Operational Amplifier Fundamentals Unit 4 – The Voltage Follower
Exercise 2 – Inverting Gain-of-One Amplifier
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to operate an inverting gain-of-one
amplifier using dc voltages. You will verify circuit operation with a voltmeter.
EXERCISE DISCUSSION
- An inverting amplifier can be configured to provide unity gain.
- Circuit gain equals R3/R1. If R3 and R1 are equal in value, the circuit gain equals one.
- Resistor R2 is added to the circuit to reduce the input bias current effect. The value of R2
equals the parallel combination of R3 and R1: R2 = (R3 x R1)/(R3 + R1).
NOTE: Input bias current flowing through R1 generates a voltage (VR1) that appears at the
output.
- Fractional gain, a gain of less than one, results if the value of R3 is less than the value of R1.
- Since the circuit output voltage is less than the circuit input voltage, the configuration acts as
an attenuator. - To determine the current distribution of the gain-of-one inverting amplifier, apply Ohm's law
(I = V/R) to the circuit. - A positive input voltage to an inverting gain-of-one amplifier generates a negative output
voltage. Electrons flow out of the op amp output terminal. - When U1 is not saturated, VO tracks VI, and VD is at 0V.
- U1 is zero-based (R2 connected to common); therefore, the inverting terminal of U1 is at
virtual ground and serves as the circuit summing junction. - If U1 is saturated (VO greater than 10 Vdc), VO cannot track VI, and VD is greater than 0V.