MOSFET drive circuitry

Base resistor is there incase the transistor came out of compliance if the 12v supply voltage drops below the vbe of the transistor.

What's the 22k for?

The 22k is there because you have (3v3 -Vbe)/22k = current through the 100k. I * 100k = voltage across the 100k.

Why not dispense with the 22k resistor...the circuit seems to work just the same without it. If you want to save on current usage you could adjust the remaining resistors: http://everycircuit.com/circuit/5411208621981696

@2ctiby you will find a typical P MOSFET will have a Vgs of -10v and a maximum Vgs of -20v the 22k is needed if the supply voltage is increased to a voltage greater than the max Vgs of the MOSFET. But yea you are correct, at 12v supply you can have a Vgs of approximately the supply voltage.

The Vgs means the voltage between gate and source. In a circuit such as this here, the Vgs could only ever be between 0 and Vsource. ... so Vsource can be expected to be greater than Vgs. If a datasheet stipulates that the max Vgs is 20 difference, then it is saying in effect that Vsource must not exceed 20v. So...if the Vsource exceeds 20v then the mosfet is damaged. The reason for stating that as Vgs max instead of stating it as a straightforward Vsource is to account for circuits which may have a secondary source which would circumvent a straightforward statement. Since this circuit here is straightforward with no complexities such as a secondary source, then we can read the datasheet as meaning...don't use a source of more than 20v. Your shown circuit would be fine by using no resistor in that emitter position up to the stipulated datasheet 20v mosfet source. (ie -20v Vgs)

Using the circuit with a source voltage higher than the MOSFETs max Vgs of 20v and putting the transistor in a switch topology; The Vgs will see the full source voltage, which is beyond the FETs rating. Using a current limit (common emitter) such as in my circuit, will limit the voltage drop across the 100k. VGs will never be greater than -10v. This will allow you to use it with a greater source voltage.

Try adjusting the supply voltage and watch as the Vgs remains constant across the supply range of 10-60v - assuming this FET has a Vds max rating of -60v. I appreciate the feedback. Thanks

@ryanstephen86, nice example! Mr 2cent can't leave its fingers from circuits which are much ahead of its knowledge. The bad about is, he's writing long lasting questions without question mark. So one get the impression he knows whats he is writting but unfortunately he does not. Keep it up!

Thank you @hurz

I have read and appreciate what you were pointing out ryan. As for the insulting Hurz, he is an obnoxious know all who is determined to prevent everyone from having a voice. It is a full time job trying to avoid being like him...it would be very easy to insult and swear like he does, but I would really prefer not to, so some of us have to keep suffering his maniacal outbursts.

Any questions 2cent?

Believe it or not Hurz, some of us that you have offended would still really like to help you. If you can't accept that then all I can suggest is that you take heed of one that is in deeper touch with you..perhaps bugsy...good luck.

A bit more respect for @ryanstephen86. Just look on your keyboard for this sign "?" Keep up guessing and messing!

My respect was shown in real terms showing a willingness to learn and accept another's view (something you never do): http://everycircuit.com/circuit/6151526845775872

Good to see you did that 2 days after your initial blabla messing.

We debate sensibly without insults, putting our opinions forward, then we go away an reflect. If we eventually consider in all fairness that our opinions were lacking an aspect which we had overlooked, then we admit it and both go away laughing together. The next time it could easily be the other person who turns the table. Such is standard academic banter. There is no need for insults etc, it should all be fun together. I would like to invite you in to this pattern which has so far escaped you, but you will need to make the decision to do a full turn around (you can call it your 'delta pie radian shift' if you like). How about it?..

Dont you see there is no debate at your unlisted links? Not even one comment! You should think about why.

Now you can help me here Hurz...What do you mean by unlisted? You have me wondering if my links are only showing on my own computer or something. I usually give exlanations with any schematic that I link.

Unlisted is readable for anybody who got the link. But you cant see it in tab "public". Its not listed

I may be doing something wrong with my EC linking. Could you do me a favour please and follow this thread to the last reply of mine to rich. I demonstrated a transconductance alteration (without actually saying so there) http://everycircuit.com/circuit/5608978322554880 Are my notes on that schematic visible where I stated "can you spot the different settings".

Nothing wrong with your linking

Many thanks. If you happened to notice errors of mine anywhere along that thread then I'll try to correct them (I could be in for a long haul....you know what I am like with some of my stupid ideas).