Schematic of class B amplifier using opamp

Hi All,
Where can i find "class B amplifier using opamp" schematics?
Thanks

Class B amplifier

A class-B amplifier has horrible "crossover distortion". Most audio amps use class-AB that has no crossover distortion because the output transistors have a small bias current

A complimentay emitter-follower pair of power transistors (one NPN and one PNP) can be biased and driven from the output of an opamp.

Re: Class B amplifier

Thanks audioguru,

Is the circuit will look like this ?(attached)
how does it operates, do you have any reference?
I am getting high third harmonics in this, what should I do

Re: Class B amplifier

Please post schematics as a GIF or PNG file type, without compression with RAR.

Your amplifier has bad 3rd harmonic distortion because its output transistors are not included in the negative feedback loop of the opamp.
Also, the output transistors have their bases driven by resistors. When the signal increases and the output transistors must conduct more current then the base current becomes less. Then the output transistors compress the peaks of the waveform. "Bootstrapping" or using a constant current source to turn on the output transistors is better than simple resistors.

I think your amplifier will have Thermal Runaway.
The driver transistors will heat which causes them to conduct more, which causes them to heat more. Then they keep conducting more and heating more.
The same thing happens with the output transistors.

Audio power amplifiers have diodes or a transistor bonded to the heatsink for thermal stabilization.

Your amplifier had extra resistors which I removed. I cropped your schematic and rewired the output transistors to be included in the negative feedback loop of the opamp.

Re: Class B amplifier

How much powerfull is this ?
Very strange shematic?

Re: Class B amplifier

zlatkoMM said:

How much powerfull is this ?

Click to expand...

The total supply voltage is only 20V.
If the output has a peak voltage of 7.0V then the peak current is 778mA when the speaker is 8 ohms.
If the output transistors have a current gain of 70 then their base current is 11.1mA.
The 180 ohm resistors with 11.1mA creates a voltage drop of 2.0V.

Then the max output is 14V peak-to-peak which is 4.95V RMS.
Then the power into 8 ohms is only 3.06W RMS.
If the output transistors have less current gain then the max output power is less.

But I think that thermal runaway will destroy the circuit.

Re: Class B amplifier

But I think that thermal runaway will destroy the circuit.
Yes you probably right.

Click to expand...

Re: Class B amplifier

Thanks audioguru,

I made a mistake while uploading the schematic.
I, did what you have suggested I removed R10,R11 and R12 and made R8 & R9 to 180Ω but there is no change.
Can you send me some material based on which I can analyze the circuit.

regards,[/img]

Class B amplifier

Did you remove R15?
Without R15 then a good opamp will reduce the distortion in the output transistors.
If a lousy old 741 opamp is used then the distortion is reduced for only low audio frequencies.

Class B amplifier

Hi!!!
I think you should have your Feedback resistor rated 1meg resistor for every transistor because it may be distorted.. do you tried to make and test this kind of circuit?

Re: Class B amplifier

The transistors are emitter-followers. They don't need a feedback resistor.
The feedback should be from the output of the circuit to the inverting input of the opamp so that the opamp can cancel the distortion in the transistors.

I simulated it and it produces up to 3.5W into 8 ohms pretty well.

Re: Class B amplifier

Thanks audioguru,

It worked with different value in the feedback circuit.
Can you please tell me about the "feedback calculations involved in it" in order to optimize the circuit.

Class B amplifier

elexlearner said:

It worked with different value in the feedback circuit.

Click to expand...

Good.

Can you please tell me about the "feedback calculations involved in it" in order to optimize the circuit.

Click to expand...

In my simulation, the transistors are included inside the negative feedback loop of the opamp. The gain of the circuit is 1+ (R9/R8) = 3.13 as shown in my simulation that has an output of 6.24V peak with an input of 2.0V peak.

Added after 1 hours 18 minutes:

R eight turned into an R smiley. R8

Added after 2 minutes:

R8 with a closed brackets after it? R8)

Added after 1 minutes:

8) 8) 8) 8) 8) 8) 8) 8)

Re: Class B amplifier

yes ... i was about to reply ... but when used quote ..
i found the difference..