In this problem we’ll explore the common-mode gain of a differential amplifier. recall that the commonmode gain gcm = ∆vout/∆vcm, where ∆vcm is an identical change in voltage applied to both inputs. in a good differential amplifier, the common-mode gain should be very low. +15v 10 kω vout v − in b 100ω c 100ω a v + in 10 kω −15v let’s start by finding the voltages and currents in the circuit when both inputs are held at 0 v. (a) find the voltages at points a, b, and c. assume for this analysis that re = 0. (b) find the current (in ma) through the 10 kω "tail" resistor. report your answer to three decimal places. (c) what is vout in this case? now let’s apply a common-mode voltage of 0.5 v to both inputs. (d) again find the voltages at points a, b, and c. (e) find the new current through the "tail" resistor. (f) what is vout now? now let’s compute the common-mode gain. (g) what is the value of gcm you calculate from your numbers? (h) what is the theoretical value of gcm? in lab, we replace the tail resistor with a current source. a current source pulls the same current through it, no matter what the voltage across it. argue why a perfect current source would lead to a common-mode gain of zero.