Page 90 - Op Amps Design, Applications, and Troubleshooting

Voltage Follower 73

2.4.2 Numerical Analysis
The numerical analysis for the voltage follower is simpler than for previous cir-
cuits because of the lack of circuit complexity. Let us analyze the circuit shown in
Figure 2.16 and determine the following values:

1. Voltage gain
2. Input impedance
3. Input current requirement
4. Maximum output voltage swing
5. Slew-rate limiting frequency
6. Maximum input voltage swing
7. Output impedance
8. Output current capability
9. Bandwidth
10, Power supply rejection ratio

For purposes of the following analyses, let us assume that the op amp in Figure
2.16 is a 741.

Voltage Gain. The ideal voltage gain of a voltage follower circuit is always
unity, or 1. This can be further demonstrated by applying the voltage gain equa-
tion, Equation (2.28), presented for the noninverting amplifier circuit. Since R F is
now 0 and R/ is infinity, our calculations become

As with other amplifier configurations, the actual gain of the circuit falls off at high
frequencies. This is further discussed, along with bandwidth, in a later section.

Input Impedance. The input impedance of the voltage follower is ideally infi-
nite because it is essentially the input resistance of the (+) input of the op amp
modified by the effects of feedback. The value may be estimated by applying
Equation (2.29) with the quantity R//(Rf + R/) considered to be unity. Thus, for low
frequencies (i.e., near DC) the circuit in Figure 2.16 will have a rninimum input
impedance of