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Clamping Split Rail Node to GND using Transistor(s)

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Mechatrommer:
hi i have this circuit (attached). at some point in time during operation when capacitors build up charges, i want to reset or "clamp" the red arrowed node to GND (0V) for a moment using transistor solution, and then release the reset condition again so it will work normally again. if anyone wonder, this is part of opamp PID circuit, and shown is the Integral part of it, the reset is to avoid Integral windup if i'm not mistaken, it is the place to do the windup reset. please tell me if i'm wrong.

if it +ve voltage only, it will be easy with just a single NPN shorted to ground when base is HI (and release clamp when LO). but this is split rails, can be +ve and -ve voltage. if i put relay to it, it will work the way i want, but this is high repetition (frequency) rate, mechanical relay wont keep up and will die in no time i suspect. i have switch IC including bidirectional type on a board that i can reuse, but i think thats specialty component probably add price to my circuit, and complexity to provide additional logic power rail. i will use it if no other better and simpler discrete transistor solution that can take up say ±15V voltage swing. so can anyone suggest the simplest transistors solution for this? thanks.

Ian.M:
To reset the integrator you need to discharge Cintg.   Grounding I- wont do that as with I+ and I- at the same voltage, the output will try to go to Open_Loop_Gain*Input_Offset_Voltage, which will probably rail it.   Therefore you need to short Cintg, i.e connect I- to Out.

The easiest way to do that without complex level shifting or drive circuits would be a PhotoMOS optocoupler + just enough series resistance to prevent the optocoupler's peak load current being exceeded when there's a full 15V+Vstg1 on the cap.

You could use an analog switch, but handling the worst case +/-15V, + providing it with a logic supply+ level shifting its control signal may be more complex that you care to implement, and also its unlikely to have anywhere near the current handling capability of a PhotoMOS optocoupler, so resets will be slower with a higher residual error.

Discrete MOSFETs would be complex to use due to Vgs max limits + the need to use either four terminal devices with a separate substrate pin or back to back pairs.  If you don't need to hold the clamped reset state, its possible you could use a pair of back to back (source to source) MOSFETs with a pulse transformer for gate drive, however a transformer that can handle a long enough reset pulse without saturating isn't going to be small or cheap.  If you need DC clamping, you need two pairs of back to back MOSFETS, one pair N channel and the other P channel so you can guarantee at least one pair is on when you take their gates to opposite rails.  You also need Zener clamping for the gates and a rather complex drive circuit to avoid injecting any current into In- when not resetting.

Symmetrical transistors with an adequate Vebo breakdown voltage to handle the worst case voltage accross the capacitor are rare like hens teeth nowadays.  Without them a BJT solution will be over-complex with a whole mess of steering diodes, which would also increase the residual offset.  The few remaining available transistors designed for reversable operation (e.g. 2SC2878 or 2SC3326) typically don't have high enough Vceo ratings for your application.  It would be a different  matter if you were using +/-5V or +/-9V rails for your OPAMP.

Mechatrommer:
thanks Ian. you are right i missed the point that opamp output will go to rail if i simply short its -ve input, i thought it simply go to 0V and discharge the capacitors. looking at digikey, photoFET optocoupler such as H11F1M is $4, quite luxurious for the circuit i'm building. coincidentally i'm waiting parts from digikey to arrive soon, among them is MOC3041 triac optocoupler as replacement damaged part of my soldering station 852AD, i bought 10 pcs so i will have extra parts to keep, less than $1 each. although it has 1.5V forward voltage, i think it can provide "limited" bidirectional reset to short capacitance, correct? your later suggestions and explanations seem to result in circuit complication...

btw, i deviced the discrete transistors bidirectional clamp to GND in spice just few minutes after making this thread (attached) but since grounding the opamp -ve input is a bad idea, so its going to trash... 4 bjt's and few resistors, it looks like even if i can cramp them in smallest space, they will be no smaller than the DIP8 footprint let alone DIP6 of the MOC3041. luckily its also pin compatible with H11F1M fet, so in case later i can justify buying the $4 part, i can simply put it in there without making new pcb. i think i am making sense here ;D no? thanks Ian for the optocoupler idea.

David Hess:
Like Ian.M says, you have to clamp across the capacitor for the reasons he gave.

In the past a JFET might be used and that is still a good solution.  Before they existed, someone probably did it with a current driven diode bridge.  Photo resistors have also been used.  Ian.M's suggestion of a photoFET or photoMOS optocoupler is one of the simplest ways.

Another way which is sometimes used is to have a second operational amplifier drive a current into the inverting input until the output reaches ground.

Ian.M:
Here's a way of doing it with a pair of back to back N-MOSFETs.   The trick is to bias the sources to the negative rail to guarantee you can initially apply enough gate drive to turn them on.  Once it starts to reset, there's no problem maintaining the gate drive. However the bias must be disconnected at the end of the reset pulse + the gate voltage needs to be held up with a capacitor to the joined sources with a resistor across it to control the rate of turnoff to avoid excessive charge injection at turnoff.  The drive circuit is therefore quite complex, verging on fugly.

AAGH!  |O The attachment went AWOL.  :palm:

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