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

 

Why do you think it's linear? You only have two data points!

 

Why do you think it's linear? You only have two data points!

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!

 

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.

 

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

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.

 

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

 

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.

 

yes I have seen that equation on various sites.

thanks for replying brownout.

 

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.

 

voltage". Is this the "gate-source" voltage on the transistor, or just the gate voltage relative to ground?

the gate voltage relative to ground?

 

the gate voltage relative to ground?

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?

 

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.