# 30W class AB amplifier

#### hntrxprt99

Joined Jan 15, 2010
5
Please help me, I need a 30W class AB transistorized power amplifier circuit design.. I have been looking around for this type of design but can't find one, our professor just wants a simple design, he rejected a lot of our designs because he says that they are too hard, and also, I need the equation for that class.. thanks a lot!. i just need it soon..

#### hobbyist

Joined Aug 10, 2008
887
post schematics of all the circuits your teacher rejected, than people can look into reducing them to a more simpler version.

#### PRS

Joined Aug 24, 2008
989
Are there any other parameters such as load? or the available power source? These are important considerations. Is there an voltage input level we need to deal with -- that is; is there any voltage gain required or are you just required to provide 30 watts to a load? If this power amp feeds an 8 ohm speaker then we have something to go by.

P = Vpp^2/(8*RL) Watts(rms)

So, in this case: Vpp = sqrt(30*64) = 44 volts.

This would be a starting point for further considerations such as the minimum supply voltage. You can see from this that you need to give us all of the parameters, not just 30 watts, if you're being asked to design a specific amplifier.

#### hntrxprt99

Joined Jan 15, 2010
5
Can somebody please help me place values on this specific circuit so that the outcome would be a 30W, this is a class AB amplifier. And also, the equations for this type of circuit. My professor called it power calculations. Thanks a lot!

#### Wendy

Joined Mar 24, 2008
21,840
What is the load, and freq response?

#### hntrxprt99

Joined Jan 15, 2010
5
i don't know.. it's an audio amplifier

#### Audioguru

Joined Dec 20, 2007
11,251

As was said in your other thread, 30W into 8 ohms is 44V peak-to-peak. The opamp, output transistors and their emitter resistors have a voltage loss so the supply must be about 50V. The peak current is 22V/8 ohms= 2.75A. The minimum current gain of the output transistors is about 30 so their peak base current from the opamp is 92mA.

But there are very few if any opamps with an allowed supply of 50V and an output current of 92mA so a 4 ohm load should be used and darlington output transistors that have a gain of 1000. Then the supply can be 40V and an ordinary 44V opamp can be used.

It is simple to calculate the resistor values.

#### hgmjr

Joined Jan 28, 2005
9,029
hntrxprt99,

I have combined your two posts on the same topic.

I would ask that you please avoid spawning multiple post on the same topic. It causes significant confusion among the members who are trying to assist you with your question.

hgmjr

#### hntrxprt99

Joined Jan 15, 2010
5
Are there any other parameters such as load? or the available power source? These are important considerations. Is there an voltage input level we need to deal with -- that is; is there any voltage gain required or are you just required to provide 30 watts to a load? If this power amp feeds an 8 ohm speaker then we have something to go by.

P = Vpp^2/(8*RL) Watts(rms)

So, in this case: Vpp = sqrt(30*64) = 44 volts.

This would be a starting point for further considerations such as the minimum supply voltage. You can see from this that you need to give us all of the parameters, not just 30 watts, if you're being asked to design a specific amplifier.
Actually i uploaded the circuit design, our professor told us to find the equations and values of the components in it, I don't know just how, I'm just new with this, I'm sorry. And also, I don't think that we need to know how great the input is, because we are going to make our own power supply, just need for the output to be a minimum of 30W, thanks again.

#### Audioguru

Joined Dec 20, 2007
11,251
As I explained, your circuit cannot use an 8 ohm speaker because its supply voltage will be too high for an ordinary opamp. It cannot use ordinary output transistors because its opamp current will be too high.

An 8 ohm speaker 30W amplifier needs a 50V supply plus darlington output transistors with a high current gain. The max allowed supply voltage for an ordinary opamp is 36V but some are 44V which is still too low.

A 4 ohm speaker 30W amplifier needs output darlington transistors with a very high current gain.

#### hntrxprt99

Joined Jan 15, 2010
5
As I explained, your circuit cannot use an 8 ohm speaker because its supply voltage will be too high for an ordinary opamp. It cannot use ordinary output transistors because its opamp current will be too high.

An 8 ohm speaker 30W amplifier needs a 50V supply plus darlington output transistors with a high current gain. The max allowed supply voltage for an ordinary opamp is 36V but some are 44V which is still too low.

A 4 ohm speaker 30W amplifier needs output darlington transistors with a very high current gain.
is it then possible that i use the circuit as i have posted above to have an output of 30W? also, do you have ym account, i need to speak to you more often. Thanks.

#### Audioguru

Joined Dec 20, 2007
11,251
is it then possible that i use the circuit as i have posted above to have an output of 30W? also, do you have ym account, i need to speak to you more often. Thanks.
No.
For the 3rd time, Your circuit cannot produce 30W to an 8 ohm speaker because its supply voltage will be too high for an ordinary opamp. It cannot use ordinary output transistors because its opamp current will be too high.
Also, your circuit cannot produce 30W to a 4 ohm speaker and use an ordinary opamp because its opamp current will be much too high.

I explained why.
You don't need to speak to me often, you just need to learn about simple Ohm's Law and read the datasheets.

Explain to your professor why ordinary output transistors cannot be used and must be replaced with darlington transistors (or Sziklai pairs).

#### Jony130

Joined Feb 17, 2009
4,930
You can use the floating power supply and then you can use the ordinary opamp.

#### Audioguru

Joined Dec 20, 2007
11,251
You can use the floating power supply and then you can use the ordinary opamp.
All power supplies have floating outputs unless you ground them.

Ordinary opamps have a max allowed supply voltage of 36V. A simple audio amplifier that produces 30W into 8 ohms needs a 50V supply which will destroy an ordinary opamp. Ordinary output transistors need a base current of 92mA to produce 30W into 8 ohms but the max output current of an ordinary opamp is only 20mA which is much too low.

#### Audioguru

Joined Dec 20, 2007
11,251
It is very strange to ground the output of the amplifier then half the power supply voltage is modulated by the signal.
Why not do it normally?

#### Jony130

Joined Feb 17, 2009
4,930
Why? because this trick works, only by change the gnd you can convert a voltage follower to a voltage gain amplifier.

#### Audioguru

Joined Dec 20, 2007
11,251
Why? because this trick works
But it is an odd way of making an amplifier.
This is the normal way to do it:

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