A quick look at this PDF might lead to think that a
Class A amplifier must use an output stage where half of it is a constant current source.
Not so, a Class A output stage is by definition using two halves that are always active, never off.
On the schematic, one can see the lower half, is inforced constant by feedback from sensing the output current of the lower half.
It only takes an easy variant to have the lower half not constant current but tracking the current of the upper half.
There are various ways to do that, the key point is to have the lower half decreasing its current while the upper half increases its own current.
Here one wants to have consistent tracking where:
The lower Delta current equals to Minus the upper Delta current.
This way upper and lower operate in a shared symmetric way. So, the speaker gets Twice the Delta current
There is no need for an exact equality in this tracking, it is only needed fairly equal.
Well that is how the Class A amp, I designed in the 60s from scratch using Germanium BJTs works.
Class A amplifier must use an output stage where half of it is a constant current source.
Not so, a Class A output stage is by definition using two halves that are always active, never off.
On the schematic, one can see the lower half, is inforced constant by feedback from sensing the output current of the lower half.
It only takes an easy variant to have the lower half not constant current but tracking the current of the upper half.
There are various ways to do that, the key point is to have the lower half decreasing its current while the upper half increases its own current.
Here one wants to have consistent tracking where:
The lower Delta current equals to Minus the upper Delta current.
This way upper and lower operate in a shared symmetric way. So, the speaker gets Twice the Delta current
There is no need for an exact equality in this tracking, it is only needed fairly equal.
Well that is how the Class A amp, I designed in the 60s from scratch using Germanium BJTs works.
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