BJT differential pair on breadboard enters saturation

I've built the following circuit on a breadboard (for the purpose of experimentation) and measured the voltages: enter image description here

IN+ and IN- are connected to ground, VCC is 10V.

As you can see it's a differential pair (using 2N3904 and 2N3906) loaded with a current mirror for differential to single-ended conversion. The input uses Q5 and Q6 to raise Q1 and Q2 base voltages so that they're turned on even when the IN+ and IN- are grounded. LTSpice simulation seems to confirm the circuit works, but on a breadboard Q5 is saturated.

I tried the following things to try to fix the circuit:

  1. Diode-connecting the other side of the mirror instead, but then Q6 would become saturated.
  2. Replacing Q3 and Q4 with same value resistors - this seemed to help with biasing, but that I think cuts the gain in half.
  3. Replacing Q3 and Q4 with some other transistor (2N2222) - this didn't make a difference.

I was hoping that it would work, even considering the obvious parameter mismatch between discrete transistors. Any ideas what am I missing?

Answers 1

  • As @user_1818839 pointed out in the comment, there was an error in measurement that I've made. The 0.9V across emitter-base of Q1 is simply out of this world, and should have made me think twice before posting - lesson learned :-)


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