Power Amplifier for 100kHz.

Clive Arthur · 2023-12-06T15:26:00+00:00

I'm not an analog design expert, but needs must.I recently adapted a Doug Self audio amplifier design for use on a 60V single supply, and to go up to 100kHz with a working ambient temperature of -20'C to 180'C.I can't show the circuit, but it was based on Fig 1a here... needs to come down for this higher frequency application, but too far and you have oscillation. Also, problems occur with slew rate limiting due to Cdom, the TR5 constant current, and the increase in hFE of TR4 with temperature.So I added an emitter degeneration resistor to TR4 to tame the hFE variation, removed Cdom and put a smaller C across Rf1 instead. Seems stable and not slew rate limiting. Took a lot longer to do than that sentence might imply.However.The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably get hotter and their Vbe would drop by more than the diodes, so it's class B. As it runs at a high temperature, I obviously want to reduce dissipation, and if I'd used say a rubber diode to get some quiescent current, I think it would be very difficult to control Iq well enough over the temperature range.So I have a circuit which works well enough, but could be better with regard to crossover distortion (though it's lower than I would have thought). In this application, the better the signal quality, the higher the data rate.Any ideas for improving crossover distortion, bearing in mind the temperature range? The signal is OFDM, so pretty much a load of 'random' steps, some of which may be small and at a crossover point.

Dec 06, 2023 Last reply: 2 years ago 48 Replies

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legg 2 years ago

Instead of just bitching about the sims, I replaced both the TL071 subckt and the LM311 subckt with a default OA and an LT1011 from the standard LTspice library so that the thing runs. ( Be sure to remove or terminate the old subcircuits to ensure easy sim startup.)

This ran with low crossover distortion at 100KHz and about 50mA bias current at xover. The bias current could be raised or lowered proportionally through comparator input adjustment.

No big OA slewing requirements.

So the concept could be demonstrated ~accurately in LTSpice.

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piglet 2 years ago

Thanks, yes I was intrigued too. So this thread doesn't grow unwieldy I am starting a new post on autobias class AB.

piglet

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Phil Hobbs 2 years ago

Yeah, I did it too, using 2N3904s and one 2N3906 for the small signal stuff, D44H11/D45H11 for the output stages of the Darlingtons, a UniversalOpAmp2 configured to look like an LF356 (4 MHz, 12 V/us), an RH111 (rad hard comparator from the LTspice 17.1 library) and a 4-ohm load.

Looks pretty good, for a car amp of that vintage. The bias circuit works well, anyway.

Cheers

Phil Hobbs

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legg 2 years ago

<snip>

JT's original zips and the generic version:

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Phil Hobbs 2 years ago

Nice arm waving there, 'Fred'. How exactly do you propose to do that?

Also it's the output current that makes the speaker move.

Gee, thanks. ;)

Not in 1977, it wasn't. The first public release of the original Berkeley SPICE2 program was in 1972, and it was pretty primitive--FORTRAN, punch cards, running on a CDC 6400. SPICE didn't actually become useful until the early '80s.

Cheers

Phil Hobbs

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Phil Hobbs 2 years ago

Sure it can, but it would take more work than you suggest.

Laurence Nagel's thesis on SPICE2 was a nice piece of work actually.

I seriously doubt that. 1977 was a _long_ time ago--even the semis were still prototyping with kit parts and doing mask design with Rubylith.

Cheers

Phil Hobbs

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Phil Hobbs 2 years ago

I went do download this, but it wants a login.

Cheers

Phil Hobbs

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JM 2 years ago

I've emailed it to you. Links posted here will only be active for a few days. If anyone else needs it try :-

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I did actually build a variation of this when it was originally posted.

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Phil Hobbs 2 years ago

Thanks!

Cheers

Phil Hobbs

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