How to bias a CS mosfet

Is there any way to bias a mosfet CS with source degeneration so that the drain is always at Vcc/2 DC? I would like to create an amplifier that works for a range of VCC's. By using a voltage divider at the gate the gate voltage will change with Vcc which will change the Q point.

It doesn't have to be perfect but within a few percent of Vcc/2 for Vcc changing over 20%.

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Archival
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Use a source current sensing resistor, the voltage developed across that is fed via an integrator resistor to a capacitor who's developed voltage biases a BJT. The BJT has a modest (a few k) collector resistor and the developed collector voltage biases the MOSFET gate via a high value bootstrap resistor (maybe 4M7 to 6M8 or so).

If needed you can increase the gain with a hefty electrolytic in parallel with the source current sensing resistor.

Reply to
Ian Field

You'd need a lot of voltage drop in the source resistor to swamp the uncertainty in gate transfer curves. Some sort of active bias circuit might be better. Like, use a common-base PNP transistor to send the drain voltage back into the gate. Or just go whole-hog and use an opamp.

John

Reply to
John Larkin

--
Talk is cheap.

Got a schematic?
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Reply to
John Fields

It's Ian... what can you say ?:-) ...Jim Thompson

[On the Road, in New York]
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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Which MOSFET? Vcc=? Could be trivial, could be more than trivial. Where's Larkin when you need his "expertise" ?:-) ...Jim Thompson

[On the Road, in New York]
--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

This moron brags about killfiling me, then whines that I'm not responding to questions.

That's way-out unclear on the concept.

Since the problem is trivial, we can expect him to post a solution in a few more minutes. Stand by...

John

Reply to
John Larkin

John Fields turns up trying to start an argument - gosh, what a surprise!

Reply to
Ian Field

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

More to the point: what can you say about a f****it who's too thick to understand a simple MOSFET biasing circuit without a simulation netlist?!

Reply to
Ian Field

We might use "useless, whining old farts" as a starting point.

John

Reply to
John Larkin

He couldn't scramble an egg without first having a working LT Spice netlist.

John

Reply to
John Larkin

Maybe his bumchum (and my creepy stalker) can come up with the goods - Oh I forgot, John Fields can't even understand a simple bias circuit without a simulation!

Reply to
Ian Field

"Bumchum" ! Wonderful.

John

Reply to
John Larkin

--
No schematic and no sim, huh, but lots of mouth?

Gosh, what a surprise!
Reply to
John Fields

--
More to the point, what can you say about someone who won't show his
"work" for fear of being found lacking?
 Click to see the full signature
Reply to
John Fields

--
So, you've both been scraping the barrel long enough that you finally
found each other?
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Reply to
John Fields

--
One thing's for sure John, I don't think anyone will make the mistake
of calling you witty. :-)
Reply to
John Fields

--
Geez, Ian, if it's that simple it shouldn't take you longer than a day
or so to get it done and post it, so why don't you knock off the idle
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Reply to
John Fields

--
... "and visions of sugar plums danced in their heads."
Reply to
John Fields

o-----------------------------------o---Vcc | | .-. .-. | | GND | | | | R | | | '-' --- '-' | --- | . . . | | ___ | | o----'------|___|--------------o | . | | | ' | | |\| ||-+ | o-o-|-\ ___ ||------|___|---------||-+ o---------|+/ | | |/| ' | | .-. .-. | | | | | | | | R '-' '-' | | | | === === GND GND (created by AACircuit v1.28.6 beta 04/19/05

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Reply to
John S

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