Question 9.10: Two op-amps are connected in cascade (series) as shown in Fi...
Two op-amps are connected in cascade (series) as shown in Fig. 9.37. Calculate the value of R where V_{1} = 3 V and the output voltage is 15 volts.
The first op-amp is connected in non-inverting mode and the second is in inverting mode. Let the closed loop voltage gain of the first op-amp be A(CL)_{1} and that of the second op-amp be A(CL)_{2}.
\frac{V_{o}}{V_{i}}= -A(CL)_{1} × A(CL)_{2}A(CL)_{1}= 1+ \frac{R_{f}}{R_{1}}= 1+ \frac{200}{50}=5
A(CL)_{2}= – \frac{R_{f}}{R}=- \frac{100}{R (kΩ)}
\frac{V_{o}}{V_{i}}=\frac{15}{3}=5
Therefore,
5= – 5 × \left\lgroup – \frac{100}{R}\right\rgroup =+\frac{500}{R}or, R= + \frac{500}{R}
or, R= 100 kΩ.
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