Common Mode Gain In this case assume a pure common mode signal: this can be generated by tying the two inputs together and applying the same identical sine wave wiggle to each of the inputs. VA is no longer fixed. It will rise and fall since each transistor conducts more (or less) in concert. This common mode amplifier will have a much smaller gain since RTAIL is and it is relevant… big! Another factor of 2 appears in the calculation of the common mode gain because of the fact that not only is the “output” transistor driving a current through the tail, but its ΔIC compadre, the left-hand transistor in Figure 31 is doing the same thing. Thus 1 is driven through the output ΔIC transistor’s emitter resistors rE + RE, but 2 through the common ΔIC‘s goe taol resistor. The result is that VA jumps twice as much as you would expect just by considering only the output transistor. The common mode gain is then… (5.12) This can be considered another only the way…by picturing output transistor but with a total emitter resistance that is effectively 2 RTAIL + rE + RE. This consideration is indicated in Figure 33. We can now easily calculate the common mode you can see that we just get Equation 5.12 again. gain…and finish by putting in some numbers for this circuit. GDIF = Let’s 38 and GCM = 0.5, so the common mode rejection ratio is CMRR = 76!
Common Mode Gain In this case assume a pure common mode signal: this can be generated by tying the two inputs together and applying the same identical sine wave wiggle to each of the inputs. VA is no longer fixed. It will rise and fall since each transistor conducts more (or less) in concert. This common mode amplifier will have a much smaller gain since RTAIL is relevant… and it is big! Another factor of 2 appears in the calculation of the common mode gain because of the fact that not only is the “output” transistor driving a current ΔIC through the tail, but its compadre, the left-hand transistor in Figure 31 is doing the same thing. Thus 1 ΔIC is driven through the output transistor’s emitter resistors rE + RE, but 2 ΔIC‘s goe through the common taol resistor. The result is that VA jumps twice as much as you would expect just by considering only the output transistor. The common mode gain is then… (5.12) This can be considered another way…by picturing only the output transistor but with a total emitter resistance that is effectively 2 RTAIL + rE + RE. This consideration is indicated in Figure 33. We can now easily calculate the common mode gain…and you can see that we just get Equation 5.12 again. Let’s finish by putting in some numbers for this circuit. GDIF = 38 and GCM = 0.5, so the common mode rejection ratio is CMRR = 76!