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How to bias a CS mosfet

--- Well, let's just take a look at who understands what, OK?

Here's his post, verbatim:

"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%."

Here's John S's original circuit, responding to the first part of the argument, with the MOSFET the OP was talking about changed out for a bipolar, the cap from the + input of the opamp to ground deleted, and fleshed out with my component values:

0-30VDC>--+-------+-----------+-->Vcc | | |1k |10k | [R4] [R1] | | | +--|-----------+-->Vcc/2 | | | | +----|-|-\ 1k C | | | >--[R3]--B Q1 | +-|+/U1 E 2N3904 [R2] |LT1037 | |10k | [R5] | | |510 GND>------+-------+-----------+

Notice that the OP was asking for something which would track Vcc and output Vcc/2 from the drain of the MOSFET (the collector of the bipolar) and you might see that the circuit above does exactly that from Vcc > ~5V.

The remainder of the OP's post is rather cryptic:

"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."

and leads me to believe that we've been trolled.

-- JF

Reply to
John Fields
Loading thread data ...

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

Actually, Jim, it appears that the Out terminal varies more than 9% as the supply voltage varies 20% (10% each side of 12V). Is 9% considered a "few" percent?

Cheers, John S

Reply to
John S

A 2N3904 is not a mosfet. And it's not a usable starting point for an amplifier. If all you want is Vcc/2, there are easier ways.

Perfectly clear, not cryptic at all. He wants to _bias_an_amplifier_

It leads me to believe that you don't know what's going on.

It's simple: he wants to bias a mosfet amplifier such that the "Q point", the quiescent bias point, has the drain at Vcc/2. That optimizes the clipping range.

His first suggestion was a voltage divider from Vcc to the gate, an a source resistor. That works fairly well; it's the classic toob amp bias scheme. Other circuits could set the drain voltage more precisely to Vcc/2, and as an added benefit not waste voltage in the source resistor.

John

Reply to
John Larkin

It is to an amateur.

John

Reply to
John Larkin

"Chided" usn't exactly "dispassionate, and logical." Especially when the problem is your inability to visualize circuits. There aren't a lot of ways to connect the parts he listed.

Since you couldn't visualize from his description, you might have

*asked* him to explain more, or to post a schematic or netlist. But you had to "chide", so naturally he didn't. So then you escalated. You must be a real pain to be around in real life.

Just count the lines.

John

(snipped the usual scores of lines of crude off-topic blather)

Reply to
John Larkin

Great idea, but I modestly note that I suggested it on August 6.

John

Reply to
John Larkin 

--
No kidding???

Do you even know the reason for the substitution?
Reply to
John Fields 

--
Since you're making the accusation, the burden of proof falls on you.
Reply to
John Fields

Finally we get to the heart of the matter: it isn't. Even Thompson understands that.

No, no, no. Totally wrong. Spice it if you don't believe me.

John

Reply to
John Larkin

Illiterate Johnny Larkin, as usual, needs his hand held as his capacity to comprehend what he reads is only 4th grade level, if that.

He was in a conversation WITH JF. From within that, he addressed YOU.

Grow the f*ck up. Like he really cares what your "answer" would be.

Reply to
Naomi Price

What makes you think this is true, idiot?

You are not even qualified to make any such conclusion. You know nothing of the ability of ANY of the participants of the thread to 'visualize' a circuit segment. You are pretty goddamned presumptuous to think you can/could.

You are so stupid that you draw stupid conclusions, then live by them as if they are fact. When you could not be more wrong. That is one of your biggest problems, Johnny.

The other one is that the maturity level of the brain with which you pull this stupid shit is that of a ten year old.

Reply to
StickThatInYourPipeAndSmokeIt

Must confess, I wasn't really paying attention - JF and his proto hominid gang have really lowered the tone.

Reply to
Ian Field 

--
That's a gratuitous jab since the OP hasn't been back to resolve the
ambiguities in his post but, expected since you'll take every
opportunity to insult unwarrantedly.
Reply to
John Fields

Probably stayed away in case you start on him next!

Reply to
Ian Field 

--
Repetition isn't validation.
Reply to
John Fields

It's not unwarranted: you are wrong. You insult without any reason at all.

I use Spice all the time. But no sensible person needs Spice to see how awful your idea is.

All you did was manually tweak the gate voltage until you got the Vout you wanted. That's really lame. It won't work if Vcc changes or if temperature changes, and it sure wouldn't work in production, with different fets. Heck, this is even worse than the mess Thompson posted. He, at least, sort of understands feedback.

The sensitivity to Vcc is extreme here. The whole point was to not have such sensitivity.

Change R1 by a tenth of a per cent and see what happens!

Ask nicely and I'll post a couple. I've already described them enough for most people.

John

Reply to
John Larkin

The netlist JF just posted is a textbook-worthy example of people who don't understand electronics doing stupid things by fiddling with Spice. Even worse than Thompson's hairball.

One giveaway is the oddball set of resistor values that were hand-tweaked to 4 decimal places. Change one of the divider resistors by 0.5% and the whole thing collapses.

John

Reply to
John Larkin

Here is John Larkin telling the world how civil and mature he is, by describing one of his insults as "warranted", and further justifying it by claiming that someone else nearby commits such acts "without warrant".

Do you know how much of a child you appear as, John Larkin?

Reply to
StickThatInYourPipeAndSmokeIt 

--
It wasn't my idea, I just posted a simple circuit to do what you asked
for.
Reply to
John Fields

Yep, That's what John L said, a divider on the gate. I let it go... Now he's attacking his own stupid scheme [shaking head]

Without a doubt Larkin is one of the most ignorant posters on this group... yet narcissistic about it.

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

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