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Old Apr 27, 2011, 04:14 PM
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Mosfet P-channel Linear Region control


Does anyone have a circuit for controlling Mosfets in the Linear Reagion.

I am locked into using a p-channel mosfet and cant use PWM.

I need to be able to controller the Mosfet with a pot and i have a heat since that will control the heat.

Thanks for all the help.

Mike
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Old Apr 27, 2011, 07:41 PM
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Voltage or Current control?
Simple single op amp would work, would need additional details (input voltage, frequency response, voltage rail, etc.)

BTW, why are you locked into a P channel?
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Old Apr 27, 2011, 07:58 PM
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just invert this for a P-fet
Old Apr 27, 2011, 08:00 PM
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Or try a linear amp!
Old Apr 27, 2011, 08:05 PM
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P-ch the arrow points out. Just modulate the gate voltage. They are just like

grids in tubes!
Old Apr 28, 2011, 10:27 AM
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I am looking for voltage control. at the moment.

I have to use a P-channel becasue that is the side i have access to.
Old Apr 28, 2011, 10:32 AM
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Quote:
Originally Posted by jglenn View Post
just invert this for a P-fet
How do you determin R1 and R3
Old Apr 28, 2011, 11:44 AM
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"Simplify, then add lightness"
Quote:
I am looking for voltage control. at the moment.

I have to use a P-channel becasue that is the side i have access to.
Why a mosfet instead of a BJT? How close to the high side rail does the output need to come?

If you want to control the load voltage and can stand the output not coming completely to the high side voltage then you would be much better off using a NPN in common collector or N-Channel mosfet in common drain configuration. That way the output voltage is equal to the input voltage minus the junction or threshold voltage.

Using a P-Channel in common source will actually control the current through the load and you would need feedback(ie an op amp) to directly control the voltage of the load.

PS Added the quote because that is all the info we have. The quality of the output depends on the quality of the input.
Last edited by jeffs555; Apr 28, 2011 at 11:53 AM.
Old Apr 28, 2011, 11:51 AM
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I need the output to be as close to the rail as possiable, that is why i dont want to use BJT. I am using a BJT now.

Can you give me a little more insight to your second line.

Mike
Old Apr 28, 2011, 06:57 PM
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How do you determin R1 and R3?

The fet gate voltage when running linear is sort of like the grid of a tube, a proportional
change affects conduction. The spec sheet will have the range, set up the pot divider
to cover that. There may be differences from device to device. And non-linearities.
Old Apr 28, 2011, 07:08 PM
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A rail to rail op amp, a FET and a few resistors. Linear, easy to scale and limit.
Simple
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Old Apr 28, 2011, 08:00 PM
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This circuit works well, driven by an opamp.

There are also power opamps like the EL2008 that can drive 1A at RF freq.
Old Apr 28, 2011, 08:12 PM
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When a MOSFET is operating in its 'linear' region, Gate voltage controls Drain current. The action is similar to how Base current controls Collector current in a bipolar transistor, but at low currents the Gate contol is close to 'pinch-off' and highly non-linear. It is also quite temperature dependent.

A pot is OK if you don't mind constantly adjusting it, but if you want a stable output voltage then you will need an amplifier and feedback loop.
Old Apr 28, 2011, 09:43 PM
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Thanks for the curves. Some find that in digital switching, the rise and fall

are actually linear. Slow ramp or slew causes heating. Too fast causes spikes.
Old Apr 29, 2011, 12:14 AM
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I was looking at using the irf4905 because I have a few. How would I start.


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