# do any linear mosfets exist!

Discussion in 'General Electronics Chat' started by rougie, Dec 15, 2012.

1. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
hello,

I currently using the pn27000 ( I bielive the
part number is) bu the problem the
vin vs I out is exponential.

I was wondering if any mosfets exist
that are linear ???

thanks for all feedback

2. ### crutschow Expert

Mar 14, 2008
19,515
5,440
All MOSFETS have a exponential relationship between Vin vs Iout but you can obtain a linear relationship by adding an op amp in a constant-current configuration such as this.

#12 likes this.
3. ### #12 Expert

Nov 30, 2010
18,078
9,616
That is a really good example of how to use google to get exactly what you want.

4. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
hi crutschow,

I haven't had yet a chance to dive into op amps, but Q1
seems to be what would work for me. The exponential relation
complicates things. But if I may ask you a question
how does the op amp attenuate the voltage's input's
amplitude so that it makes the MOSFET linear ....???

Also, can I use an Lm324 as opposed to an OPA333?

I should really put some time in testing op amps !!!

anyways thanks so much for your help.

Last edited: Dec 15, 2012
5. ### #12 Expert

Nov 30, 2010
18,078
9,616
The basic concept of an opamp is that it does whatever is necessary to make both of its inputs equal. The opamp is responding to the voltage from the potentiometer and providing whatever is required to get its other input to the same voltage. In this case, the opamp is changing the voltage on the gate of the mosfet just enough to get it to pass enough current to get the right voltage across the load resistor.

6. ### nepdeep Active Member

Sep 14, 2011
140
0
Now if I may ask you a question, where do you want to use mosfet in linear region....can you share the schematic please??

7. ### crutschow Expert

Mar 14, 2008
19,515
5,440
The op amp doesn't really make the MOSFET linear, but it does make the output versus input linear as stated by #12. It uses negative feedback to make the non-linearity very small (basically divided by the open loop gain of the op amp).

Yes, an LM324 should work for this purpose.

8. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
After all the schematics i have posted and worked out
with this forum during the months of August, Sept and
Nov... I must admit I am a little exhausted.... You guys
have been a tremendous help and very patient with me
as I am not a very good hardware guy.

I know if I post the schematic, there will always be something
wrong with what I am doing. Right now though everything
works as I need it. So I would rather go on with what I have
in terms of hardware for now and continue my software
part of the project.

Rest assure though, I try every weekend to get an hour
or two in on some electronics theory.... and will get back
with schematics regarding my issues. For now not much
time in my hands for this though... and therefore I try my best...

r

Last edited: Dec 16, 2012
9. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
Very well explained #12. This gives me at least an idea
of what's happening. During christmas holidays I will
*try* to put some time in basic op amp theory...

thanks again!

10. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
okay.... thanks so much for your circuit... really appreciated

r

11. ### #12 Expert

Nov 30, 2010
18,078
9,616
Just a note...the first mistake noobies make with an opamp is in choosing the polarity of the output, and you choose that at the input. The opamp doesn't have the sense to know if it should hunt at a higher voltage output or a lower voltage output to get its inputs to equal each other. You are solely responsible for designing the feedback circuit and choosing whether the "+" input or the "-" input should be attached to the incoming signal. If you get it wrong, the output of the opamp will be "stuck" to one of the voltages you gave it from the power supply. Now, if you see this condition, you will know where to look.

12. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
thanks for the warning #12

r

13. ### MKCheruvu Member

Nov 20, 2012
30
6
A MOSFET when used as switch will be operated in cutoff(OFF) - Vgs<Vt
and Ohmic(ON)- Vgs>> Vt conditions . and when operated as Amplifier is operated in in current Saturation region - Ids = K{(Vgs-Vt) Square}/2 ie the Ids follows Square relation but in reality it gets raised to a value between 1 & 2.

As an Amplifier Ids = Gm. Vgs, ie a small change in Vgs(input) will be magnified linearly by Gm times (to realize Ids of the MOS FET Current.)
So a MOSFET when used as Amplifier(Ids=K Vgs) is used as Linear Element (with certain amount of built in nonlinearity due to second order effects)between input(vgs) and output (Ids)