Page 137 - Op Amps Design, Applications, and Troubleshooting

120 AMPLIFIERS

2.9.2 Numerical Analysis
Let us now numerically analyze the behavior of the circuit shown in Figure 2.32.
Appendix 1 shows the data sheet for the 741 op amp, and Appendix 2 shows the
specifications for an MJE1103 transistor. First, we know from our basic transistor the-
ory that the impedance looking into the base will be approximately equal to $ times
the resistance in the emitter circuit. Thus, the op amp sees the load resistance as

In the case of Figure 2.32, the emitter resistance appears as

R, = 750 x 4 O = 3000 O

We can now refer to the 741 data sheet and determine the worst-case saturation
voltage when using a ±15-volt supply and driving a 3000-ohm load. This value is
listed as 10 volts.
The transistor data sheet indicates that the base-emitter voltage drop is 2.5
volts or less. Thus, the highest (worst-case) voltage that we can expect at the load is

For the circuit in Figure 2.32, we have

v 0(peak) = 10 V - 2.5 V = 7.5 V

The unusually high value of V BE stems from the fact that the MJE1103 is a
high-current Darlington pair. With Ohm's Law, we can calculate the maximum
instantaneous (i.e., peak) load current as

4 Q
= 1.88 A

The op amp must supply a current that is smaller than load current by a fac-
tor of ft. That is,