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Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
If I shouldn't be asking this question here let the moderators say so and either lock or delete the thread. I have a question about calculating transconductance from experimental measurements of a mosfet. In a college course I'm taking we have a lab where we diode-connect a mosfet, apply a variable voltage across it and measure the current. Then we calculate threshhold voltage and transconductance from the data. I'm okay with the threshhold voltage but I have a conceptual problem with the transconductance. Basically, since Gm is the partial derivative of drain current with respect to gate-source voltage, that implies holding the other variable, drain-source voltage, constant. But we get our data by varying drain source voltage. So I'm wondering if this is a valid procedure for calculating transconductance from experimental data. Can you calculate transconductance the way the lab suggests? The instructor is not really the person to pose this question to, because I would be basically challenging his understanding of the subject, and since I'm not sure about this, it would be better to find out on my own whether I'm right to question it. Here's some of the wording from the lab:
Procedure 1. Place a 2N7000 n-channel MOSFET in your breadboard, and connect the gate lead to the drain lead. Connect the +25V power supply terminal to the drain terminal of the MOSFET and connect the COM terminal of the supply to the source terminal of the MOSFET. Starting at 0V very slowly and carefully increase the voltage in small steps, recording both voltage and current from the power supply, until the power dissipation is the maximum specified for this device. Repeat this step for both 2N7000 devices in both lab kits.
Analysis
Using the data from the first procedure, plot ID versus VGS, and then find the derivative of this curve at five points along the curve. Tabulate these values of the transconductance.
Edit: Ok, now I'm realizing that connecting the gate to drain keeps the mosfet in the saturation region, which means variation in the drain voltage has a fairly small effect on drain current. There is only a small error is due to lambda. So it seems my concern was unfounded.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi Kell
I'm the instructor for some similar analog electronics labs, which got cancelled at the last moment because the 2N7000s were so temperature sensitive.
The hidden assumption is that transconductance doesn't depend strongly on Vds. That as far as I know is reasonable: it's mostly a function of the drain current, as you figured out. In fact it should be linearly proportional to the drain current, because Id in a FET is proportional to the square of the gate voltage.
We got the students to do the simulated lab in Pspice instead, where the temperature stays constant and you can get the measurement of the threshold voltage to stand still. Cop-out, I know.
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
Transconductance would be linearly proportional to drain current at a constant gate voltage, according to the equation gm=2Id/(Vgs-Vtn). But we have the gate tied to the drain so its voltage moves when the drain moves. So we're going to find the slope of the voltage-current curve to calculate gm. That also frees us from having to know Vtn. Of course the Vtn negative temperature coefficient is still lurking in the background, and maybe that will have an effect even though Vtn itself won't show up in the calculation. Maybe that means that the transconductance has a temperature coefficient, too. Probably worth looking into.
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measuring mosfet transconductance
