Page 138 - Op Amps Design, Applications, and Troubleshooting

High-Current Amplifier 121

In our case, the calculations are

This current is well within the range of op amp output currents, even though the
load current itself is nearly two amps.
In order to ensure that we have a maximum symmetrical swing for the out-
put signal, we will establish a positive DC offset in the output. The value of this
offset should be midway between the two extremes. One extreme is the value of
VSAT, or +10 volts. The other extreme is the minimum turn-on voltage for Q} and is
2.5 volts. The DC level on the output of the op amp must then be

In the present case, we have

The maximum voltage swing at the output of the op amp will be

We will get this same swing at the load, but the DC level will be reduced by the
amount of V BE.
If the output of the op amp is allowed to go more positive than +V s#r (esti-
mated here as +10 volts), trie waveform will be clipped on its positive peaks. This
clipped waveform will also appear across the load.
If the output of the op amp is allowed to go below the minimum turn-on
voltage for Q 1 (estimated here as +2.5 volts), the waveform will be clipped on its
negative peaks. The load voltage will also have a clipped waveform.
If the amplitude of the input signal remains fixed, but the IX offset voltage
in the output of the op amp is changed, then similar waveform clipping can occur.
That is, if the instantaneous value of the combined AC and DC voltages on the
output of the op amp goes more positive than +V SAT or less positive than the turn-
on voltage for Q l7 the output waveform will be distorted.
The sketch in Figure 2.33 clarifies the relationships between the output
waveforms (op amp and load), the bias level, and the clipping levels.
In order to establish the 6.25-volt IX bias at the output of the op amp, we
need to adjust potentiometer R P to the necessary level. Although this would have