Simplest class B Amplifier

[Miles Prower]

Lol, the PNP transistor in the push-pull output stage on that CW radio amplifier is in backwards.

Yeah, I have to redraw that someday. Anyway, that is not how I built it. It's two complementaries.

By changing that amplifier to work with negative global feedback, it would be not bad.

Wouldn't do much good for what it's used for. The total gain of the entire audio strip is 120db(v). Since this is a TRF design, that's where all the gain comes from. So to add global feedback, and maintain the gain, the open loop gain would have to be even greater, and for a useage where fidelity isn't necessary. Almost all longwave work is CW, and so all it has to do is go: "beep, beep". The DBM also will cause the same sort of distortion on AM that you'd get with SSB. So I designed for stability, which I've got (exception: open input, then you get a 2.5KHz oscillation) and as little tendency for microphonics as possible. It does what it was meant to do.

Edit: Here's a corrected schematic.

[mod_evil]

Hi friends, he necessite a simple amplifier because in rio de janeiro, we arent found a diverse components.

Portuguese version;
Olá amigos, ele necessita de um simples amplificador, porque aqui no rio de janeiro, nós não achamos diversos componentes"

[amplifierguru]

Hi Andrew T,

I don't follow you're comments. Rest assured everything is in the right place and it will deliver serious power at low THD. the FETS are driven from the low output Z complementary EF and the the predrivers all in a local capacitative local loop to rolloff gain to unity at VHF. The main (global) FBloop is 22K//39p=5K1 to the 1K
at the chip input. All BJT's are BC546/556. DC coupled.

[Duo]

Miles Prower: I understand what you're saying. I just mentioned global feedback as in the last amplifier stage there because it would improve the performance for audio listening.

I am an amateur radio operator and I know you don't need/want good fidelity for listening on CW.

[Miles Prower]

OK, Duo, I see what you're saying. I thought you meant global feedback around the entire audio strip. As for the PA and driver, usually it would be best to take the feedback for the op-amp from the output. I simply didn't do it. Regardless, you're not going to get outstanding performance with simple, 741-type op-amps anyway.

For whatever the original poster wanted to do, I would endorse that mod. Still, he wanted "simple", and I gave that to him.

[Duo]

lol, with a 741 you'll need all the help you can get

[AndrewT]

Hi Amplifierguru,
If the output is EF then the fets are transposed.
If its complementary follower pair then the gate should be on the collector of the driver.

[amplifierguru]

Hi Andrew T,


It is indeed CFP with gain rolling to unity at VHF and an interposed complementary EF driver! Elegant simplicity.

[AndrewT]

Hi amplifierguru,
CFP ok.
Why is the gate on the emitter of the driver?
The driver has a Vce of about 56v-V(100r/2) or about 50v
The FETs will have this 50v across their gate/source connections.

[amplifierguru]

Hi Andrew T,

It is early morning in Scotland? Splash some water and look again. It is CFP (with gain) with interposed complementary EF driver (pair BC546/56 as dissipation typ 50mW). The top MOSFET is Pch , bottom Nch.


Cool. Yes it is. many +++ es. Can you see it?