anyone familiar with this topology?

[justin]

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 µA741, this was in the
early '80!

Anyone know where to find design info or such?

Thanks,
j.

 

[Tony Williams]

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 µA741,
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. General Electric 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.

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.

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.

 

[Kevin Aylward]

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. General Electric 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),

Oh dear...you have my sympathies...

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.

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. The transfer
function is not well controlled or modelled, in addition to poor hf
response. 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.

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.

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.

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...

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Tim Wescott]

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 ?A741, this was in the
early '80!

Anyone know where to find design info or such?

Thanks,
j.

I'm generally deeply suspicious about using parts outside of their
original intent -- what happens when the process (or temperature)
changes, or purchasing cleverly buys someone else's chips to save a few
pennies?

Certainly this may do for a clever one-off, but I'd do more than
hesitate before putting it into production.

 

[Pooh Bear]

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 µA741, this was in the
early '80!

Anyone know where to find design info or such?

Yup, seen that design concept.

Give it a wide berth. It's full of problems. Not even fitted with emitter
resistors for DC bias stability on the output devices !

Truly horrid. It belongs back in the early 80s too.

Graham

 

[Winfield Hill]

Kevin Aylward wrote...
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.

Right, Tony, it looked very bad.

Ahmmm...

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

 

[Kevin Aylward]

I'm generally deeply suspicious about using parts outside of their
original intent -- what happens when the process (or temperature)
changes, or purchasing cleverly buys someone else's chips to save a
few pennies?

Certainly this may do for a clever one-off, but I'd do more than
hesitate before putting it into production.

I wouldn't hesitate at all, to put it in the bucket...

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Genome]

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 µA741, this was in the
early '80!

Anyone know where to find design info or such?

Thanks,
j.

The proper way to do it is to give the op-amp a local gain of about 3 and
ask it to drive a low value resistor. Then you take feedback from the power
stage output to the op-amps output.

Add some emitter degeneration for the darlingtons.

Use a TL074. Implement a Vbe multiplier with two of the op-amps driving each
other and scale things through the bias chain. Use one as a DC servo and use
one as the main amp.

SEE ABSE

DNA

 

[Michael A. Covington]

I've seen this concept before -- an op-amp whose range is extended by
putting it in the middle of a swinging power supply. I think I've even seen
it in a TI book, but I'm not sure.

 

[Jim Thompson]

Yup, seen that design concept.

Give it a wide berth. It's full of problems. Not even fitted with emitter
resistors for DC bias stability on the output devices !

Truly horrid. It belongs back in the early 80s too.

Graham

Oh, Bull Puckey!

In the early '70s I built many a hybrid power amplifier that way,
using, of all things, a 741 as the core.

And a variant that allowed a low dropout regulator, with regulated
zener current.

In the '60s this was done at Motorola with a standard NPN chip
(P-substrate) and a PNP chip (N-substrate) so that both good NPNs and
PNPs were attained.

...Jim Thompson

 

[Roy McCammon]

I'm generally deeply suspicious about using parts outside of their
original intent -- what happens when the process (or temperature)
changes, or purchasing cleverly buys someone else's chips to save a few
pennies?

Certainly this may do for a clever one-off, but I'd do more than
hesitate before putting it into production.

I inherited the sustaining on such a design. Its been rock solid
and free from problems for years. I have to admit that although I
understand it in the dc-low freq sense, I don't understand it's
stability criteria sufficiently well to be comfortable.

 

[John Larkin]

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 µA741, this was in the
early '80!

Anyone know where to find design info or such?

Thanks,
j.

I do this as a transimpedance stage in all my NMR gradient amps. These
are inherently current-output amps, so the opamp rail currents
(cascoded, as shown here) then drive output current mirrors, more
precise versions of the PNPs and NPNs used here. I use power mosfets
in closed loops as the mirrors. The biggest one I do this for is +-180
volts, 100 amps (18 kilowatts) peak out.

I don't know who did this first, but it's a very old trick. It's in
National appnote AN-31 by Robert Dobkin, February 1978.

John

 

[Fred Bloggs]

I do this as a transimpedance stage in all my NMR gradient amps. These
are inherently current-output amps,...

The type of output is what is regulated by feedback-and that looks like
voltage to me. That 4n7 at the output looks more like a wide band
negative feedback loop around the OA output stage to cascode to
Darlington at high frequencies.

 

[John Larkin]

Yup, seen that design concept.

Give it a wide berth. It's full of problems. Not even fitted with emitter
resistors for DC bias stability on the output devices !

