This is a differential amplifier built with two transistors. The output equals the voltage difference between the two inputs. Input 1 is a 40 Hz signal, and input 2 is a 40 Hz signal with a 200 Hz signal added. (In this simulation the two signals are added simply by connecting two sources in series, which is convenient but unrealistic.) The output is a 200 Hz signal. The two inputs are connected to the bases of the two transistors. The emitter of each is a diode drop below the base. In the differential-mode case, when input 1 rises and input 2 falls by the same amount, there is more voltage across the left 1 kΩ resistor and therefore more current. Across the right 1 kΩ there is less voltage/current. It gives the same voltage and current through the 75 kΩ. The decrease in current through the right transistor causes the output to rise. When input 1 falls and input 2 rises, the output falls. In the common-mode case, when input 1 and input 2 rise together, this means more voltage/current across both 1 kΩ resistors, which also means more voltage/current across the 75 kΩ. But the 75 kΩ is a large resistor, so a small increase in current results in a large increase in voltage. Thus, the change in current through the right transistor (and the change in voltage at the output) in the common mode is much smaller than in the differential mode.
This page is a utility for simulating дифференциальный усилитель online with specified initial values.
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