# Calculating the gain in a differential amplifier [closed]

I need to calculate the gain of this amplifier. However, when I do this, I get the value two times bigger than the simulation of this circuit, which I run in Multisim, suggests.

My line of thought is this: The gain should be equal to $$R_1/((r_{e1}+R_2)+(R_4 \parallel (R_3+r_{e2}))$$ where $$\r_{e1}\$$ and $$\r_{e2}\$$ are the thermal resistances of the corresponding transistors.

Calculating gives me the value of approximately 6.2, when the simulation suggests that the gain is 3.3. I have double-checked and the simulation circuit has no errors in it.

• hFE only 20? try 200. did U choose f so Zc1=~0 Commented Oct 17, 2016 at 22:54
• @Tony Stewart. EE since '75 My source frequency is 1GHz, it's in the range where resistncae in negligible. I can't change the hFE values, as the task I'm given specifically states I should use 20. Even if I do, the change is just a small fraction of the result.
– Mu3
Commented Oct 17, 2016 at 23:06
• ok I never expected 1GHz with 1uF due to ESL Commented Oct 17, 2016 at 23:34
• With R4=1K it works for me at low f with a gain of 4. But you never specified R4 Commented Oct 18, 2016 at 0:19
• R4 is 129 Ohms. Frankly, the frequency is not given either. I just assumed 1GHz to eliminate the capacitive resistances.
– Mu3
Commented Oct 18, 2016 at 13:49