In spite I took the pain to match QA and QB for Vbe and hFE, after testing a just assembled class AB amplifier I found that the output offset is quite high: 125 mV. (The attached .pdf shows the parts of interest).
Based on the measurements I took, with 0V at the input, the reason of the imbalance seems to be the difference in value between Rin and RfA//RfB.
My questions:
1 - Is the above correct? If so, why to bother in matching the pair after all if we have to resort to compensation?
I see several possibilities to reduce the offset voltage to a small value:
2 - Using Pot A to equalize currents. Read that it is bad practice (but I know it works). Not very professional...? In fact it was the first thing I did when playing with a long tail pair, more than 20 years ago.
3 - Using Pot B (or Pot C - one of them only) to equalize the currents in the pair.
4 - Changing the value and ratio of Rf A and Rf B (avoiding to mess with Rin).
5 - Using a compensation in the form of an offset voltage applied to the bottom of Rf B provided by a low Z source.
This idea (totally new to me for a long tail pair) I found it in a project from Dick Cappels,
here
I cannot avoid the feeling that matching transistors was a waste of time, vis a vis the necessity of offset compensation.
Yes, my experience in analog designs is very limited and certainly rusty.
Can anyone help with comments?
Gracias.