I've been playing around with a common emitter amplifier with an active load. I made up the following circuit and have measured some of its characteristics, as shown in the attached image.
I'm not familiar with the use of active loads, and I'm somewhat unclear about what is going on here:
1) What determines the quiescent operating point of Vout?
Note that my Iref and Ic don't match too well, so I'm assuming that Vout is being developed as (Iref - Ic) * r_out, where r_out is the output resistance of either Q1 or Q2. But if so, what determines r_out? (bearing in mind that this is a DC analysis, and I can't assume a small signal model for Q1/Q2 will be appropriate).
2) I have read in a few places that this kind of arrangement needs some kind of negative feedback - I have none but even so the circuit works just fine as an amplifier, with an unloaded voltage gain of about 34.
In addition, it seem pretty stable, thermally at least - if I disturb Iref by heating up Q2 or Q3, I can move the operating point so that the circuit clips either at the top or bottom - but it always returns to the 3.7 V quiescent point at V_out.
OTOH, I believe that ideally quiescent V_out should be 0 V when I_ref=Ic - I've been unable to balance these nicely, and I guess this could be done via feedback from Vout to VB - but how is this implemented specifically?
So: is it vital to operate this arrangement with negative feedback? If yes, then where does the stability that I'm seeing come from?
Anyone familiar with this stuff? I'm having difficulty finding any kind of detailed analysis of this arrangement that answers my questions.