# Low Power Class AB Amplifier

Joined Jun 16, 2023
128
I found this low power class AB amplifier schematic and did a sim. Is resistor RF a DC negative feedback
resistor? How would a designer decide what value this resistor should be?

Thanks

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#### Ian0

Joined Aug 7, 2020
8,947
I found this low power class AB amplifier schematic and did a sim. Is resistor RF a DC negative feedback
resistor? How would a designer decide what value this resistor should be?

Thanks
Yes - it is a DC feedback resistor.
A real designer wouldn't use that circuit, as it would set the DC level at Vbe+V(RB2) without the ability to change it.

Joined Jun 16, 2023
128
Yes - it is a DC feedback resistor.
A real designer wouldn't use that circuit, as it would set the DC level at Vbe+V(RB2) without the ability to change it.
LOL. Now that we have established that the designer wasn't real, what would the appropriate circuit be?
It always amazes me that so-called experts put circuits like this out there. It makes it hard when you are
trying to learn.
RS

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#### Ian0

Joined Aug 7, 2020
8,947
There would be another resistor between the base of Q1 and ground so that Vout*Rnew/(Rf+Rnew)=Vbe

Joined Jun 16, 2023
128
There would be another resistor between the base of Q1 and ground so that Vout*Rnew/(Rf+Rnew)=Vbe
Thanks Ian. So If I set the circuit up like the attached, the voltage on the base of Q1 should be 0.7 volts?

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#### Audioguru again

Joined Oct 21, 2019
6,445
Your circuit is biased wrong and has a very low gain.
I fixed it but the transistors are overloaded.
I improved it with more powerful little transistors.

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Joined Jun 16, 2023
128
Your circuit is biased wrong and has a very low gain.
I fixed it but the transistors are overloaded.
I improved it with more powerful little transistors.
Once again...nothing.

#### MrChips

Joined Oct 2, 2009
29,850
This is a circuit I threw together quickly. It is almost identical to your circuit originally posted. It works well enough for my purposes.
If yours does not work, check the wiring or the transistor leads identification.

#### Audioguru again

Joined Oct 21, 2019
6,445
The amplifier shown on Sept 10 produces a medium headphones level when it is slightly clipping:

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Joined Jun 16, 2023
128
This is a circuit I threw together quickly. It is almost identical to your circuit originally posted. It works well enough for my purposes.
If yours does not work, check the wiring or the transistor leads identification.

View attachment 302515
Thanks Mr Chips. I will try this out.

Joined Jun 16, 2023
128
Your circuit is biased wrong and has a very low gain.
I fixed it but the transistors are overloaded.
I improved it with more powerful little transistors.
Hi AG

Thanks for all your work. I ran the sim and found that if I replace the 2 diodes
with a 2N4401 and a 2N4403 in a diode configuration and increased the power
supply voltage to 15 volts. The THD decreased from 11.8% to 1.6%.

I do have some questions regarding some component values:
1) How did you arrive at the values for R2, R3, R4 and C4?
2) What does R1 do and how did you know what value to use?
3) Is C2 just used to provide additional power supply filtering?

Again, thanks.

RS

#### Ian0

Joined Aug 7, 2020
8,947
Hi AG

Thanks for all your work. I ran the sim and found that if I replace the 2 diodes
with a 2N4401 and a 2N4403 in a diode configuration and increased the power
supply voltage to 15 volts. The THD decreased from 11.8% to 1.6%
What happens to the bias current?

Joined Jun 16, 2023
128
What happens to the bias current?
Hi Ian:
The bias current increases which I suppose creates an issue because IbQ2 and IbQ3 are clipped on the bottom.

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Joined Jun 16, 2023
128
This is a circuit I threw together quickly. It is almost identical to your circuit originally posted. It works well enough for my purposes.
If yours does not work, check the wiring or the transistor leads identification.

View attachment 302515
Thanks. I ran a sim and there is 7% THD (some crossover) and very little output (240 μW).

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#### Audioguru again

Joined Oct 21, 2019
6,445
R1 increases the input impedance so it is not simply the base-emitter diode of Q1 (as a signal rectifier) and roughly sets amplifier gain.
Your Sept11 schematic has a very low input level of 38mV peak and with the gain of only 10 times, the output power is only
0.009W.
I used simple calculations and a bit of cheating for resistances and capacitances.

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Joined Jun 16, 2023
128
R1 increases the input impedance so it is not simply the base-emitter diode of Q1 (as a signal rectifier) and roughly sets amplifier gain.
Your Sept11 schematic has a very low input level of 38mV peak and with the gain of only 10 times, the output power is only
0.009W.
I used simple calculations and a bit of cheating for resistances and capacitances.
The ouput is 125mW and the THD is 6%? Wasn't this circuit a 1 watt amplifier?

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#### Audioguru again

Joined Oct 21, 2019
6,445
The output is 125mW and the THD is 6%?
No. The output of my amplifier on post #15 is undistorted 4V peak which produces an output power of 2W peak which is 1.0W continuously into the 8 ohms speaker. The distortion is fairly low for such a simple amplifier.

Years ago I built an amplifier for a TV. I used an LM390 amplifier IC that is similar to an LM386 but it uses bootstrapping for a higher output voltage swing. it produces 1.13W into 8 ohms at low distortion with a 10V supply.

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Joined Jun 16, 2023
128
No. The output of my amplifier on post #15 is undistorted 4V peak which produces an output power of 2W peak which is 1.0W continuously into the 8 ohms speaker. The distortion is fairly low for such a simple amplifier.

Years ago I built an amplifier for a TV. I used an LM390 amplifier IC that is similar to an LM386 but it uses bootstrapping for a higher output voltage swing. it produces 1.13W into 8 ohms at low distortion with a 10V supply.
Here is what I get. Don't know why mine is different.

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#### Audioguru again

Joined Oct 21, 2019
6,445
Your amplifier has a 0.5V peak input signal which causes the output to have bad clipping.
My amplifier in post #6 has an input of 0.38V peak then its output has no clipping.

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