Re: anyone familiar with this topology?

From: Kevin Aylward (salesEXTRACT_at_anasoft.co.uk)
Date: 09/12/04 

Date: Sun, 12 Sep 2004 08:25:03 GMT

Winfield Hill wrote:
> Kevin Aylward wrote...
>>
>> Tony Williams wrote:
>>> justin <justin.c@se.net> wrote:
>>>
>>>> http://www.armory.com/~rstevew/Public/Ccts/4oma120w.gif
>>>>
>>>> I'm sure I have seen this concept before, I think in one of
>>>> those TI or NS data books but built around 741, yes the ľA741,
>>>> this was in the early '80!
>>>
>>> I originally saw it as a reader's idea in Electronics, about
>>> the late 70's to early 80's. "Getting Power and Gain out of
>>> the 741-type op amp." By Pedro P. Garza, Jr. GE Co, Apollo
>>> and Ground Systems, Houston, Texas.
>>>
>>> I later pinched the idea and pushed it to a 1250VA amplifier
>>> (8+8 Mosfets in the o/p stage, fan cooled), used in a test
>>> jig to stimulate wound components at full power.
>>>
>>> Winfield Hill also tried out the idea last year, with a high
>>> voltage power amplifier. I don't know what the final outcome was.
>
> It worked out very well, Tony. Remember the schematic I sent you?
>
>> Personally, I think this type of method is a truly dreadful way to
>> design an amp. There are so many issues with it. The output devices
>> bias is completely undefined, unless it set to essentially zero.
>
> With the crummy circuit in the gif, perhaps, but not with a sensible
> circuit. In fact it's remarkably easy to define the class A current.
> And control the crossovers, because one is using current-source mode
> to drive the output, with drive circuitry pegged to the rails, rather
> than an awkard floating bias circuit.
>
>> The transfer function is not well controlled or modelled, in addition
>> to poor hf response.
>
> It has a well behaved transfer function,

Oh? care to present the *full* effect of all the opamp poles?

>although the load figures in.
> Which in the end may limit bandwidth, but allows good output linearity
> and is especially well suited for current drive amplifiers.
>
> In the case of Mark Alexander's current-feedback design (Analog
> Devices AN-211), a much wider bandwidth was achieved than with common
> circuits.

This has little to do with using the op-amp. In fact, its in spite of
the op-amp. As I already explained in
http://www.anasoft.co.uk/EE/currentfeedbackmyth/currentfeedbackmyth.html,
the fundamental reason fo the speed of a current mode amp is due to the
class ab input stage.

The basic issue is that an op-amp has *lots* of poles, as it has many
stages. Minimising the number of stages is usually the best thing to do.
A discrete design might easily have, say 4 less poles. So, in *general*
I don't recommend using an op-amp in a discrete power amp. You can
usually make a much better optimised design because an op-amp is
designed with general compromises to make it a general purpose part. In
a discreet design, you can ignore many opamp characteristics, and
optimise the circuit better. For example, when you do a full custom
analogue design, you essentially never use a redesigned available
catalogue part internally. There are simply not optimum for more
specific requirements.

>
>> Its really grungy, like finger nails down a blackboard sort of thing.
>> Those that use this type of circuit should be shot on sight, and have
>> their entrails thrown to the vultures.
>
> Why don't you tell us how you really feel?
>
>> In general, putting an op amp in the feedback loop is bad news. It
>> justs gets you extra poles that you could avoid with a discrete
>> design.
>
> Actually, not here,

Oh?

> because with the opamp acts as a voltage-current
> converter, via the opamp power-rail cascode transistors.

Of course, to first order.

> The current
> transfer function is determined by resistor ratios: the high-voltage
> rail resistor, the opamp's output load resistor (and capacitor!), and
> the Darlington output-transistor's emitter-degeneration resistor.

Now do a full, detailed analysis than includes the op-amp exactly.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
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