component of Vo', causing the final output Vo of the circuit to be a purely AC amplified copy of the input waveform, or Vo = (-RF/R1) Vin.

Figure 2.  Input and output waveforms of the AC amplifier in Figure 1

Figure 2 graphically shows an example of how an AC input voltage is inverted and amplified by the AC amplifier shown in Figure 1. The op amp's output, Vo', is an inverted, amplified copy of the input voltage, shifted upwards by a DC component equal to Vcc/2. The final output Vo of the circuit is just an inverted and amplified copy of the input waveform, since the DC component has already been removed by Co.

See Also:  Non-Inverting AC Amplifier;  Operational Amplifiers

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