(a) For the circuit in Fig. 13.39, show that the loop gain of the common-mode feedback

Chapter 13, Problem 13.84

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(a) For the circuit in Fig. 13.39, show that the loop gain of the common-mode feedback loop is A Ro9 Ro7 re7 +R7 Recall that the CMF circuit responds only to the average voltage VCM of its two input voltages and realizes the transfer characteristic VB = VCM + 0.4. Ignore the loading effect of the CMF circuit on the collectors of the cascode transistors. (b) For the values in Example 13.8, calculate the loop gain A. (c) In Example 13.8, we found that with the CMF absent, a current mismatch I = 0.3 A gives rise to VCM = 2.5 V. Now, with the CMF present, use the value of loop gain found in (b) to calculate the expected VCM and compare to the value found by a different approach in Example 13.8. [Hint: Recall that negative feedback reduces change by a factor equal to (1+A).]