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

You just built a smoke generator. You have no way to compensate for component variations or for temperature. You'll have similar problems with series diodes. Typical solution for bipolars would be emitter resistors. It's more complicated with fets because of the variations in threshold voltage. You're describing a voltage amplifier circuit using the term "current gain stage". How many is "high" MHz.? THD is certainly a valid term, if you put a number on "better", but it may obscure your objective. What are you trying to accomplish? Is your load always purely resistive

100 ohms? The devil is in the details.

The dominant 'stray' capacitances will be Cgs and Cdg, regardless of the number of parts you hang onto the gates, whether the power device being controlled ia a mosfet or an IGBT (as your schematic actually indicates).

With a very fast op amp and no local compensation, you'll run into the situation where the Op amp will try to charge the gate capacitances through the zener body diode, something that will not happen with a diode string.

Cross-over distortion is not a 'static' characteristic; I'm guessing you're just looking for a linear transfer and stable DC bias. Unless you know why the first technique was used (with all those 'extra' components), it's not really possible to speculate on the benefits or advantages of a modification.

RL

As Mike pointed out, this circuit's bias is unstable. Also, you'd need very low voltage zeners for most recent MOSFETs. In addition, you'll need a very high voltage op amp to make use of those +-30V supplies.

With 10k resistors on the gates, the MOSFET capacitance will prevent the gate following your op amp very fast in the direction towards drain potential, so if you really try to go at megahertz rates, you'll see a lot of distortion due to slew rate limiting.

One cute method is to put resistors in series with the op amp's supply pins, and drive the gates from there. (The sources go to the supplies, i.e. the PMOS device goes on the high side.) In this case, you could put the zeners in series with the supply pins, so you don't blow up the op amp. A resistor to ground from the op amp output, plus one from the op amp output to the booster output (the FET drains) gives you some local feedback that makes it easier to frequency-compensate the whole thing.

(Source resistors on the FETs will help stabilize the bias, either way.)

I wouldn't use the fancy method for a general-purpose amplifier, though, because its quiescent current isn't too well controlled and because it tends to be vulnerable to reactive loads, which can make it oscillate or blow it up.

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

In other words, use a bipolar output stage... MOSFET's are for people who _want_ ethanol in their gasoline ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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I love to cook with wine.     Sometimes I even put it in the food.

Think of it as a petrol Martini. ;)

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

This time of year, Gin & Tonic is the afternoon cocktail of choice :-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

As others have mention it's better done with BJT. FETs have way too large a production tolerance in the thresholds. It'll either distort badly or go phssst ... *PHUT* because of excessive quiescent current.

Another common trick of the trade is to have a resistor between opamp output and load, to (somewhat) tide over the dead zone.

--
Regards, Joerg

http://www.analogconsultants.com/

I like power fets in linear amps, but you've got to believe the SOA curves.

Another trick: use a large source resistor and shunt it with a diode or two. That lets you use a big source resistor for stable idle current, but lets a lot of load current through when you need it.

Yet another trick: use an opamp per fet to remove individual Vgs variations.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

Same here, I use big FETs in linear mode a lot.

Yes, one can do that. It's just that the threshold of FETs can vary more than a volt from lot to lot.

But the cost, the cost :-)

--
Regards, Joerg

http://www.analogconsultants.com/

I was afraid it would be something like this. I'll try to elaborate some: With current gain stage I mean the push pull output, as it's ideally a follower. The speed question is more difficult. The load for the amp is a piezoceramic with a capaciance of around 90nF. The resistor in the schematic was just for the purpose of my simulation. I use a series resistor with the piezo to decouple the reactive load from the piezo. Now the overall bandwidth should excell 1MHz piezo drive. In most cases the output will not oscillate, but follow another controllers signal. To achieve 1MHz output however I've found (simulation and board testing) that i need quite fast opamps (400MHz GBP up) so that i can afterwards silence the oscillation of the whole amplifier with capacitive load with the series resistance. With deadbugging I've noted that the THD with unbiased fets is around -35dB at 10kHz, which i would like to reduce with biasing.

I hope that clears it up some.

The opamp i use is in a suspended rail configuration, so up to +-100V output swings are no problem. I don't think i understand your suggestion yet, but it sounds complicated to do as my supplied to the opamp are already floating. Since my load is inherently purely reactive (dampened some by the addition of a series resistance) the blowing up is always a concern.

I initially used BJTs as output instead of the mosfets, but as the output load can spike to 6-10A easily at full speed and full swing i've found that I achieve higher bandwidths with a fet output, at the cost of some headroom and unfortunately also some crossover distortion.

That is what I'd call a "serious load". You'll probably have to pre-bias. Assuming that you transmit ultrasound pulse trains you should have that running in a servo in order to mitigate temperature drift.

Then I'd add some sort of fast current sense so you can detect an overload before molten solder and TO220 pieces splatter about. Or maybe even a simple SWR-bridge so you can detect if the transducer loses coupling to the medium, to avoid putting a big crack into the piezo.

Question: Why do you need to drive a tranducer with a linear amp? Even PZT-5H with a good backing material isn't more than 50% in BW.

--
Regards, Joerg

http://www.analogconsultants.com/

Take a look at TCA0372. It's a 1 MHz, dual, 1 amp opamp, good for +-20 volt rails. Use both sections to bridge-drive both ends of the piezo for more voltage swing, or run the sections in parallel with ballast resistors if you prefer more current. Costs 50 cents.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

Ideally E'd run it in a charge amplifying configuration to even linearize the piezo motion which has some hysteresis. The reason for using a linear amp is that I'm not interested in transmitting power, but for the use as a nanopositioning device. It's a part of a homebuilt atomic force microscope that is supposed to allow for at least 100kHz, ideally even up to 500kHz topography feedback. I can already drive the load with the version of the amplifier that I already have, however i stagger around 300kHz and I'm currently redesigning it to allow for ideally >1MHz operation. I am fully well aware that driving a 100nF load at MHz is a more than serious load and I know that doing full swings at that frequency is utopic, but with 6-10A output transistors it's possible. The only thing I'm as mentioned stuggling with is getting dc precision, bandwidth and signal integrity to a compromise. The actual integrated up power load on the amp is not that staggering, but the peak loads are a bit extreme.

I need +-60V upwards unfortunately, but cost is really not an issue, i can spend as much as i want as long as i get it to work.

That makes it a lot easier. How about a Cirrus (nee Apex) PA85?

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

As much as you want? Now that is a nice situation to be in :-)

Well, then ...

Digikey had them in stock. But sit down before looking at the price. Of course, the usual precautions for capacitive loading apply but that is the case for any amplifier.

--
Regards, Joerg

http://www.analogconsultants.com/

Yeees, that's just the thing. They're not fast enough. That's where I started out, using

but with them i never got the noise as far down as with my suspended rail highspeed opamp. Also i've had trouble getting more than 200kHz closed loop bandwith out of them. That's the reason i started building my own amplifier in the first place.