Help understanding (circuit wise) this predriver-driver configuration


2012-02-27 5:02 am
I see this configuration used all the time in class AB schematics, and what to understand it's mode of operation:


For now, lets just look at the class A output configuration on the right.

I understand that Q5 is the pre-driver to Q6 to supply the base current, but kind of stage is Q5? I think think a pre-driver would need to be configured common collector, but it looks to be common emitter gain stage.

How is the bias for Q5 set? And what function does the 730uA current source play?
Q5/6 form a complentary feedback paid (CFP) or compound pair.
Its simplest to just think of it as a single transistor of the predriver (Q5) gender.
Q6 supplies the bulk of the load current, the Q5 bias is set by sinking the required Q6 base current plus the 1K bias resistors current ~0.8/1K. That is in this case with 2A total emitter current and assuming a Q6 beta of 100 then 20ma is its base current for a total Q5 collector current of 20.8mA. if Q5 has a beta of 100 then it needs 200uA of base current.

The 730uA current source is there to supply some of this bias, with the DC coupled input voltage and series resistor as drawn this current source isnt really needed.

The compound pairs give you lots of beta gain but they do form a local feedback loop and can be thus difficult to use.
With no load current all the current will be supplied by Q5 until the bias across the 1K resistor is enough to turn on Q6 then you stay within this local feedback operation (Q5 collector current is essentially Q6 base current).