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

a

That will make the output impedance go down, making it a voltage source instead of a current source. That's OK I guess. Loop dynamics will change some. I sort of like the current source into piezo integrator thing.

I am assuming that there is some sort of front-end circuit off to the left, like a DAC and an error amplifier, with some pole-zero stuff of its own. In an AFM or STM, there's feedback from a photosensor or the tunneling current or whatever, closing the overall loop.

I've used this sort of architecture several times when I wanted a super accurate true current source.

The other piezo driver that I like is totem-pole high voltage optoisolators, but that's too slow for this app. Unless....

Incidentally, you can buy AFM tip/flexures fairly cheap. They might be fun to do something with, like make an electrostatic laser deflector or something.

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John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
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Reply to
John Larkin
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[...]

This doesn't have to be an opto-isolator. You can also pump RF in the GHz range into a tiny RF transformer, rectify on the other side and use that to control the FET. Modulating the RF on the primary side is easy, the signal into the modulator is your control signal. On the rectifier side things must be fast and loaded so there will not be a slow decay.

The capacitance between the windings of such a transformer can be in the low single-digit picofarads.

[...] 
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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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Piezos tend to have high-Q resonances, though, and if the OP needs speed, that generally means using a notch filter to keep the whole thing from oscillating. Lower output impedance makes filters a lot easier to build.

Back in the palmy days, when I built the IBM SXM scanned probe microscope workstation, I was using piezo bimorphs to excite cantilever vibrations in the tip as well as doing the Z axis servo. That bimorph had resonance at 30 kHz with a Q of about 30, meaning that with a 1-pole rolloff, the loop BW had to be 1 kHz or less. Using a simple LC notch got me a 10 kHz loop bandwidth, which was a big win.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net
Reply to
Phil Hobbs

If your final stage amplifier used PNP/NPN complementary transistors, a few heatsink-mounted diodes would perform this function. Diode thermal characteristics match the offsets in that case. If you want something adjustable for MOSFETs, consider:

------------ (V+)---|PNP mirror| --+------+-- | | (Radj) +------->gate1 | | | R | | | >--+ | | | R | +------->gate2 --+------+--- (V-)---| NPN mirror| -------------

This will allow you to make fine adjustment. Zeners are only available in kinda large voltages, with no good way to match the offset to an installed pair of MOSFETs. Capacitor bypassing of the resistors (R) might be useful. I showed the input at the R center tap, that's to balance R-Cgs delays...

As others have mentioned, there's usually emitter resistors (in the MOSFET case, source resistors) in this kind of bias scheme, to stop thermal or other runaway current problems.

Reply to
whit3rd

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If you try to get high reaction speed out of a high-Q transducer that usually ends with a shattered transducer and a puff of smoke wafting away from the amplifier :-)

How do they get fast movements if there isn't any stiff backing material? The PZT transducers I dealt with (except for CW Doppler) all had 6dB bandwidths between 30% and 60%. The only way to get there is to have a lot of tungstens and stuff in the backing.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Weell... make that two or three units at first. The circuit as proposed by John Larkin seems to me to be the currently best option i have. I'll be trying to combine that with the rest of the amp (voltage swing) and a fast overall feedback and see if I can get a better amp. The power handling capability of mosfets is after all quite stunning.

Reply to
Adrian Nievergelt

I am well aware of these problems as I already have a simpler version of this physically running. Slew limiting i do simply with an RC lowpass at the entrance. The startup i do using a relays to ground the input initially and after the rails have been established (window comp) together with a time delay (zener and RC) i turn on both rails simultaneously. This already works reproducably.

Reply to
Adrian Nievergelt

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Ah yes this is where the whole thing becomes increasingly interesting. If you use dither piezos that are near MHz range with their resonances you don't need much any drive circuitry, you need neither linearity nor much power, as cantilever in air have a resonance Q of easy 300 or more. I use cascaded PID loop control for the z feedback. The samples are essentially mounted on a stack of piezos with increasingly higher resonance frequencies and in turn decresing maximal deflections. This scheme allows to boost z-feeback to well over 500kHz if done right, but it takes a long time to set up and control for. Also total z deflection measurements become a project of their own as you'd have to sum up all single hysteresis-ridden deflections. The current goal is using optical sensors instead.

Reply to
Adrian Nievergelt

But, don't get misled with those high amp ratings.. Heat is a killer of MOSFETS.

Jamie

Reply to
Jamie

That's just the beauty of it all. Using piezos as fast nanopositioning devices requires crazy currents of multiple amps at peak, but since you don't have steadystate oscillations the actual power is limited. Granted I will have to care for heat dissipation, but with the current amp scheme i can scan around 100kHz feedback without the DPAK cased hexfets going much above 30 maybe 35C even with deadbugging and without a heatsink.

Reply to
Adrian Nievergelt

Yeah ... but ... if it's swinging back and forth all the time you have to make a proper time allowance in the SOA and then the power handling capabilities are quite a bit reduced. Don't push it :-)

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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! 
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John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Reply to
John Larkin

SOA, too. Probably not a problem driving a piezo.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Reply to
John Larkin

load

which

parasitic

level

be a

an

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These ones were bimorphs, i.e. they bend when you put a voltage on them.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net
Reply to
Phil Hobbs

In theory that looks good. But.. Watch out for FET bias variations over temperature and the fact that zener voltage variations over temperature go the other way..

Reply to
Robert Baer

Check; my horse would rather have the corn than the booze...

Reply to
Robert Baer

I don't understand the circuit well enough yet, but isn't it a problem attaching +-60V supplies this way with an opamp that can only take 30V? The suspended rail configuration i drew earlier hardwires the rails to a fraction of the supply range above and below the opamp. That one i've already tested on board and it works really quite nicely. I might draw the board flexible enough to try both schemes. If i understand your first suggestion correctly, setting the dividers at the input correctly will already keep each transistor always just slightly conducting, eliminating any crossover distortion as well.

Reply to
Adrian Nievergelt

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

Reply to
Adrian Nievergelt

The DC-DC converter is a isolated type and the common is being attached to HV side output. This way the op-amp supplies will only see +/- 15 volts because they are basically riding ontop of the HV supply.. the output will look like a common/ground to the op-amp no matter the level.

This poses a problem however, the common for your input signal.. Something just don't look right there.. SOmething tells me the inputs also need to be isolated.. They do make isolated op-amps where the common rail for the output side can attached in this manner and the inputs can use a low voltage common.

Jamie

Reply to
Jamie

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.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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