There's many ways to understanding a circuit - here's how I'd describe it in a few simple steps:
(1) Start with a single transistor, common emitter gain stage, and an emitter resistor. Output impedance is high (you basically control the collector _current_, not voltage.)
(2) Add a feedback resistor from collector to base. Now the feedback makes it a voltage amp. Very common circuit.
(3) Decouple the feedback from this gain stage by adding an emitter follower (darlington, even). Makes it a better voltage amp, doesn't change much of the closed loop maths. (except for the BE voltage drop, and extra poles)
(4) Now, instead of using the darlington as an emitter follower, "open" its collector-to-supply connection, and use this as an output. The darlington now acts as a voltage-to-current converter, using the first transistor's output voltage as its control voltage.
This is perfect for adjusting the current thru the choke or output transformer. But to make it a voltage amp again:
(5) Add feedback from the "current source" output (collector of darlington) back to a point where you get negative feedback (emitter of 1st stage).
Important detail: Make this feedback path AC coupled. Then you have a voltage amp again for audio, but you still have a current source for DC, i.e. perfect control over your bias current.
I'm aware that I've left out some important little details, and made sloppy use of the term "current source". But this step-by-step approach helped me to understand the basic function of the circuit, so maybe it will help others as well.
JH.
(1) Start with a single transistor, common emitter gain stage, and an emitter resistor. Output impedance is high (you basically control the collector _current_, not voltage.)
(2) Add a feedback resistor from collector to base. Now the feedback makes it a voltage amp. Very common circuit.
(3) Decouple the feedback from this gain stage by adding an emitter follower (darlington, even). Makes it a better voltage amp, doesn't change much of the closed loop maths. (except for the BE voltage drop, and extra poles)
(4) Now, instead of using the darlington as an emitter follower, "open" its collector-to-supply connection, and use this as an output. The darlington now acts as a voltage-to-current converter, using the first transistor's output voltage as its control voltage.
This is perfect for adjusting the current thru the choke or output transformer. But to make it a voltage amp again:
(5) Add feedback from the "current source" output (collector of darlington) back to a point where you get negative feedback (emitter of 1st stage).
Important detail: Make this feedback path AC coupled. Then you have a voltage amp again for audio, but you still have a current source for DC, i.e. perfect control over your bias current.
I'm aware that I've left out some important little details, and made sloppy use of the term "current source". But this step-by-step approach helped me to understand the basic function of the circuit, so maybe it will help others as well.
JH.
