Version 4SHEET 1 884 680WIRE 176 64 80 64WIRE 304 64 176 64WIRE 496 64 400 64WIRE 592 64 496 64WIRE 752 64 672 64WIRE 176 96 176 64WIRE 496 96 496 64WIRE 752 128 752 64WIRE 176 208 176 176WIRE 352 208 352 128WIRE 352 208 176 208WIRE 496 208 496 176WIRE 640 208 496 208WIRE 704 208 640 208WIRE 640 224 640 208WIRE 80 304 80 64WIRE 176 304 176 208WIRE 496 304 496 208WIRE 640 304 640 288WIRE 80 416 80 384WIRE 176 416 176 384WIRE 176 416 80 416WIRE 496 416 496 384WIRE 496 416 176 416WIRE 640 416 640 384WIRE 640 416 496 416WIRE 752 416 752 224WIRE 752 416 640 416WIRE 80 480 80 416FLAG 80 480 0FLAG 80 64 VccFLAG 496 64 Vcc/2SYMBOL res 160 80 R0SYMATTR InstName R1SYMATTR Value 1000SYMBOL res 160 288 R0SYMATTR InstName R2SYMATTR Value 1100SYMBOL npn 304 128 R270SYMATTR InstName Q1SYMATTR Value ZTX1048ASYMBOL res 480 80 R0SYMATTR InstName R3SYMATTR Value 1000SYMBOL voltage 80 288 R0WINDOW 3 24 104 Invisible 0WINDOW 123 0 0 Left 0WINDOW 39 0 0 Left 0SYMATTR InstName V1SYMATTR Value PULSE(0 30 0 .1)SYMBOL res 480 288 R0SYMATTR InstName R4SYMATTR Value 1000SYMBOL nmos 704 128 R0SYMATTR InstName M1SYMATTR Value IRF1312SSYMBOL res 688 48 R90WINDOW 0 0 56 VBottom 0WINDOW 3 32 56 VTop 0SYMATTR InstName R5SYMATTR Value 100SYMBOL voltage 640 288 R0WINDOW 3 24 104 Invisible 0WINDOW 123 0 0 Left 0WINDOW 39 0 0 Left 0SYMATTR InstName V2SYMATTR Value SINE(0 .25 1000)SYMBOL cap 624 224 R0WINDOW 0 -42 6 Left 0WINDOW 3 -36 38 Left 0SYMATTR InstName C1SYMATTR Value 10µTEXT 88 448 Left 0 !.tran .1-- JF

I'm sure Jim Thompson will approve.John

mosfets.---So, for an audience which you'd like to capture, you're trying to setup 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 admitto your errors before you start looking for control by castingaspersions on anyone else.------Nonsense. My circuits succeed or fail on their own merit, and I'd expect thatJim, or anyone else, would critique them for errors and, unless thecircuit was particularly clever, eschew responding to its functioningproperly. You, on the other hand, delight in subterfuge and cheating, and Inotice you've had nothing to say about my last circuit entered on thisthread.Care to comment?-- JF

mosfets.---Is this what has you flummoxed? ZTX1048A Q1 100Vcc>--+-----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.------From the OP's post, I understood that he wanted a drain _supply_ ofVcc/2 for a MOSFET amplifier, and a voltage divider he could connectto the MOSFET gate in order to set the quiescent bias point asrequired to do whatever he wanted.--- ---Only because it was replying to the amateurish (and incorrect)statements you made in the post immediately prior to my reply.Even though I've responded repeatedly to your false accusations in aneffort to quell them by correcting your misconceptions,...

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>--+-----------+--------+--------+---Haven't you been paying attention?I posted a sim earlier, and while what you say is true, it's what theOP wanted in order to try out various amplifiers (Q2) with a Vdd ofVcc/2 and an adjustable gate bias source, R3 and R4.Think of R3 and R4 as a pot and you'll see that it's easy to changeQ2's bias as Vdd changes.------You're using Jim as a hierarchical corroborating reference? Maybe you're finally beginning to see the light.------Close, but no seegar.The consensus was, originally, that the Vcc/2...

How to bias a CS mosfet, a different approach

J

John Larkin 14 years ago

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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J

John Fields 14 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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J

John Larkin 14 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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J

John Fields 14 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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StickThatInYourPipeAndSmokeIt 14 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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J

John Larkin 14 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:

----------

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%.

----------

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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J

John Larkin 14 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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Pomegranate Bastard 14 years ago

He/she won't try because he/she has no knowledge of electronics.

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J

John Fields 14 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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J

John Fields 14 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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J

John Larkin 14 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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J

John Fields 14 years ago

No, thanks.

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How to bias a CS mosfet, a different approach

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