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High voltage push-pull output stage

S

Stefan Heinzmann

Jan 1, 1970
0
Winfield said:
John Larkin wrote...



Most of my designs have a single conveniently-placed resistor
that eliminates this reasonable but apparently-bogus fear.

You're referring to the technique used in current dumping amplifiers, I
assume. The resistor would go from the OpAmp output to the Anode of D1,
at least conceptually (some level shifting might be needed).

Cheers
Stefan
 
W

Winfield Hill

Jan 1, 1970
0
Stefan Heinzmann wrote...
You're referring to the technique used in current dumping amplifiers,
I assume. The resistor would go from the OpAmp output to the Anode of
D1, at least conceptually (some level shifting might be needed).

.. basic high-voltage MOSFET dc amplifier
.. by Winfield Hill
..
.. + supply rail -----------------+-------,
.. | |
.. R11 |
.. | D Q4
.. +--+--G
.. C2 | | S
.. ,--R4--||-, | C |
.. IN | | | B--+ Q5
.. o--R3--+--|+\ | | E |
.. | >---+--R5--, X | R10
.. ,--|-/ | | | | out
.. | E | '----+-+--R12--o
.. | gnd -+- B Q1 | D1 |
.. | | C +--|<|----+
.. | | | +---R13---+
.. | | | | |
.. +---------- | -- | ------- | -+--R2--+
.. | | | | | |
.. R1 R13 | C3 | '--||--+
.. | | E ,-||--+ C1 |
.. gnd +- B | | |
.. | Q2 C R8 | R9
.. | | | D Q3 |
.. R14 +---++--G C4
.. | | | S |
.. | R6 C | gnd
.. | | B--+
.. | | E | Add a zener across
.. | | | R7 each FET gate!
.. | | | |
.. - supply rail --+----+----+----'

No, it's R13 above, in parallel with D1. To choose its value,
consider the minimum R11 current, when the output is near the
+ rail. Let's say the nominal R11 current is 1mA (or 1.6 watts
max with +/-400V supplies). At +350V out this drops to 125uA,
so we can allow R13 to draw say 25uA at its maximum operating
voltage, which is Vgs(linear) for Q4, or about 3V. This gives
us 120k for R13, a high value that insures modest effect, just
a little current pathway when both Q4 and D1 are off. Note,
this is partially a feel-good part, since the amplifier seems
to work pretty well without it, probably because the feedback
resistor is usually drawing or sourcing some current. Of course
any such current disappears near zero volts out.

Thanks,
- Win

whill_at_picovolt-dot-com
 
J

John Larkin

Jan 1, 1970
0
No, it's R13 above, in parallel with D1.


A similar case is an opamp followed by complementary emitter
followers. In that case, the (??? - we need a name for it) resistor
can be a pretty low value, the transistors need no biasing stuff at
all, and the crossover distortion really does go down. People go to a
lot of trouble to bias complementary output stages, and to avoid
thermal runaway, when this is so simple.

The PNP-boosted 3-terminal regulator works sort of the same way... a
power stage that only kicks in when times are tough.

John
 
S

Stefan Heinzmann

Jan 1, 1970
0
John said:
A similar case is an opamp followed by complementary emitter
followers. In that case, the (??? - we need a name for it) resistor
can be a pretty low value, the transistors need no biasing stuff at
all, and the crossover distortion really does go down. People go to a
lot of trouble to bias complementary output stages, and to avoid
thermal runaway, when this is so simple.

That is exactly what I meant. I associated it with the current dumping
amplifier by P.J. Walker, but maybe it has been used before. I haven't a
clue whether there's a name for the resistor. I think of it as a
small-signal feedforward resistor. It provides the output signal when it
is so low that the complementary "current dumpers" are both off.

A low value for this resistor reduces the demands on the slew rate of
the OpAmp, so in effect you compromise between OpAmp slew rate and drive
current.

Cheers
Stefan
 
J

John Larkin

Jan 1, 1970
0
That is exactly what I meant. I associated it with the current dumping
amplifier by P.J. Walker, but maybe it has been used before.

I invented it independently in very roughly 1970, but I'm sure
somebody must have thought of it before that; it's pretty obvious.

I *did* invent the dual-slope integrating ADC when I was in high
school, but Fairchild patented it and made all the money.

John
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <791huvg7vt8n4juipvngd8h06o2untte4c@
4ax.com>) about 'High voltage push-pull output stage', on Tue, 23 Dec
2003:
A similar case is an opamp followed by complementary emitter
followers. In that case, the (??? - we need a name for it) resistor
can be a pretty low value, the transistors need no biasing stuff at
all, and the crossover distortion really does go down.

Where does this magic resistor go? Between the op-amp output and the
emitters of the followers?
 
S

Stefan Heinzmann

Jan 1, 1970
0
John said:
I invented it independently in very roughly 1970, but I'm sure
somebody must have thought of it before that; it's pretty obvious.

That's about the time when Walker worked on it, too.
I *did* invent the dual-slope integrating ADC when I was in high
school, but Fairchild patented it and made all the money.

