BJT Diferantial Amplifier

Determine the operating mode and output voltage (V0) of the given BJT Differential Amplifier Circuit provided on the side. ri1=ri2=20k.ohm β1=β2=75

by musethor
June 18, 2023

If you want to use the simulator to find the answer, a few things have to be corrected.

First, the emitter of Q2 should be down, not up. It is a mirror of Q1. Technically, you will need to change their beta_forward to 75 and their internal resistance to 20k.

Next, you cannot do any computation with labels. The 9V and -9V must be sources, not labels.

To communicate with the program that you are interested to get the voltage at the V0 point, change the label for a NODE name.

You could then run a DC simulation to get the voltages. By the voltages Vce and Vbe, you should be able to answer to the question about the operating mode of the BJT (that is, blocking, forward linear, forward saturation, or forward deep saturation, as exemple).

Take note of the voltage at V0 and rerun with a light difference of voltage for Vin2. As example, change the 2mV to 2.1mV. Observe then how much V0 becomes, and compute the difference you get for V0 by comparison with the first test when the sources Vin1 and Vin2 were equal, and divide that new difference output by the difference of 0.1mV for one of the input, to get the amplification. After all, the circuit should amplify the difference between Vin1 and Vin2, isn't it? That is not part of your question, but a very legitimate one (unless you haven't seen that matter yet in your classroom).

Oh, and rename one of the Rc. We cannot what the exact same name for two different resistors. I suggest to use RC1 and RC2.

by vanderghast
June 19, 2023

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