Power Amplifier (Darlington) with Power and current limitation
I'm more interested in the resistors R3 &amp; R4 as they are for current and power limitation and I don't really understand how it works and how I calculate them.


For Power Amps the main design tradeoffs are heat rise and non-linearity at current range. The hFE is a critical factor as it can vary 10:1 over a very wide range of currents as well as  Vce &amp; T rise. It is also interesting to know that BJT's perform better hot and MOSFET's perform better cold. But the current limiter here is to prevent Q2 from burning up from a short circuit fault.

In order to detect current Vbe sense the voltage drop across Re so to regulate this limit, with proportional feedback (PID theory) the limit error is inversely related to the loop gain and drift in the voltage reference.  

Since R3/(R3+R4) ratio forms a feedback attenuator, that reduces loop gain and increases error.  For optimal heat protection, you want R4=0 to increase the current loop gain and reduce the limit error above your Current limit setpoint of Vbe/Re = 1A,  then R3 is redundant.

Yet, BackEMF is stored load energy. So the L/R or ESR*C of an overcurrent load must be defined or at least considered, like a pumpmotor or a supercap being charged or a woofer being connected. Thus R4 can be useful for Q3-Ib protection but R3 makes current limiting softer and is just a loop gain attenuator.  So you must decide between Thermal margin and compression distortion while limiting current.  This has been the basis for another class of design with current booster transistors added to the basic Class A-B design.

Recommendation

Let R4 = 12V/100mA ~ 120 Ohms where I want Q3-Ib to handle a 100mA transient. Then do not install R3.







simulate this circuit &ndash; Schematic created using CircuitLab

Caveat

Since Re senses current limit it also attenuates the signal 0.4/8ohms = 5%  So I am speaking about additional attenuation from Zs/Zin ratio to provide linear gain and current limiting. There are many details not discussed.