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Chapter 18

Q. 18.7

In Fig. 18-27, R_1 = 1 \ kΩ and R_2 = 51 \ kΩ. If a 741C is used for the op amp, what is the voltage gain of the circuit? What is the closed-loop output impedance? What is the shorted-load current of the circuit if each transistor has a current gain of 125?

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Verified Solution

With the equations given in Fig. 18-26, the voltage gain is:

A_{V}= \frac{-51 \ k\Omega }{1 \ k\Omega }=-51

The feedback fraction is:

B= \frac{1 \ k\Omega }{1 \ k\Omega+51 \ k\Omega }= 0.0192

Since the 741C has a typical voltage gain of 100,000 and an open-loop output impedance of 75 Ω, the closed-loop output impedance is:

Z_{out(CL)}= \frac{75 \ \Omega }{1 + (100,000)(0.0192) }= 0.039 \ Ω

Since the 741C has a shorted-load current of 25 mA, the boosted value of the shorted-load current is:

I_{max} = 125(25 mA) = 3.13 A