Truly horrid. It belongs back in the early 80s too.

Graham

I sell a million dollars or so a year of amps that use this as a key
stage. It works beautifully to solve an otherwise very nasty
current-splitting drive problem.

John

 

[Ken Smith]

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 µA741, this was in the
early '80!

Anyone know where to find design info or such?

Some op-amps have a nearly constant no-load supply current that is fairly
low. If the op-amp's supply current is not well controlled or is at all
large, this circuit will be hopeless.

This sort of circuit only works where you don't care about the distortion.
The output section is running class C. If you try for AB operation the
slop in the supply current spec will make your design unreproducable.

 

[John Larkin]

The type of output is what is regulated by feedback-and that looks like
voltage to me. That 4n7 at the output looks more like a wide band
negative feedback loop around the OA output stage to cascode to
Darlington at high frequencies.

Right; I don't do that. I just use the opamp as a unity-gain follower,
with an extra resistor from output to ground. This output resistor
defines the transimpedance. So the input signal voltage becomes opamp
supply rail currents, with essentially perfect crossover behavior.

I'm driving inductive loads, and I want my output impedance to be as
high as possible, and this topology does that nicely. But for audio or
motors or whatever, I'd go for a topology with low open-loop
impedance, so loop dynamics wouldn't be too load dependent. But then I
don't design audio!

John

 

[Jim Thompson]

Some op-amps have a nearly constant no-load supply current that is fairly
low. If the op-amp's supply current is not well controlled or is at all
large, this circuit will be hopeless.

This sort of circuit only works where you don't care about the distortion.
The output section is running class C. If you try for AB operation the
slop in the supply current spec will make your design unreproducable.

This thread sets the record for so many wrong statements about a
circuit configuration.

It's class-B, not class-C, but class-AB would indeed be difficult
(without some extra servoing).

The open-loop dead-band can be made reasonably small, and distortions
way under 1% are easy to attain... based on hands-on experience with
this configuration ;-)

Except for the OpAmp Q-current, there is really no difference between
this approach and the Sziklai connection (AofE, p95) commonly used in
power amplifiers.

...Jim Thompson

 

[John Larkin]

This thread sets the record for so many wrong statements about a
circuit configuration.

It's class-B, not class-C, but class-AB would indeed be difficult
(without some extra servoing).

The open-loop dead-band can be made reasonably small, and distortions
way under 1% are easy to attain... based on hands-on experience with
this configuration ;-)

I run this, I suppose, AB, in that I run a modest quiescent current, a
few percent of peak, in the output fets. The opamp supply currents
drive mosfet-based current mirrors. As the signal goes positive from
zero, the P-side fets increase their current from the idle value, and
if the sig goes negative the N-side fets increase. There's really no
crossover distortion, basicly perfectly sharp corners around the
zero-signal point. That's a little hard to think about, but it works.

My amps settle to under 10 PPM in 50 usec (limited partly by current
shunt eddy effects) and have noise and distortion of a couple of PPM.

This circuit paid for my house and is putting The Brat through
Cornell, so I have to defend it.

Except for the OpAmp Q-current, there is really no difference between
this approach and the Sziklai connection (AofE, p95) commonly used in
power amplifiers.

If I were given unlimited funding to design the perfect current
splitter custom linear IC for my gradient amps, I'd probably wind up
with the opamp I'm using.

John

 

[Ken Smith]

This thread sets the record for so many wrong statements about a
circuit configuration.

It's class-B, not class-C, but class-AB would indeed be difficult
(without some extra servoing).

I still maintain that "the output section is running class C" by which I
mean
that the op-amp's output current needs to be greater than about

1.4V/680 - 1.4mA = 0.66mA

in either direction to turn on one of the output transistors. This means
that there will be a dead band while the internal nodes of the op-map
slew. If the dead band was small enough to be called zero, I'd agree that
it is a class B output.

The open-loop dead-band can be made reasonably small, and distortions
way under 1% are easy to attain... based on hands-on experience with
this configuration ;-)

I have used basically the same configuration and yes the distortion can be
small well below the frequency limit of the circuit. As you go up in
frequency, the dead band time does not decrease as fast as the period does
so the distortion gets much worse.

 

[Winfield Hill]

John Larkin wrote...

I sell a million dollars or so a year of amps that use this as
a key stage. It works beautifully to solve an otherwise very
nasty current-splitting drive problem.

Hear, hear. I haven't seen your circuit, but I attest to the
viability of the architecture; very powerful if properly used.