Steve,
Thank you. I was trying to drive the MOSFET in the linear region. I
figured gm would be accurate for the linear region and that bias
adjustment would be accurate for a particular initial condition. I was
speculating that even given those two points the shape of the drive
curve might not be identical. Is this true??
I have a dual trace scope, one channel connected to the gate and one
drain. The result is a pretty clear picture of what the bias and drive
points are.
Thanks,
Dan
Steve Thompson wrote:
On Wednesday 08 September 2004 14:44, Dan Sawyer wrote:
This is a circuits question from a 575 curve tracer being used to match
MOSFETs for an HF amp so it is a least close to topic. The problem is
the difference in granularity between the 'steps' circuit and the 'zero'
adjust circuit. The bias of the power MOSFETs is high enough so that
only higher numbered steps get displayed the granularity per step is too
high. The result is only 2 steps per trace get displayed.
The circuit that produces this is a voltage adder with the steps input
coming through a 90K Ohm resister and the bias coming through a 600K
ohm. My question is can I substitute a smaller resister for the bias? I
was thinking of 150K ohm. This would effectively increase the bias to
the point where it was turning on the MOSFET and the steps drive could
be reduced to a low enough value where multiple steps were observed.
Is this a feasible modification??
I'm not familiar enough with the 575 to properly answer your question,
although I imagine the proposed mod should work.
Let me suggest an alternative matching method that was used when I was
involved in manufacturing rf fets:
measure the turn on point by tying the gate and drain together and applying
volts between that and the source - go up to a few 100s mA. There's no big
deal about the configuration - it's just quick easy way of doing it. Use the
curve tracer, or just a PSU and voltmeter.
Use the curve tracer to measure the gm at a higher current using the step
function.
If you have control of the circuit design, put in separate bias adjustment for
each device, and do away with step 1.
Steve
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