Page 93 - Op Amps Design, Applications, and Troubleshooting

76 AMPLIFIERS

Bandwidth. The bandwidth (i.e., the upper cutoff frequency) of a voltage fol-
lower circuit may be estimated by the following equation:

For the circuit shown in Figure 2.16, we can compute the upper cutoff frequency
and/or bandwidth as follows:

bw=f UG =1.0 MHz

At lower frequencies, the voltage gain will be nearly equal to the calculated value
of unity. As the frequency approaches the upper cutoff, the voltage gain begins to
decrease. Once the input frequency exceeds 1.0 megahertz (for a 741), the overall
circuit gain will decrease dramatically.

Power Supply Rejection Ratio. The change in output voltage (%) for a
given change in DC power line noise voltage (V N) is computed for the voltage fol-
lower with the following equation:

where v o, V N, and PSRR are the values of the output noise signal, the noise signal
on the DC supply lines, and the power supply rejection ratio, respectively. The
manufacturer's data sheet in Appendix 1 lists the power supply rejection ratio
(PSRR) as ranging from 30 to 150 microvolts per volt. The worst-case effect on the
output voltage for the circuit in Figure 2.16 is then

In other words, the amplitude of the power line noise (V N) will be reduced by
a factor of 0.000150. This means, for example, that if the DC supply lines had noise
signals of 100 millivolts peak-to-peak, we could anticipate a similar signal in the
output with an amplitude of about

2.4.3 Practical Design Techniques
The design of a voltage follower circuit is fairly straightforward because of the
lack of circuit complexity. Let us examine the design procedure by designing a
voltage follower with the following characteristics: