[Amps] a 'circuits' question

[Dan Sawyer]

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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