How can this circuit work? -- LF351(Jfet op amp) drive tip127, tip122 amplifier.

PickyBiker

Joined Aug 18, 2015
101
I was searching the web for a push-pull audio amp circuit that could produce 10 watts. I came across on on circuittrue.com with an op amp feeding TIP122 and TIP127 transistors. It looked like what I wanted, but as I looked at it, I couldn't see how it could possibly work. The op amp output does not feed the base of the transistors. The transistor bases are connected to the + and _ power inputs of the op amp.

Can someone please explain how this could actually work?

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Dodgydave

Joined Jun 22, 2012
11,303
It's using the volt drop across R3 and R2 to feed the transistors,

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PickyBiker

Joined Aug 18, 2015
101
It's using the volt drop across R3 and R2 to feed the transistors, also C1 is far too low to drive the speaker, try 47uF.
Isn't the speaker being driven by the transistor collectors?

Dodgydave

Joined Jun 22, 2012
11,303
Yes, the transistors will provide the current in the speaker.

Veracohr

Joined Jan 3, 2011
772
Even 47uF is too small. More like 2200uF if you want a full range amp with an 8 ohm speaker.

Jony130

Joined Feb 17, 2009
5,488
Do not forget about op amp internal circuit.

Even 47uF is too small. More like 2200uF if you want a full range amp with an 8 ohm speaker.
No, you do not need to change C1 value. C1 has a different task to do.

AnalogKid

Joined Aug 1, 2013
11,045
Yes. C1 is not the output coupling capacitor. This circuit does not have one because it has bipolar supplies.

R1 is the key. Any current flowing through it in either direction must come from one of the two IC power leads because there are no other sources. That current goes through R2 or R3 depending on polarity, and when the voltage drop across the resistors exceeds Vbe, the transistors start to turn on and move current to the load. The overall feedback loop keeps the base current low enough that the transistors amplify linearly.

There is a problem with the circuit in post #1. Negative feedback is taken from the output of the opamp, not the overall circuit output as it should be. The way the circuit is drawn, the high power output transistors are completely outside the feedback loop, and will saturate. For the correct way to do this, and see several variations, see this thread:

ak

PickyBiker

Joined Aug 18, 2015
101
Do not forget about op amp internal circuit.
View attachment 95018

No, you do not need to change C1 value. C1 has a different task to do.
That's what I thought, the speakers get power from the transistor collectors, not C1.
Yes. C1 is not the output coupling capacitor. This circuit does not have one because it has bipolar supplies.

R1 is the key. Any current flowing through it in either direction must come from one of the two IC power leads because there are no other sources. That current goes through R2 or R3 depending on polarity, and when the voltage drop across the resistors exceeds Vbe, the transistors start to turn on and move current to the load. The overall feedback loop keeps the base current low enough that the transistors amplify linearly.

There is a problem with the circuit in post #1. Negative feedback is taken from the output of the opamp, not the overall circuit output as it should be. The way the circuit is drawn, the high power output transistors are completely outside the feedback loop, and will saturate. For the correct way to do this, and see several variations, see this thread:

ak

PickyBiker

Joined Aug 18, 2015
101

PickyBiker

Joined Aug 18, 2015
101
Not too happy with this circuit. There is a lot of what I think is called crossover due to no bias. I cant really see exactly how the common collector Push Pull circuit would get the bias either.

Here's a look at the input sign wave and a really distorted output.

Is this circuit worth pursuing?

AnalogKid

Joined Aug 1, 2013
11,045
The power transistors in post #1 are Darlingtons! This changes significantly the circuit bias resistors. Look up the Vbe for the transistors and adjust R2 and R3 accordingly.

ak

PickyBiker

Joined Aug 18, 2015
101
I have to put this project aside for a couple weeks for vacation. Back to it in a couple weeks.

Bordodynov

Joined May 20, 2015
3,180
The electronic circuitry is no good. Recent transistors are not covered by the feedback DC. See the results of the simulation. In the second amplifier feedback is taken from the output.

Jony130

Joined Feb 17, 2009
5,488
Did you connect the right side of R4 to the output (between Q1 and Q2 collectors)?
Is this circuit worth pursuing?
Find another circuit.