Purpose of back to back transistors in front of a JFET input stage

[setq]

With respect to this circuit, which is an extracted and redrawn input stage from something, what is the purpose of the back to back transistors? The "rest of circuit" has a current sink and is using this as a high impedance follower by the looks.



I am making a vast assumption here but are these very low leakage clamps to stop the JFET blowing up if the input range is exceeded? So three questions:

1. Am I correct?
2. If so, what sort of leakage current and what transistor characteristics should I look out for in this space?
3. What happens to the BJT if I exceed this?

For ref in the actual design they are specified as 2n3393 which has a Vce(max) of 25v but no leakage specified which is absent on all transistor datasheets.

[Ian.M]

The B-E junction is being used as an amplified (by the reverse HFE) Zener clamp.  Matching the characteristics would be difficult, and it may have required one particular make of 2n3393 to work as intended.

What's it out of?

[setq]

Thanks for the info.

It is from a Heathkit IM-16 FET VOM. I'm looking at input stages for simplified devices purely for the sake of education.

[T3sl4co1l]

It was a common way to get ESD protection at low leakage, back in the day.

Nowadays, ICs use snap diodes, a similar construction.  (A BJT exhibits a breakover effect if B-E is biased slightly off; or a full 4-layer (SCR or DIAC) structure is used, which is available commercially as TVS devices for surge clamping.)

Tim

[setq]

Thanks for your reply. That makes sense.

[zlymex]

Be sure not to apply current of more than a few hundred micro amps to the input because the reverse C-E junction usually has negative slope(negative resistance) that will result in oscillation(when paralleled with a capacitor such as the input capacitor of the input amp). I often use two B-E junctions connected in reverse series for bi-directional Zener clamp to avoid this.

[Cerebus]

Yep, as already stated, that's a cheap and quick way to get a low-leakage Zener clamp on the front end.  The failure mode of these transistors will be to fail shorted (most of the time), so that will (hopefully) protect the FET.  Used in this way, these transistors will probably not survive very much energy (the base-emitter "Zener" is very fragile), so you want to be very careful how you use this circuit.

Nowadays, you would use a TVS diode with the anode connected to the most negative voltage, and the cathode connected to the most positive voltage (with the TVS diode's voltage appropriately rated).  Then, you would connect some very low leakage diodes from this TVS diode (top and bottom) to your signal.  A low leakage diode can be the base-collector junction of a BJT, or a low-leakage JFET with the source and drain connected (and the gate is the other side of the "diode").  You can buy these in a single package-- for example, the Littelfuse SPA series (and Littelfuse has these for higher voltages too).  These have fairly low leakage and capacitance on the protected pins, but will do a good job protecting against fairly large ESD strikes.  The idea is to get this very close to the connector that goes outside of the case (to reduce inductance).  I have used these before and they have done a GREAT job for me in the past.

Bear in mind that the particular part you've called out has ~1nA leakage which could be several orders of magnitude too high for the kind of JFET input that the OP was looking at, depending on the exact circuit parameters (i.e. it has an unspecified gate resistor which might make this caveat purely academic).

If your input calls for < 5pA or so of leakage then you're stuck with selected diode wired BJTs or JFETs as you suggested or wildly expensive low-leakage diodes like the DPAD5. My experience is that the humble 2N3904, depending on manufacturer, can make an excellent low leakage diode and, being cheap as chips, are inexpensive to select the good ones out from a big bag/reel of them. I've had good results from Fairchild ones.

[dom0]

Usually leakage specs on small diodes, FETs etc. are much much higher than the actual typical value, because leaking testing is a slow and costly test. So they just put "50(0) nA" on a lot of stuff that normally performs at sub-nA leakage.

[setq]

My experience is that the humble 2N3904, depending on manufacturer, can make an excellent low leakage diode and, being cheap as chips, are inexpensive to select the good ones out from a big bag/reel of them. I've had good results from Fairchild ones.

This is where I ended up actually. I've got a bag of Fairchild 2n3904s. Problem I have now is how to measure leakage at picoamp levels. Had my head in Linear data sheets for the last two days working out how they do it. All of my kit loads it too heavily.

[Cerebus]

My experience is that the humble 2N3904, depending on manufacturer, can make an excellent low leakage diode and, being cheap as chips, are inexpensive to select the good ones out from a big bag/reel of them. I've had good results from Fairchild ones.

This is where I ended up actually. I've got a bag of Fairchild 2n3904s. Problem I have now is how to measure leakage at picoamp levels. Had my head in Linear data sheets for the last two days working out how they do it. All of my kit loads it too heavily.

If you don't have a dedicated picoammeter, the easiest way to do this is to construct a little battery powered transresistance amplifier you can use with a multimeter. Dead bug construction style is the norm for this so as to eliminate leakage paths. The classic op amp to use for this is an LMC6001 (25fA input leakage guaranteed, 10 fA typical) but they're expensive - around £10. A cheaper alternative ( < £1) is the LMC662 (2-4 pA guaranteed, but 2fA typical input leakage) and hope you get a good one or select from a few. Next hurdle is a good stable high value resistor for the feedback path - 1G or 10G. The last hurdle is you're going to want to protect that highly sensitive input, which means selecting some low leakage diodes/JFETs/BJTs. Recursion - see recursion.

Once you've built something you'll enter into the fascinating world of low level measurement where moving near your bench can change readings, you start fussing over humidity and temperature and the cat becomes even more of a hazard to work in new and interesting ways.

If you haven't already read a copy I recommend the Keithley "Low Level Handbook" downloadable from Keithley's (Tektronix) website I believe.

Finally a two part article from Paul Grohe (The late Bob Pease's technician) in EDN to get you started:

[danadak]

Measure to femotampere simple jig.

http://electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow


Regards, Dana.

[setq]

Thanks both - much appreciated. Lots of reading ahead