Audio amplifier with output Power stage

Audioguru again

Joined Oct 21, 2019
6,672
The original circuit is missing a resistor or the preamp transistor at the base of Q3 to Vee.
It is also missing AC and DC negative feedback that all audio amplifiers have.
The simulation should show the DC voltages at the emitters of the output transistors instead of power, then you will see that there is no way the output transistors will have their base voltages near 0V.
Here is a similar simplified circuit I simulated but the little transistors might be overloaded.
 

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Thread Starter

BUZZARD1n

Joined Apr 20, 2022
24
The original circuit is missing a resistor or the preamp transistor at the base of Q3 to Vee.
It is also missing AC and DC negative feedback that all audio amplifiers have.
The simulation should show the DC voltages at the emitters of the output transistors instead of power, then you will see that there is no way the output transistors will have their base voltages near 0V.
Here is a similar simplified circuit I simulated but the little transistors might be overloaded.
Hi, thank you for this. I have updated the circuit and changed the current mirror to rather use resistors for efficiency (attached above), but I see that you mentioned the push-pull AB amp with diode biasing. I love that idea and I will definitely implement it. I would like to, however, still try to make the vbe multiplier AB amp work with what you suggested in the first image. Thank you
 

Thread Starter

BUZZARD1n

Joined Apr 20, 2022
24
The original circuit is missing a resistor or the preamp transistor at the base of Q3 to Vee.
It is also missing AC and DC negative feedback that all audio amplifiers have.
The simulation should show the DC voltages at the emitters of the output transistors instead of power, then you will see that there is no way the output transistors will have their base voltages near 0V.
Here is a similar simplified circuit I simulated but the little transistors might be overloaded.
I have quickly attempted the current matching you showed. It is working a lot better with much less distortion. I am still struggling to get the gain right with this one as well. There is a big offset at the collector of Q6 and I assume it is a problem.
 

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Audioguru again

Joined Oct 21, 2019
6,672
Your new currents are FAR from being matched.
Since you do not show the DC at the emitters output then you do not see that the current source is a saturated
very low impedance that cuts the output signal to only 600nV peak which is a huge signal loss.

The 10uF output capacitor feeding 8 ohms on the circuit with many transistors cuts many low and mid audio frequencies.

Again you have no AC and DC negative feedback.
 

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Thread Starter

BUZZARD1n

Joined Apr 20, 2022
24
Your new currents are FAR from being matched.
Since you do not show the DC at the emitters output then you do not see that the current source is a saturated
very low impedance that cuts the output signal to only 600nV peak which is a huge signal loss.

The 10uF output capacitor feeding 8 ohms on the circuit with many transistors cuts many low and mid audio frequencies.

Again you have no AC and DC negative feedback.
I require a current of 100mA as my bias current for the circuit image attached. So I tried to make the current mirror imitate the 100mA bias as close as possible. I do however understand what you are saying and I will try to implement it in my design. Thank you. I also apologise for my lack of understanding. I know how to design all the circuits separately, but this is the first time I made a power output stage. The output stage works without being connected to the amplifier and it is just connected to a sinewave input.

I attached an image of the circuit I have redesigned now. It does not use the current mirror config to apply a 100mA bias but rather resistors.
 

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Last edited:

Ian0

Joined Aug 7, 2020
9,667
Just because you would like 100mA through the output stage doesn't mean you need 100mA through the driver stage.
You are setting the bias current with a Vbe multiplier, not a current mirror.
 

Thread Starter

BUZZARD1n

Joined Apr 20, 2022
24
UPDATE: Thank you for everyone's help and information. I plan to use the knowledge gained to solve the problems I face regarding the circuit and my lack of knowledge thereof. I am going to take the problem on my own from now on and won't be posting any more questions in this thread, only updates. Sorry for my sour attitude yesterday. Good luck with everyone's project challenges.
 

k1ng 1337

Joined Sep 11, 2020
940
UPDATE: Thank you for everyone's help and information. I plan to use the knowledge gained to solve the problems I face regarding the circuit and my lack of knowledge thereof. I am going to take the problem on my own from now on and won't be posting any more questions in this thread, only updates. Sorry for my sour attitude yesterday. Good luck with everyone's project challenges.
Now that's what I like to see except for one little part..
 

k1ng 1337

Joined Sep 11, 2020
940
Sorry if something came across as upsetting. Could you please tell me what bothers you?
I'm glad you asked. In post #35 you stated that you will stop asking questions and I think that's just plain silly. ;)

When jumping in late, read the thread and you shall find that this was handled 17 posts ago.

ak
Yes you are right. I have a tendency to comment when I see disrespect instead of sifting through the next 3 pages of potential bickering. Also, it is my view a person is more likely to change their unpleasant ways if they are confronted from multiple people so I don't think a reminder in the same topic is much of a bad thing. And I am no exception, if I make an ignorant or uneducated post or response, I want to know about it so I can correct myself and move on..
 
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