# Differential pair with current mirror - unequal currents

I designed a basic audio amp for learning purposes and now I'm trying to upgrade the differential stage with current mirror. However, I encountering a problem - the currents on both branches of the differential amp are different beacuse of the additional current from T1's base (the mirror itself works as expected tough)

In fact, I achieved better symmetry with simple resistor rather than a current mirror. Am I doing something wrong?

I've marked the problematic currents in yellow squares on picture below. As You can see, the difference is about $$\23µA\$$ which is quite high comparing to $$\3µA\$$ difference I achieved using a simple feeding resistor.

Agreed - this is just a simulation (and not the best one for sure) but still...

The circuit can be found here!

• Zobacz co sam Douglasa Selfa ma do powiedzenie na ten temat: "Note must be taken of the base current the VAS transistor will draw from the input stage; it must not be allowed to unbalance the input transistor collector currents significantly. A high VAS beta helps, but will not be found in company with a high Vce(max). The EF-VAS is in my experience wholly free of this problem because of the extra stage of current amplification." – G36 Apr 4 at 6:18
• Also, notice that the input stage tail current values are 4mA to 6mA in Douglas designs. – G36 Apr 4 at 6:28
• @G36 yep, I missed that part in Selfs book. Emitter follower is the answer. Thank You! – Dawid W Apr 6 at 9:02

• Uhm thanks but it's not 300$uA$ and 294$uA$ that bother me but rather 300$uA$ and 323$uA$ - the mirror works fine but the way it's connected to the differential amp makes it pretty useless since there is still lack of symmetry present at the differential amplifier collectors. I'm not sure if it supposed to be connected this way and if such difference is acceptable (I think not). – Dawid W Apr 1 at 17:51