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https://www.circuitlab.com/circuit/8v5qy8/differential-amp/

The single ended differential amplifier has an unexpected 6.6V dc offset. The gain seems correct, but I don't understand why there is the offset.

Any ideas?

@moogie,

Because both transistors are identical, this simulation is of a nearly perfectly symmetrical long tailed pair.

The only source of imbalance is the difference in Vce and Vcb due to the collector load in Q2 and not in Q1.

Therefore, the current flowing through the emitter resistance of approximately 1.4mA, is split almost exactly equally between the two transistor emitters.

Neglecting the reduction due to hFE, that means the collector currents of each transistor are almost exactly 0.7mA.

Therefore the collector voltage of Q2 is:

15V - 0.7mA*10kR = 8V

:)

Hello Signality,

Thanks for responding to my question. That is exactly what I was seeing. What first led to my confusion was the transient analysis. On my scope I did not observe this offset, and that confused me. Then I realized that I was using AC coupling on my scope, so that would remove the 8V dc offset. So for the simulation piece to look like my scope, I added a 1 uF cap and 1 MHohm resistor in series and redid the transient analysis and the output now looks like my scope output with AC coupling on.