Here's a nice example of a circuit that pulls itself up by its bootstraps - a high-voltage opamp follower featuring electrometer-style high input impedances.
My design is meant to replace expensive high-voltage opamps, like the PA97 offered by Apex, with low-cost superior-performance low-voltage opamps, like those offered by Analog Devices (Stephan Goldstein should enjoy this one). My goal is for an amplifier with an offset voltage under 65uV and less than 1pA input current, yet operating over a 430-volt signal range.
On a sunny day (9 Oct 2007 19:58:08 -0700) it happened Winfield Hill wrote in :
I am curious about your 'input protection', would there not be a very short input current spike if you applied a step from 0 to +400V ? That could cause problenms in some circuit.
Yes indeed. For example, with its infinite Zin and vanishingly-low input current, if a probe is used to measure +400 volts, after removal the amplifier will simply stay there. Then if the probe is touched to ground, the amplifier's input will experience an instantaneous -400-volt spike.
The easy fix is a pair of back-to-back diodes at the input to the circuit's "supply-ground" output, along with some current limiting input resistance. I chose two 1k resistors in series at the input, along with one more between the diode node and the opamp to limit its current during the fault. The low value was chosen to preserve the opamp's 8.5nV voltage noise. But 2k input resistance allows for a 200mA fault current, so I added a depletion-mode MOSFET circuit to further limit the fault current to about +/- 0.5 to 1mA.
I used my favorite Supertex LND150 or LND250 parts. The protection diodes are low-leakage low-capacitance BJT base-collector junctions.
Well, in its favor, the APEX does have lots of features mine lacks, such as high output drive. But they would make for expensive HV-voltmeter high-impedance voltage followers. We needed two inputs for a differential measurement, so I decided to design this circuit. There are several applications where it can be used.
You could also do a flying-capacitor thing, with a dpdt relay, if static measurements were enough. It could report the differential input almost 1:1, or you could dump into a bigger cap and get a voltage-divider effect.
My input current-limiting circuit, transient protection for the MOSFETs, a small isolated capacitor to absorb fault charge, a protected guard circuit, and series resistors to limit the fault currents if a MOSFET fails.
You have an extra zener with cap, unless you want to add an active PMOS pulldown circuit to increase negative slew-rate capability. That's my next version and pushes the parts count up to 38.
John, and the switches for the high voltage being ? There are several conditions to it. Such as low leakage, low charge injection. Win was talking about an electrometer input, meaning there cannot 1uF at the input.
The depletion Fets are single sourced ? I only heard about one manufacturer once, and even forgot about it. I'd prefer enhancement MosFets. A schematic is one thing and some estmations, simulations or measurements are another. What are expectable slew rates ?
There are at least three manufacturers, although Supertex is the most prolific and committed one.
Unh Huh. Be my guest trying to create a simple effective circuit like this with an enhancement- mode MOSFET. You need ugly things, like a pullup resistor that has to provide sufficient current over a two-decade range of signal voltages, or a HV current source (which is best made using a depletion-mode transistor), etc. I've done it, multiple times, but it's a true pleasure to be able to use depletion-mode instead. Supertex is a reliable manufacturer, and now that Mouser is stocking their parts there's little excuse to avoid them. We all work with sole-source parts all the time. Just maintain a good inventory.
What! You question the operational efficacy of my circuits? :-) Actually, this one has been reduced to elegant practice, with a nice SMD-component PCB.
Pullup slew rates are very fast, but in my simple class-A circuit (again below), the fixed pulldown current source is a limitation, against the combined Coss of the pullup + pulldown MOSFETs, plus any load capacitance, such as a triaxial guard cable, etc.
If the load is just a resistive divider, the circuit can be reasonably fast. Supertex dn3145 and dn2540 are candidate MOSFETs, with a Coss of about 12pF at 25V, so let's calculate for C = 25pF (for the pullup plus pulldown CS mosfets). dV/dt = i/C = 60V/us, which means it takes 6.6us to slew 400 volts, and can follow a 50kHz 400Vp-p sine wave.
My full circuit, 2nd rev, adds a p-channel pulldown MOSFET, which I expect to double the negative slew rate and maintain this speed into a triax guard capacitance.
Actually the preferrence was more towards something buyable, not against depletion as itself.
Supertex ist having a strange policy. Their minimum order quantity apparently is a wheel of 1k pieces. Yes, they do samples and can be asked for lower quantities, but with our distributor in between they are somewhat unwieldy. Perhaps I should look at Mouser, which haven't ordered yet from.
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