Re: anyone familiar with this topology?

From: Winfield Hill (Winfield_member_at_newsguy.com)
Date: 09/11/04 

Date: 11 Sep 2004 10:07:24 -0700

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, 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.

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

>> Interestingly, neither of us felt comfortable with that capacitor
>> from output back to the opamp output pin. It gives wide bandwidth
>> but an uncomfortable feeling of an oscillator, just waiting for an
>> excuse.

 Right, Tony, it looked very bad.

>> I used a capacitor from the opamp output pin to 0v, which Win
>> later analysed and remarked that it formed a nice and tidy
>> Zobel Network with the opamp output Z.
>
> Ahmmm...

 Right, a beautiful addition suggested by Tony, nicely enhancing the
 transfer function. 

-- 
 Thanks,
    - Win
 (email: use hill_at_rowland-dotties-org for now)

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