How can I test a MOSFET for Transconductance (gFS) and Forward Admittance on my curve tracer?
Transconductance (gFS) and Forward Admittance
What It Is:
Transconductance is the ratio of ID to VGS. The I/V ratio is commonly referred to as gain.
On the curve tracer, the Collector Supply drives the drain and the Step Generator drives the gate.
What The Display Shows:
The display shows VDS on the horizontal axis, and the resulting ID on the vertical axis. With the Step Generator providing gate drive, the curve will be displaced upward from the horizontal axis as the gate drive causes a proportional ID. The specification is met when, at either the specified VGS or the specified ID, the ratio of ID to VGS is equal to or greater than the specified minimum.
How To Do It:
1. Set controls:
A:Max Peak Volts to the lowest setting above the specified VDS
B: Peak Power Watts at the lowest setting to satisfy (ID x VDS)
C: Collector Supply Polarity to (+DC) for N-channel or (-DC) for P-channel
D: Horizontal Volts/Div to display the specified VDS between the 5th and 10th horizontal divisions
E: Vertical Current/Div to display the specified ID between the 5th and 10th vertical divisions
F: Number of steps to minimum (zero)
G: Step Generator to Voltage
H: Step Generator Polarity to apply forward bias (+ for N-channel), (- for P-channel)
I: Step/Offset Ampl to approx 1% of the specified VDS
J: Pulse to Long
K: Configuration to (Base/Step Gen, Emitter/Common)
L: Variable Collector Supply to minimum % (full ccw)
M: DotCursor ON
2. Apply power to the MOSFET:
A: Position the Left/Right switch as appropriate
B: Slowly increase the Variable Collector Supply % until the specified VDS is reached
3. Adjust to parameters:
Press and hold Offset Aid until an appreciable vertical displacement of the curve occurs. It will be necessary to readjust Variable Collector % to maintain VDS. Continue adjusting Step Offset and VDS alternately until the specified operating point is reached.
4. Calculate transconductance (gFS):
Read gFS directly from the cursor readout
5. Compare to data sheet specifications:
Check that the value is equal to or greater than the specified minimum
Forward admittance is an alternative way of expressing transconductance and is measured by setting the curve tracer up to measure transconductance (as above), switching Horizontal Volts/Div to STEP GEN, using SWEEP to complete the curve, then changing the cursor to F line and positioning the slope of the F line until it’s tangent to the curve.
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