circuit.
I post all sorts of stuff, from conceptual sketches to PDF schematic sheets of real, current commercial products.
Sounds like you're in a crabby mood again.
John
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- posted
12 years ago
How to bias a CS mosfet, a different approach
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- posted
12 years ago
mosfets.
-- So, for an audience which you'd like to capture, you're trying to set up an imaginary hierarchy with me on the bottom, Jim in the middle, and you on top? Really, John, you need to take stock of your own screw-ups and admit to your errors before you start looking for control by casting aspersions on anyone else.
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- posted
12 years ago
mosfets.
The one from Aug 16? Insane, makes no sense at all.
The OP wanted to _bias_an_amplifier_ so that its drain voltage is reliably Vcc/2. You don't seem to be getting that.
A previous post, where you fiddled resistor values to set the gate voltage, was equally amateurish.
And Jim won't comment on any of them. And he rags me for doing perfectly sound beta biasing, which can be proven to be safe in some situations.
What's the collective noun for a cluster of old hens? Brood? Peep? Maybe Lurk?
John
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- posted
12 years ago
mosfets.
--
Is this what has you flummoxed?
ZTX1048A
Q1 100
Vcc>--+-----C E----------+--[RL]--+
|1000 B | |
[R1] | [R3] |
| | 10µF |1000 D
+-------+ +--[C1]--+------G IRF1312S
|1000 | | Q2 S
[R2] [ACIN] [R4] |
| | |1000 |
GND>--+-----------+--------+--------+
If it is, then just ask politely and I'll post a circuit description.
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- posted
12 years ago
You keep missing (deliberately?) the fact that he posted the circuit as YOU described it. AND he even told you so.
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- posted
12 years ago
mosfets.
It doesn't make sense, and doesn't do what the OP wanted.
There is a very small range of Vcc over which the gate of this fet is properly biased to make this amplify. Change the fet, or change Vcc, and it breaks. Try it in your own simulation.
Fixed-voltage biasing the gate of a mosfet amp is way, way worse than beta biasing a bipolar. Ask Jim if you don't believe me.
The original post:
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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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Get it? He wants the dc ***DRAIN*** voltage to be Vcc/2 for a range of Vcc. That makes sense as a thing to do, and as a problem to do well. He sensibly mentions source degeneration, which gets close to solving his problem.
He didn't want Vcc to be Vcc/2! And you didn't even get that right.
The dumb gate divider makes no sense, and doesn't do anything like what was requested. A gate divider plus a source resistor starts to work, but you didn't do that.
You keep posting wrong stuff. Several varieties of wrong stuff so far.
It's not my fault if women avoid you.
John
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- posted
12 years ago
I said it wouldn't work, and it doesn't.
Why don't you take a crack at the problem?
John
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12 years ago
He/she won't try because he/she has no knowledge of electronics.
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- posted
12 years ago
bias mosfets.
-- Here then, let me annotate it a little to make it clearer: . ZTX1048A Vdd (Vcc/2) . Q1 / .Vcc>--+-----C E----+-----+--[RL]--+ . |1000 B | 100 | . [R1] | [R3] | . | | 10µF |1000 D . +-------+ +--[C1]--+------G IRF1312S . | / | | Q2 S . [R2] Vb [ACIN] [R4] | . |1100 | |1000 | .GND>--+-----------+--------+--------+ > >There is a very small range of Vcc over which the gate of this fet is >properly biased to make this amplify. Change the fet, or change Vcc, >and it breaks. Try it in your own simulation.
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- posted
12 years ago
---
A little source degeneration works wonders!
. ZTX1048A Vdd (Vcc/2) . Q1 / .Vcc>--+-----C E----+-----+--[RL]--+ . |1000 B | 100 | . [R1] | [R3] | . | | 10µF |1000 D . +-------+ +--[C1]--+------G IRF1312S . | / | | Q2 S . [R2] Vb [ACIN] [R4] | . |1100 | |1000 [R5] . | | | |10 .GND>--+-----------+--------+--------+
-- JF
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- posted
12 years ago
Vdd should be the DRAIN voltage, not the DRAIN SUPPLY voltage, per the original request. The whole Q1 thing is silly. Get rid of that and we can discuss what's left.
John
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- posted
12 years ago
-- No, thanks.