So did you disclose your invention to them or did they come up with it
independently?

Cheers
Stefan
 
J

John Larkin

Jan 1, 1970
0
I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <791huvg7vt8n4juipvngd8h06o2untte4c@
4ax.com>) about 'High voltage push-pull output stage', on Tue, 23 Dec
2003:

Where does this magic resistor go? Between the op-amp output and the
emitters of the followers?

Yup. It adds a nonzero gain slope in the crossover region when the
power transistors are off.

v+
|
c
+----------b
| e
| |
in --------------+-----r-----+---------out
| |
| e
+----------b
c
|
v-


No Vbe multipliers, no temp comp, no extra current sources!

Of course, you have to have no religious prejudices against overall
negative feedback.

John
 
W

Winfield Hill

Jan 1, 1970
0
John Larkin wrote...
Fairchild has an optocoupler with a 400 volt output transistor, which
makes a cool 400 v p-p amp using just a pair of optos as the output
totem pole. CTR is a little low, but usable. Too bad nobody makes a
kilovolt photofet!

There are several 600V parts available. E.g. from Aromat NAiS.
All the optomos SSRs I've seen have Schmitt action, darn it.

I'm not so sure about the Schmitt aspect, but the designers do
have a difficult problem in removing the charge from the large
FET gate capacitance at turnoff, which can be solved by various
techniques, including an SCR-like pulldown structure. It's not
a scene that lends itself well to analog linear control. FETs,
even small ones, simply have too much capacitance. :>(

Thanks,
- Win

whill_at_picovolt-dot-com
 
W

Winfield Hill

Jan 1, 1970
0
Winfield Hill wrote...
. basic high-voltage MOSFET dc amplifier
. by Winfield Hill
.
. + supply rail -----------------+-------,
. | |
. R11 |
. | D Q4
. +--+--G
. C2 | | S
. ,--R4--||-, | C |
. IN | | | B--+ Q5
. o--R3--+--|+\ | | E |
. | >---+--R5--, X | R10
. ,--|-/ | | | | out
. | E | '----+-+--R12--o
. | gnd -+- B Q1 | D1 |
. | | C +--|<|----+
. | | | | |
. +---------- | -- | ------- | -+--R2--+
. | | | | | |
. R1 R13 | C3 | '--||--+
. | | E ,-||--+ C1 |
. gnd +- B | | |
. | Q2 C R8 | R9
. | | | D Q3 |
. R14 +---++--G C4
. | | | S |
. | R6 C | gnd
. | | B--+
. | | E | Add a zener across
. | | | R7 each FET's gate.
. | | | |
. - supply rail --+----+----+----'

... some kind of magical network is needed at point X to isolate
the desired fast crossover from the Q5 FET's pullup capacitance.

Several possibilities are possible for element X. For example,
I have used a common-emitter BJT stage to good effect (weird),
and I've even seen circuits emplying LM317L regulators to control
the Q4 pullup FET (even more weird).

The most simple approach is a small fixed voltage drop to reduce
the R11_Q4-gate slew distance required to go from D1 conducting
to Q4 conducting and back again. One easy way to get this drop
is a low-voltage zener, but there are problems. As Q4 heats up,
its Vgs drops (for fixed Id), so that your carefully-arraigned
biased-off class-B condition can become an ugly power-consuming
class-A condition. For example, an irfBG20 1kV mosFET conducts
about 10mA with Vgs = 4V, at 25C, but increases dramatically to
about 100mA at 150C. This means we'd want to choose the zener
voltage carefully, even make it adjustable, which means we don't
want to use a real zener! Aha, a classic Vbe multiplier is well
suited for the task, especially if the transistor Q6 can roughly
track the FET temperature. Too bad one can't purchase FETs with
embedded temperature-sensing transistors!

.. + supply rail
.. -----+--------,
.. | |
.. R11 |
.. | D Q4
.. +---+--G
.. ,----+ | S
.. | | C |
.. R16 | B--+ Q5
.. | C E |
.. Q6 +--B | R10
.. | E | | out
.. R15 | '----+-+--R12--o
.. | | |
.. '----+ D1 |
.. +---|<|----+
.. |
.. |

Thanks,
- Win

whill_at_picovolt-dot-com
 
J

John Larkin

Jan 1, 1970
0
John Larkin wrote...

There are several 600V parts available. E.g. from Aromat NAiS.


I'm not so sure about the Schmitt aspect, but the designers do
have a difficult problem in removing the charge from the large
FET gate capacitance at turnoff, which can be solved by various
techniques, including an SCR-like pulldown structure. It's not
a scene that lends itself well to analog linear control. FETs,
even small ones, simply have too much capacitance. :>(

Thanks,
- Win

whill_at_picovolt-dot-com

Hi, Win

Do you have a part number or a link to a 600 volt coupler? All I can
find on the (typically terrible) Aromat web site is SSRs (and Massage
Loungers and Personal Hygiene Systems.)

http://www.aromat.com/p_aromat_products.htm


John
 
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