Using reverse biased zeners to correct for mosfett turnon voltagein a push pull stage

[Joerg]

From all i've gathered only for Vt > 5V, below zener diodes have a NTC
as have FETs.

Doesn't matter too much. The zener diodes remain relatively cool while
the FETs will become hot and vary a lot in temperature.

In the old RF amp days when they used bipolar transistors they used to
thermally couple a diode to the heatsink so that it moves with the
transistor. With FETs you can't really do that.

 

[Adrian Nievergelt]

Interesting idea, stacking piezos like that. Not the simplest feedback
control problem in the world, I imagine.

My gizmo used an 80 MHz heterodyne interferometer as the readout, which
worked pretty well. (This was the first commercial force microscope,
back in 1988.)

Cheers

Phil Hobbs

As it turns out the feedback is simpler than the actual data
reconstruction. The simples controller layout is a cascaded PI loop
where your controller input from your lock-in or beam deflection signal
controls directly the fastest piezo on the stack for a
deflection/amplitude setpoint. Every further piezo takes the output of
the next faster PID and tries to control for zero voltage there, meaning
no deflection of the piezo or in other words keeping the small ranged
fast piezos centered around their range. Actually setting up the
controller in the lab is still another story

 

[Tim Williams]

The horizontal deflection transistors are vanishing, too. Beautiful
1400 volt things. Their c-b diodes made wonderful drift step-recovery
diodes, some nice doping coincidence. Apply 48 volts in the forward
direction, wait a while until you get 50 amps or so, then reverse them
hard. SNAP!

My experience (limited, since MOSFETs are just that much nicer!) is that, at
least without a huge amount of reverse bias, they tend to break down at
Vceo, which is a paltry 600V, versus the 1500V Vcbo. This is kind of
unusual as BJTs go, because I made that measurement at Vbe = 0 (inductive,
pulsed), a condition where the average BJT goes pretty close to Vcbo.
(2N3904 goes to 80V or so with a moderate B-E resistor, and also exhibits
avalanche behavior which can be of use.)

I'm surprised they snap well, I'd think the capacitance and lead inductance
would hamper things greatly.

That 1kV pulser you made, you ended up with a very particular selection of
part number, manufacturer and reel, of some common rectifier (1N4007 or some
such), right? 50A through a HOT, if it snaps well enough (given its
capacitance), should do about as well. Hey, if you can push it to 1500V in
a split second, that's even more into 50 ohms than the 1kV you had!

Tim

 

[legg]

Doesn't matter too much. The zener diodes remain relatively cool while
the FETs will become hot and vary a lot in temperature.

In the old RF amp days when they used bipolar transistors they used to
thermally couple a diode to the heatsink so that it moves with the
transistor. With FETs you can't really do that.

This is still common practice in audio with discrete bipolar pass
transistors, where the Vbe multiplier is a thermally coupled small
signal transistor. From the sixties.

No reason why the concept couldn't be used with mosfets. You'd have to
have the same part number for the signal as for power though, just to
ensure that the tempco's are similar - too much process variation
otherwise.

RL

 

[Joerg]

This is still common practice in audio with discrete bipolar pass
transistors, where the Vbe multiplier is a thermally coupled small
signal transistor. From the sixties.

No reason why the concept couldn't be used with mosfets. You'd have to
have the same part number for the signal as for power though, just to
ensure that the tempco's are similar - too much process variation
otherwise.

With FETs it isn't a BE junction where a distinct voltage drop develops,
you'd have to design a wee bit of electronics around that. It won't work
to well even then because your sense FET will not see tens of watts of
dissipation slamming into its die but the FET to be protected and
bias-regulated does.

 

[josephkk]

This time of year, Gin & Tonic is the afternoon cocktail of choice

...Jim Thompson

You might try a mojito instead. Very tasty.

?-)

 

[josephkk]

That one may be easier to frequency-compensate if you add a resistor
from U1's output to the FET drains. That adds some local feedback that
(I believe) bypasses the main frequency compensation of U1. Local
feedback will also push the output pole to higher frequency.

Cheers

Phil Hobbs

I would be rather careful with the idea as the final output swings quite a
bit farther than the opamp rails.

?-)

 

[Joerg]

That's why the thermal coupling is used.

The issue in a basic schematic, as drawn by the OP, is the unavoidable
process variation between P and N-chanel parts. This wasn't an issue
with bipolars, as the Vbe multiplication could be trimmed, and all had
the same tempco.

That'll be a lot more difficult than with BJTs. Another major impediment
will be those lot-to-lot variations. With FETs they are huge. With BJTs
its mainly the beta that varies a lot but that has little impact on the
quiescent current if it's all done right.

A Vgs multiplier also has an interesting impedance, when imbedded in a
signal train.

In this case I'd prefer clamping. Stop the microscope every so often,
current drops down to Iq, regulate Iq back to where it should be,
re-start the microscope.

 

[legg]

With FETs it isn't a BE junction where a distinct voltage drop develops,
you'd have to design a wee bit of electronics around that. It won't work
to well even then because your sense FET will not see tens of watts of
dissipation slamming into its die but the FET to be protected and
bias-regulated does.

That's why the thermal coupling is used.

The issue in a basic schematic, as drawn by the OP, is the unavoidable
process variation between P and N-chanel parts. This wasn't an issue
with bipolars, as the Vbe multiplication could be trimmed, and all had
the same tempco.

A Vgs multiplier also has an interesting impedance, when imbedded in a
signal train.

RL

RL

 

[josephkk]

It forms a voltage divider, which is what one needs to control a HV
output from a LV amplifier.

The general scheme works great.

Cheers

Phil Hobbs

I was discussing this particular circuit, not one of Adrians whose supply
is servoed.

?-)