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Mosfet Current mirror, but???

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Terrano

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Hi, I have reed a few PDFs about mosfet current mirrors and they talk about:

https://www.electro-tech-online.com/custompdfs/2005/04/ch09.pdf -> page 4

µ
Cox
W
L
λ

Can't find the values for those constants in the datasheet of the mosfets that I bought.

How do I find the values for W, L, λ, µ and Cox?...in the PDF file they get a value of 700 for µCoxW/2L.

I just want to use 2 mosfets and connect them to get a mosfet current mirror.

when I try to calculate the R, I get across the constants mentioned above.

Thanks
 
Perhaps someone who is designing integrated circuits can give a better answer, but this is what I think: You can't make a current mirror with two discrete mosfets because the threshold voltage has to be identical for both. That is easy to do in an IC but you would have to measure a lot of mosfets to find two identical. The parameters you listed are for the integrated circuit mosfet, L = length, W = width, etc.
 
Terrano said:
in the PDF file they get a value of 700 for µCoxW/2L.

that's because µCoxW/L is known as Kn (for an NMOS that is... for a PMOS it's called Kp). if you look at the saturated mosfet equation, it is Id=(Kn/2)*(Vgs-Vt)^2*(1-λVds)
and Kn/2 is the value they are giving in the datasheet.

λ=1/Va (Va is the "early voltage")

the value µCox is also known as Kn' (prime)
therefore, Kn = Kn' * W/L

the (1-λVds) term is often left out for estimations.

in a current mirror, the Vgs values are the same (and typically you assume the Vt and Kn' values are the same also) so the current ratios depend only on W/L. if you include the last term, then the current ratios are dependent on W/L and the drain-source voltage.
 
I found a chip with mached pair...but they r quite expensive

Russlk -> "you would have to measure a lot of mosfets to find two identical"

Measure how?

Can't I just buy two mosfets and use them?

i.e if the two mosfets r the same ...they should have the same W & L

Is there a way of finding the value if Id wihtout the need of W & L?

Thanks
 
Those r surface mounted I need them bigger to be bale to place them on the BreadBoard.

Can't I just use 2 same mosfets?

How and what can I measure to see if the 2 mosfets r identical?

Thanks
 
Why do you need a current mirror? It seems to me that you would be much better off just using an opamp to be your current source.
 
you can get current-mirror like diodes, they behave like zeners BUT constant current
 
bmcculla -> "Why do you need a current mirror? It seems to me that you would be much better off just using an opamp to be your current source."

Because Mosfet or BJT current mirror is much smaller in size than an OpAmp

Styx - > "you can get current-mirror like diodes, they behave like zeners BUT constant current"

Never seen a current mirror using diodes...can u direct me to a webpage where I can see the circuit?

Thanks
 
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