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Old 27th February 2012, 12:23 PM   #1
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Default My First Super Simple Class AB Amplifier

Hi guys. So, i need some help to do a circuit analysis about the circuit i'll show below. The goal is to build a Class AB amp as simple as possible, aplying the learned concepts (that isn't too much) to require something around 5-10W on the output...

Here follows the schematic:

Click the image to open in full size.

About this schematic... something is wrong? should be fixed? How much power will i get on the output? and the current gain about the 3th stage?
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Old 27th February 2012, 12:41 PM   #2
Mooly is online now Mooly  United Kingdom
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It's not ideal, although great for learning.

The TL072 isn't suited to this type of circuit due to very limited output current capability. At the very least you would need darlington outputs.

I haven't calculated the bias network but that looks non ideal too. You need emitter resistors to provide some stability (0.47 ohm for example).

R9 I take it is to enable the opamp to drive the load at low levels thus minimising crossover distortion. In practice it will just load the opamp to much.

As I say, it's great for learning but you can do so much better if you want it for music reproduction.
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Old 27th February 2012, 12:54 PM   #3
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what opamp would you recommend? the bias network i see isn't stabilized, i'll put the divider to provide some 0,7~0,9V on the bases.

I tought about darlingtons too, but they don't have some problemas with temperature, like excessive heating? what you think about connect more tip31 on parallel? like base to base..

And, of course, thanks by the tip about emmiter resistors, will do that.

My point here is really develop a circuit as simple as possible, with some considerable sound quality, for learning, and for use here on my PC.
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Old 27th February 2012, 01:37 PM   #4
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Like mooly said your circuit need emitter resister for output transistor.

Opamp has current limited if you use darlington output you can get more current gain and it's not main reason to make your amp hot(if you adjust idle current of output transistor right). Output bias current is main reason for heat dissipation.

If you want to make pc amp tip31 can delivery 10 watts without problem(class AB). no parallel need no big heatsink need (class AB) .

If you want to build simple amp and sound good(not the best but still good) OTL 10 watts current feedback is your best choice.
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Old 27th February 2012, 02:23 PM   #5
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That bias network isn't going to work. Have you ever calculated required base current for 10W/4ohm, and the resulting voltage drop across a 2k2 resistor?
I suggest you use a diode (1N400x) + ~47 ohm resistor for each of the 2k2s, preferably with a ~10..100 electrolytic going from Q1 base to Q1 base. With 0.47R emitter resistors, that should give you a Q1/Q2 idle current of about 10x diode current.

And yes, you can run multiple transistor pairs in parallel. Not indefinitely, but 2 should be feasible.

I don't see the point of R6-C2. Reducing high-frequency gain only increases the risk of oscillation in such a setup. If necessary, plan on installing a little (2-digit pF range) capacitor between opamp output and inverting input.

Oh, and 40 dB of gain seems excessive. Do you really think you need a ~60 mV input sensitivity? Normally about 26..30 dB will do fine.

The opamp should be a 5532 at minimum, maybe even a 4556.
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Old 27th February 2012, 08:35 PM   #6
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Quote:
Originally Posted by sgrossklass View Post
preferably with a ~10..100 electrolytic going from Q1 base to Q1 base.
Oops. Make that "from Q1 base to Q2 base".

Oh, and you can leave out R9. It gains you absolutely nothing with an AB buffer stage at reasonable quiescent current, though it tends to be beneficial if you're running in class B.
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Old 27th February 2012, 09:06 PM   #7
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http://www.ti.com/lit/an/snoa600a/snoa600a.pdf
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Old 28th February 2012, 02:10 PM   #8
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Here is my suggestion for a super simple amp.
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Old 28th February 2012, 06:01 PM   #9
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Ingenious, could you explain the roll of the constant current source device(and the 15k resistor attached between the colector and regulation terminal)? and the capacitor after that, and the capacitor on the 5534
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Old 28th February 2012, 06:12 PM   #10
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Quote:
Originally Posted by ingenieus View Post
Here is my suggestion for a super simple amp.
Have you run the simulations to get any idea how it performs ?
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