# Class AB Amp With Class Driver

#### crutschow

Joined Mar 14, 2008
25,132
Here's a better design that uses a differential input to provide both DC negative feedback to stabilize the output bias, and AC negative feedback to determine the gain and reduce distortion.
It also has a bootstrap driver to R12, to increase the positive output drive voltage before clipping.

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• 5.3 KB Views: 9

#### Jony130

Joined Feb 17, 2009
5,176
The input signal is 10mV while the output is only 45mV. Av = RB3 / RB4 = 1K / 100Ω = 10. The output power is almost zero.
Have you tried to understand why is that?

#### newbie2019

Joined Apr 5, 2019
95
Have you tried to understand why is that?
Yes. I always try to figure things out on my own before I post.

#### Jony130

Joined Feb 17, 2009
5,176
I simply want to know what do you think about this "poor circuit". And do you know why the gain is less than 10? That all.

#### newbie2019

Joined Apr 5, 2019
95
I simply want to know what do you think about this "poor circuit". And do you know why the gain is less than 10? That all.
Not enough current drive?

#### Zeeus

Joined Apr 17, 2019
597
Here's a better design that uses a differential input to provide both DC negative feedback to stabilize the output bias, and AC negative feedback to determine the gain and reduce distortion.
It also has a bootstrap driver to R12, to increase the positive output drive voltage before clipping.

View attachment 179732
dude..you see we are struggling with 1 transistor and you bring diff amp

#### Zeeus

Joined Apr 17, 2019
597
Not enough current drive?

Look post 11

#### crutschow

Joined Mar 14, 2008
25,132
dude..you see we are struggling with 1 transistor and you bring diff amp
Dude, who's "we".
It's newbie2019's thread and I was posting for him, so don't hijack the thread.
If you want an explanation for the circuit, then you need to start your own thread for that.

#### newbie2019

Joined Apr 5, 2019
95
Here's a better design that uses a differential input to provide both DC negative feedback to stabilize the output bias, and AC negative feedback to determine the gain and reduce distortion.
It also has a bootstrap driver to R12, to increase the positive output drive voltage before clipping.

View attachment 179732
Thanks crutschow. This circuit is very interesting. I may have to bug you with alot
of questions in the near future.

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#### Audioguru

Joined Dec 20, 2007
11,249
Newbie, it seems that you do not understand how a simple transistor works. Look at these two circuits that have vastly different gain.
Do you see the difference? Do you know why the gains are so different.
Do you know why the one with high gain also has high distortion?

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#### Audioguru

Joined Dec 20, 2007
11,249
Newbie, you should not have given me a nasty personal message when I am trying to help you understand the simple circuit that you said was a "poor circuit" with low gain and almost no output.
I simply added a single capacitor to it and asked you if you understand why it increased the gain and output 18.5 times.

#### Jony130

Joined Feb 17, 2009
5,176
dude..you see we are struggling with 1 transistor and you bring diff amp
https://electronics.stackexchange.com/questions/416762/power-amplifier-does-this-circuit-work/417035#417035
https://electronics.stackexchange.com/questions/363367/function-of-specific-capacitor-in-audio-amplifiers-negative-feedback/363376#363376

Not enough current drive?
What you need to understand is that to be able to drive a low resistance load (speakers) we need power ( high currents and high voltages).
So, we need a current and a voltage amplifier. This is why we are connecting the load (speakers) directly or via a large capacitor to a complementary emitter follower.

And the job we give this emitter follower is to provide a necessary output current for the load (current gain).
But unfortunately, emitter follower does not provide any voltage gain. Therefore we need a voltage amplifier stage (VAS) that will be able to provide a high voltage gain and needed output voltage swing for the spraker. Hence we are adding CE amplifier befor our output stage.
You may ask why we do not use CE amplifier and drive the speakers directly from CE amplifier. The main disadvantage is high output resistance of a CE stage ( low load resistance will kill all the needed voltage gain).

So we have at least a two-stage amplifier. But to isolate the VAS stage form the input signal and to be able to apply global negative feedback we are adding input stage a diferential amplifer.

Thanks to this three-stage amplifier we can independently "control" each stage of amplifier performance.

Not enough current drive?
Have you thought about the loading effect? The output transistors (emitter follower) current gain is not equal to infinity but it has some finite value.

Hence the load resistance seen by a CE stage will be equal around Rc||(β+1)RL. Therefore the voltage gain will be lower than Rc/RE.

#### Bordodynov

Joined May 20, 2015
2,630
See the alternative output and input stage. A more complex input stage sets the same transistor currents, which reduces distortion.

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#### Zeeus

Joined Apr 17, 2019
597
wow..took a while to understand this and just first circuit (anyways, little comfort, last circuit by Bordodynov "looks" surprisingly not hard to understand )

This circuit now, the AC signal that goes to speaker, comes through R3 and C2? normally, one side of speaker is to dc ground : this different : nice

and we assume the down part of the speaker (R5 in image) is also 5V?

If understand circuit then you did not do well lol...I post better circuit in new thread..lol, thanks ofcos! as always

btw, please explain C2...in post, you mentioned bootstrap

no no, wrong ..You are master, me student...can't outdo the master just yet..too soon...in years to come

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#### Zeeus

Joined Apr 17, 2019
597
Thanks crutschow. This circuit is very interesting. I may have to bug you with alot
of questions in the near future.
@newbie2019 @crutschow

Newbie2019, crutschow been waiting for your circuit