# Mosfet gives linear results???

Discussion in 'General Electronics Chat' started by rougie, Dec 20, 2012.

1. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
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2
Hello,

A simple thought!!!!

I am using the 2N7000 mosfet.

I am varying the input voltage from 2V to 4V which allows 0 to aproximately 35ma through the mosfet's drain to source.

Can it be that the variations of the input voltage vs output current relation is linear instead of exponential!!!

Can this happen due to the fact that I am using a very small part of the exponential curve in that to me it seems linear?

thanks for all feedback!
r

2. ### Brownout Well-Known Member

Jan 10, 2012
2,373
1,003
Why do you think it's linear? You only have two data points!

3. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
well, i am varying from 2-4V but within this range
I have logged down 8 coordinates for voltage/current. The graph seems to be
linear. Well okay I actually logged the measurements of voltage
to distance perceived by my sensor... (on a v/inches graph).
And this does not seem exponential at all!

4. ### Brownout Well-Known Member

Jan 10, 2012
2,373
1,003
You shold log the output current vs input voltage. There is no distance. You should not find an exponential function. The device should obey the square law for transconductance. Make your measurements from cutoff through the entire active region.

5. ### WBahn Moderator

Mar 31, 2012
23,396
7,100
You need to provide the circuit you are using. The behavior will be determined by the intersection of the MOSFET's loadline and the loadline of the rest of the circuit.

6. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
okay... then it's not a parabola... it's more of a square
so ignoring the load .... so to speak, for ex:

so 1v over the threshold = 1^2 ma = 1ma
so 2v over the threshold = 2^2 ma = 4ma
so 3v over the threshold = 3^2 ma = 9ma

and so on... is this correct?
thanks

Last edited: Dec 21, 2012
7. ### Brownout Well-Known Member

Jan 10, 2012
2,373
1,003
Someting like that. The output current is proportional to (vgs - vt)^2. It's equal to that times a constant factor in the active mode. There should be lots of sites that gives the equation.

8. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
yes I have seen that equation on various sites.

9. ### WBahn Moderator

Mar 31, 2012
23,396
7,100
When you gave your data you referred to the voltage as being the "input voltage". Is this the "gate-source" voltage on the transistor, or just the gate voltage relative to ground? It makes a huge difference, which is why it would be helpful for you to provide a sketch of your circuit.

10. ### rougie Thread Starter AAC Fanatic!

Dec 11, 2006
410
2
the gate voltage relative to ground?

11. ### WBahn Moderator

Mar 31, 2012
23,396
7,100
Voltage is always a measurement between two points. When you say that the gate voltage is 4V, what is that the voltage between? The gate and what other point in the circuit?

12. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
675
You need to post a schematic of how you are measuring the voltage AND current. Show the transistor, the power supply (with its voltage), the ammeter, and the voltmeter, and any other components in the circuit (resistors, etc.).
The make and model of your ammeter is also important.