Page 323 - Op Amps Design, Applications, and Troubleshooting

Ideal Biased Clipper 301

7.3.2 Numerical Analysis
Let us now extend our understanding of circuit operation to include the numerical
analysis of the circuit shown in Figure 7.8. We will compute the following charac-
teristics:

1. Minimum and maximum clipping levels
2. Highest frequency of operation
3. Maximum input voltage swing
4. Input impedance
5. Output impedance

Clipping Levels. The clipping level in the circuit is determined by the voltage
on the wiper arm of Pj. When the wiper arm is in the extreme right position, the
reference voltage (and clipping level) will be 0 volts, as the wiper arm will be con-
nected directly to ground. The maximum dipping level will occur when the wiper
arm is moved to the leftmost position. Under these conditions, the reference volt-
age is determined by applying the basic voltage divider formula:

Highest Frequency of Operation. Because the feedback loop is essentially
open-circuited for a majority of the input cycle, the highest frequency of operation
is more dependent on slew rate than on the bandwidth of the op amp. As the fre-
quency increases, the output will begin to develop some degree of overshoot. That
is, the output will rise beyond the clipping level momentarily and then quickly
drop to the desired level. This overshoot is caused when the output of the op amp
switches more slowly than the input signal is rising. Thus, we continue to see the
full input waveform at the output until the amplifier actually switches. The
switching time, of course, is determined by the slew rate of the op amp. There is
no precise maximum frequency of operation. The upper limit is determined by the
degree of overshoot considered acceptable for a particular application. For pur-
poses of our analysis and subsequent design, we will consider a 1-percent over-
shoot to be acceptable. With this in mind, we can estimate the highest frequency of
operation as follows: