Page 140 - Op Amps Design, Applications, and Troubleshooting

High-Current Amplifier 123

If we drive the amplifier with a signal greater than 0.325 volts RMS, we can expect
clipping to occur in the output.
The input impedance of the circuit is approximately equal to the value of R }.
We can apply Ohm's Law to compute the current supplied by the AC input source
under maximum input voltage conditions.

The usefulness of the circuit should become very apparent after this last cal-
culation. A signal source delivering a peak current of 17 microamps is driving a
load resistance that requires 1.88 amps peak current.
2.9.3 Practical Design Techniques

Much of the design procedure was covered in our numerical analysis discussion
in the preceding section. Let us now design a high-current amplifier that will per-
form according to the following:

1. Input voltage of 1.0 volt RMS
2. Input resistance > 10 kilohms
3. Input frequency range between 10 hertz and 2.0 kilohertz
4. Load resistance of 50 ohms
5. ±15-volt supplies to be used
6. 741 op amp to be used if practical

Select the Output Transistor. There are basically five transistor parameters
that must be reviewed to select a transistor:

1. Forward current transfer ratio (h FE) or current gain (/3)
2. Base-emitter voltage drop (V BE)
3. Emitter-collector breakdown voltage
4. Maximum collector current
5. Power dissipation

In some cases, the frequency characteristics of the transistor must be evaluated,
but in most cases the transistor performance exceeds that of the op amp and can
be ignored.