Operational Amplifier Fundamentals Unit 7 – The Difference Amplifier
Exercise 2 – Difference Amplifier AC Operation
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to operate a difference amplifier using
ac voltages. You will verify your results with an oscilloscope.
EXERCISE DISCUSSION
- The difference amplifier combines input waveforms and accounts for amplitude and phase.
- If R1, R2, R4, and R5 are equal in value, Vo equals V2 - V1.
- If V2 and V1 are identical (in amplitude and phase) ac waveforms, Vo is essentially 0V.
- Vo is 0V because of the common-mode rejection capability of U1.
- The common-mode rejection capability of an op amp is an advantage because a common-
mode input voltage has a small effect on circuit output voltage. - If V2 has greater amplitude than V1, Vo is in phase with V2 and lower in amplitude then
either V2 or V1. - If V2 has less amplitude than V1, Vo is in phase with V1 and lower in amplitude than either
V2 or V1. - If V2 and V1 have equal amplitudes and are 180° out of phase, then Vo is in phase with V2.
In addition, Vo equals 2 x V1 or 2 x V2. - When V2 and V1 are out of phase, V1 (the inverting input) adds to the magnitude of V2 (the
noninverting input). V2 dominates V1, and Vo is in phase with V2. - Based on the V2 - V1 relationship of a difference amplifier, out-of-phase inputs (not
common-mode) are added.