Diode connected transistor in output stage of SMU - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: March 29, 2018, 03:53:58 am
content: Hey everyone, I’ve been looking at the schematics for the Keithley 236 SMU output stage when I spotted a bunch of diode connected BJTs. Here’s the schematichttps://imgur.com/a/ZXB8WAny idea what’s up with q28, 24, 22? I don’t immediately see anything that they need to be matched to (current mirror etc) and I don’t immediately see why leakage current would be a serious issue where they’re positioned, but I could be missing somethingThanks everyone!

author: ezalys
author_rank: Frequent Contributor
timestamp: March 29, 2018, 04:03:24 am
content: Sorry here’s a better quality upload

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 29, 2018, 04:08:38 am
content: Most likely something to do with how general purpose diodes don't actually behave much like perfect silicon diodes at the voltage they just start to turn on. It is also worth noting that chips used to read "thermal diodes" can be used with diode connected transistors in order to read temperature on a point of a PCB, but an actual signal diode will not work as expected.

author: Cerebus
author_rank: Super Contributor
timestamp: March 29, 2018, 04:45:55 am
content: Well, the lowest output range in current sourcing on the 236 is +/- 1nA, with a 100fA step size. I think that answers the question of whether leakage might be an issue. Note that the output is bipolar.I presume they are just there to protect the totem pole MOSFET gates if a significant reverse bias develops because somebody externally applied too high a voltage to the output. It's difficult to see immediately from the full schematics, but the output of that totem pole is connected to the outside work by no more than the current sensing resistor(s), so it needs some protection.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 29, 2018, 11:03:27 am
content: The transistors can work like a low leakage zener diode at around 6-8 V to protect the gate. It might be just BOM simplification not to use normal zeners.

author: ezalys
author_rank: Frequent Contributor
timestamp: March 30, 2018, 03:56:34 am
content: Okay this all makes sense so far. Would someone mind explaining why you'd cascode a fet above a BJT as was done in this design?

author: Cerebus
author_rank: Super Contributor
timestamp: March 30, 2018, 04:29:04 am
content: Quote from: ezalys on March 30, 2018, 03:56:34 amA MOSFET might be a bit simpler than a BJT in terms of the absence of a base current contribution to the output current, no small secondary non-linear effects as the totem pole swings up or down. That is however, just conjecture.The cascoding is, as I assume you realise, just to spread the dissipation out over three rather than one transistor. Dumping 150V @ 100mA across 1 transistor is asking for it to get a bit hot.

author: ezalys
author_rank: Frequent Contributor
timestamp: March 30, 2018, 06:06:32 pm
content: Would you mind explaining how this bias circuit enables the transistors to evenly share the load?

author: Cerebus
author_rank: Super Contributor
timestamp: March 30, 2018, 07:12:52 pm
content: Current is obviously evenly shared by virtue of the transistors all being in series. All that remains is to ensure that they take a rough division of the total voltage drop across them.The MOSFETs are just source followers, their output voltage is set (minus VGS) by the gate voltage.The gate voltages are derived from a voltage divider chain between the power rail and the output level with the relatively unusual feature of a zener diode at the bottom of it to set the minimum VCEacross the bipolar output transistor. That guarantees that they have some voltage to 'work against' and aren't pushed into saturation because of low VCE. The capacitors are in the divider chain to make sure that it operates as a divider as effectively at AC as it does at DC - a compensated divider, note that the RC product for all sections are the same.

author: ezalys
author_rank: Frequent Contributor
timestamp: March 30, 2018, 08:49:35 pm
content: Okay. Now the reason for using mosfets becomes very clear. BJTs would draw current from the divider network and would make it harder to design to ensure Vds for each fet is equal.

author: Circlotron
author_rank: Super Contributor
timestamp: March 30, 2018, 10:11:38 pm
content: Quote from: ezalys on March 30, 2018, 03:56:34 amAs the BJT emitter goes up, the mosfet above it pulls the collector up by almost the same voltage. This means the BJT operates with practically a constant voltage across it. That would contribute to better linearity and frequency response.