Design Ideas: November 9, 1995

The amplifier in Figure 1a is a well-known circuit that provides voltage gain of +1 or –1, depending on the logic voltage (0 or 5V) you apply to the MOSFET gate. By adding one resistor (Figure 1b), the amplifier can provide gain of +A_V or –A_V with the same logic control. The gain is ±R₃/R₂, and R₁ is (R₃R₂)/(R₃–R₂.). You need a driving source with low output impedance, and the input impedance of the circuit is different for the two states. In Figure 1b, the chosen resistor values yield gains of ±2. The SD215 DMOS transistor has a separate connection for its substrate; connecting it to the negative supply prevents conduction of the device’s drain-source parasitic diode. As the on-resistance of the FET slightly reduces the inverting gain, you can reduce the noninverting gain to a matching value by increasing R₁ by a small amount.

The scope photo in Figure 2 shows gain-switching performance for a ±2V sinusoidal input. (DI #1787)