I need someone to revise/suggest improvement for the audio power amplifier I am designing. This PA is going to be used for driving the horn speaker (8ohm 100W). I decided to start design from scratch instead of just copy/paste existing design (of course this is still not something new, you can find the same topology on Internet, Hitachi has similar concept in app note). As you can see, it is composed of two stage differential amplifier. The first differential amplifier uses Wilson current mirror as a current source, and the second the "standard" current mirror as a load. The second differential amplifier provides bias voltage and drives the output stage. For the output stage, I selected mosfet approach that is composed of 4 mosfet transistors (complementary). I must emphasize that I am not much in analog design field, so does not have strong sense if this design is any good. This is the part of bigger project with the rest of digital logic. The main aims of this PA is to be stable, and to use in wide temperature range (preferably in range of -20°C – 75°C), if possible. Therefore, I need your revision and improvement suggestion. I highlighted a couple of design doubts:
- I am not sure about this kind of bias (the red circled resistor would be potentiometer). Is this stable and safe (not get in situation to blow up mosfets while adjusting bias)?
- Is this design stable (not oscillate)? Of course, I am planning to add Boucherot cell (Zobel network) at the output.
- How to improve temperature operating range?
Please note that I used available bjt transistors from the LTspice lib. Maybe some improvement can be done also in that part.
The following are screenshots of schematic, transient analysis, and FFT analysis performed in LTspice.
EDIT: According to my simulations, I noticed that the mosfet bias current change with temperature is mostly caused due to change of emitter current of driving differential amplifier (that also changes the voltage drop on R4 and sets mosfet bias). I decided to implement current source based on current in emitter of the driving differential amplifier instead of simple resistor. I got better results in therm of bias stability, if I am correct. There are also some changes due to suggestions. The revised schematic is following: