W
Winfield Hill
- Jan 1, 1970
- 0
Jim said:The scheme correctly "stiffens" the emitters of Q1,Q3, but
blows it by taking the outputs from the emitters of Q2,Q4.
The proper way is left as an exercise for the student ;-)
Perhaps you can spell it out for us.
There are other configurations, but this looks right to me,
for this one. Q2 serves to keep Q1's current constant, more
or less, as does Q4 for Q3. This means the changing signal
current, Vin/R9 (where R9 includes the variable part), goes
through Q2 and Q4, rather than the input transistor pair.
The input pair's nearly constant current means a nearly
constant Vbe voltage, hence the low distortion. I'd like
to see current sources in place of R2 R6 to improve CMRR,
and a maybe mirror for R4 R8, but this is a good start.
There's another scheme I like by Barrie Gilbert, but it
requires six transistors. I'm not sure it performs any
better at low audio frequencies.