Simplest class B Amplifier

[Mr Evil]

Originally posted by PicancoNet
I made this circuit. Is it realy can give 50W at 2V@4ohms in input ?

I expect that would work ok.

Originally posted by Upupa Epops
Why are you all still fascinated by some simply ***** ? Wake up guys, now is the time, when useable transistors cost only 5 cents ! Simply Zen, simply GC, simply ... Beauty is in complicated connections, only with them you can make right amp.

Simple is beautiful too Ok, so most of these amps aren't going to give the greatest quality, but they are useful. Like I said, I find the one I posted is useful when I need a quick test amp. Simple amps are also the best place to start learning.

[Dozuki]

Simple CAN also sound good.

see:

OPMOSFET Amp

Not much more complicated, very adaptable (I've used it with IRF devices). I also made a version using 2 green LEDs in place of the Vbe multiplier. No Global Feedback, only local. I would add Gate stopper resistors tho. Especially with lateral Mosfets. If the Output seems to be too hot for what its doing, then its oscillating and the gate stoppers will stop this. SK1058/SJ162 seem to need higer values for the stoppers. 220 should be fine for IRF Devices and 470 for SK1529/SJ200 seems OK. The Cap between the Opamp and the Output is probably not necessary, just a little protection.

-D.

[Dozuki]

Here's a bare bones Basic AB circuit. R2 sets the bias for the amp and should be set so that 1/2 the supply voltage (~12V) is seen between the + side of C2 and Ground.

No Global Feedback and 2V should send it to clipping. Will post Distortion Spectrum Later.
1V in will swing about 13Vpp. I figgure that is about 20W into 8Ohms with a 1K source impeadance.

C1's polarity is reversed. sorry.

[Dozuki]

I did some real measurements on the above amp:

1000 Hz sine input at .66v (1v out of source unloaded)
Unloaded output of amp : 12.7V
Output with 8 Ohm speaker: 10.5V
13 Watts output

Distortion Spectrum in db with 8 Ohm speaker load:

[AndrewT]

Hi,
your calculator needs new batteries.
13Vpp is 2.6W into 8R
10.5Vpk is 6.9W into 8R.
Is that a typo 2sj163?
can this amp really swing to within 1.5v of the rails with that drive circuit?

[amplifierguru]

Good morning Andrew T,

You have figured out my offering yet? It's audio cryptography - the seed.

[AndrewT]

Hi Amplifier Guru,
I planted some currents into the circuit & calculated the standing voltages. It seems to indicate that the right level of voltage appears across the CFP 2nd stage.
But all the CFPs I've seen before had a collector resistor on first stage & used the volts drop across the R to generate the driving voltage for the second stage.
Is yours connected to the emitter because of the voltage amplification? or is that a separate issue?
I still think previous circuits with voltage amplification had a local gain loop ireturning to the emitter but your's does not.

[PicancoNet]

Originally posted by Dozuki
I did some real measurements on the above amp:

1000 Hz sine input at .66v (1v out of source unloaded)
Unloaded output of amp : 12.7V
Output with 8 Ohm speaker: 10.5V
13 Watts output

Distortion Spectrum in db with 8 Ohm speaker load:

Hello,

What program did u use to made this measurements ?

[richie00boy]

Quick guess on Amplifierguru's circuit. Is the voltage gain achieved by the first transistor stage, then the second transistor stage is to buffer the the gate drive requirements. The MOSFETs are connected apparantely upside down to instigate an inverting action so that the overall feedback is negative, as the first transistor stage causes an inversion. Also to facilitate rail-to-rail swing.

[Dozuki]

your calculator needs new batteries.
13Vpp is 2.6W into 8R
10.5Vpk is 6.9W into 8R.
Is that a typo 2sj163?
can this amp really swing to within 1.5v of the rails with that drive circuit?

2.6W and 6.9W RMS, yes (although 13 Vpp would be 10.6W RMS). I guess I should have clearified that I was measuring Peak Voltages and Peak Power, not RMS.

I=E/R and W=I*E in which case

10.5e/8r=1.3125i, 1.3125i*10.5e=13.78 Watts Peak to Peak

Yes, that's a typo, should be 2sj162

And since I was measuring Peak to Peak, the amp stays within 5 volts of the positive rail and 5 volts of ground with about .66v input.

What program did u use to made this measurements ?

An Oscilloscope that shows voltages
and Signal Scope for the distortion specs.
With the circuit built on a breadboard.

Edited Schematic, and input cap is correct: