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

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
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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AnalogKid

Joined Aug 1, 2013
8,223
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:

http://forum.allaboutcircuits.com/threads/output-power-boost-for-op-amps.117595/

ak
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
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:

http://forum.allaboutcircuits.com/threads/output-power-boost-for-op-amps.117595/

ak
Thanks for the advice. I will adjust the circuit accordingly.
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
IMG_1071.JPG IMG_1072.JPG 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
8,223
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
 

Bordodynov

Joined May 20, 2015
2,454
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 Ampl3.png is taken from the output.
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
I Think you are right Jony130. A better quality class AB audio amp should not be hard to find.

I'll look for one when I return from vacation in a couple weeks.
 
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