# Power amplifier circuit analysis

Discussion in 'Homework Help' started by UpDownStrange, Feb 2, 2015.

1. ### UpDownStrange Thread Starter New Member

Feb 2, 2015
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0
Hey everyone!

So I have been given a circuit diagram and tasked with figuring out what it is and how it works. I was told only that this circuit is found in an audio amplifier, but I've done some research and I believe it's a power amplifier / output stage circuit.

As far as I have figured out, the circuit amplifies current in order to create enough power to drive a speaker, however I am struggling to figure out exactly how it works. Would anyone be able to assist me in understanding this circuit? I would ideally like to know what the purpose of each component is.

From what I have gathered so far, R1 and R2 act as a potential divider to evenly split the input signal, and the capacitors act to reduce noise. I don't know if any of that is correct, I'm really struggling to understand this circuit.

I am also told that:
R1 = R2
C2 = C3
C1 = C4

I have attached a JPG of the circuit diagram.

This is my first post so if I'm doing something wrong then sorry. I did have a read of the 'please read' thread.

Thanks!

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2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Google the following terms:

Emitter Follower
Transistor Biasing
Coupling capacitor

Finally, C1 is redundant (not needed)

Feb 17, 2009
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4. ### UpDownStrange Thread Starter New Member

Feb 2, 2015
3
0
Couldn't figure out how to delete this.

5. ### UpDownStrange Thread Starter New Member

Feb 2, 2015
3
0

Thanks for the help. So far I have gathered the following information:

• It is a class B amplifier circuit
• It is comprised of an NPN and a PNP transistor that work together to create a push-pull circuit
• Only one transistor conducts at a time, and when a transistor is conducting, it is an emitter follower, providing minimal output impedance to the speaker/load
• The capacitors reduce noise
• The resistors create a potential divider to evenly split the input signal
Is all of this accurate? Also, I'm having trouble figuring out what the diodes are for. Could anyone help me there?

Cheers

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Good work.

To correctly bias the PNP and NPN to class AB1 or class B, which implies that both transistors are just barely forward biased (weakly turned on), what should their respective Vbe (Voltage from base to emitter) be? Think about measuring these base voltages with respect to the emitters.

How does the forward voltage drop of a diode relate to the Vbe of a transistor?

Why diodes instead of a resistor? If the diodes were replaced with a single resistor, how would you calculate the required resistor value so that the two Vbes cause the NPN and PNP to be weakly turned on.

Why two diodes instead of a resistor? (Hint, think about what happens as the temperature changes)