I have a few questions about how to derive the differential gain and common mode gains:
Taken from Art of Electronics - "Imagine a symmetrical input signal wiggle in which input 1 rises by v_in (a small signal variation) and input 2 drops by the same amount As long as both transistors stay in the active region, point A remains fixed."
I don't follow how A would be fixed?
However taking it to be true (no current through R3) I get the voltage gain as follows
(V_in1 - 0.6)-(V_in2-0.6) / 2*(R_E+r_e) = 0-V_OUT / R_C
V_OUT / Vdiff = -R_C/(2*R_E+r_e)
but in the AoE they have G_dff = R_C / 2*(R_E+r_e) what happened to the (-) sign?
Common Mode Gain:
Common Mode signal = (V_in1 + V_in2)/2 = V_in2
Following the suggestion to split the pair into 2 sections (I'm looking at section on the right)
(V_IN2 - 0.6)/ (R_E + r_e + 2*R3) = 0-V_OUT / R_C
This is as far as I get - I don't see how I can get rid of the 0.6V to get the right answer of -R_C/(2*R3+R_E).
Thanks in advance !