KEITHLEY 617 input stage question

[harrimansat]

Can someone explain Q311 function?
Thanks
Regards

[JohnnyBerg]

input protection, anti parallel diodes.

[harrimansat]

Thanks for your answer JohnnyBerg, I don' t understand because they don't used something like diodes and use a matched pair transistor.

[codeboy2k]

input protection, anti parallel diodes.

True, but more than just that, the real point is that they are using the base-emitter junction of two transistors that are on the same die.

The base-emitter junction of many BJTs have just a few pico-amps of reverse leakage. This is very important here because you don't want the protection diodes to leak current, since the electrometer has such a high input impedance at 200T ohm.  Any leakage currents in the input path will cause measurement errors, so they must be minimized and controlled.

So yes, these are anti-parallel input protection diodes, but the main reason for using a BJT here instead of an average diode is the lowest possible reverse  leakage current . They possibly need to be on the same die to keep them matched for temperature  (or maybe they selected them for lowest possible leakage as a pair).

[harrimansat]

Thanks Codeboy2k!!!

[dom0]

That'd be my guess to. The matched pair ensures very closely matched leakage currents, so they will nearly cancel each other out.

[codeboy2k]

You're welcome.  You'll also sometimes see FET's used this way too, as for example as I highlighted here in the Keithley 196 DMM
(click for bigger picture)

[codeboy2k]

On closer inspection, those base-emitter diodes are back-to-back, as in a clipper. So they are there to clip the signal to one diode drop. This makes the rest of the circuitry much easier since it only has to deal with max 500 mV input signal, even if the actual input is 1000V.   The leakage parts still apply though. That's the main reason for ever using a BJT as a diode.

The voltage divider for basic DC measurements is at the top of the page, so the clipping circuit and the following JFET op-amp and push-pull driver is for other measurements.

[rastro]

Also R348 is the input current adjustment.  It appears this is to correct for any mismatch/leakage between the the two BJT junctions.  In the manual 7.4.9 of the procedure is looking for 1.5fA specification. 

[codeboy2k]

Thanks.

I was curious about R348 and also the two thermistors.  I didn't read the manual yet, but I'm guessing the thermistors are for maintaining the set point over temperature.

I'll have to read it myself soon