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%.
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.
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
It's Ian... what can you say ?:-) ...Jim Thompson
[On the Road, in New York]
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]
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
John Fields turns up trying to start an argument - gosh, what a surprise!
"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?!
We might use "useless, whining old farts" as a starting point.
John
He couldn't scramble an egg without first having a working LT Spice netlist.
John
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!
"Bumchum" ! Wonderful.
John
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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