Help with current mirror simulation

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sjamil02

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Hi all,

I am running a basic current mirror circuit as shown in the attachment. Reference current is set to ~ 24uA. To set the output current as twice the reference, I need to increase W3/L3 twice bigger than W1/L1. Here's the problem.

In my simulation, W1/L1=3u/3u, M=0, I set W3/L3=6u/3u, M=0. Refer to waveform2 --> Io = 80uA #

Now with same setup, W1/L1=3u/3u, M=0 and I set W3/L3=3u/3u, M=2. Refer to waveform1 --> Io = 50uA # (Close to expected value).

Why there is a discrepancy between those two simulation. My understanding is W3/L3=6u/3u , M=0 same with --> W3/L3=3u/3u , M=2 .......Is it not?

Im confused!

Appreciate any explanation.

cheers
sj
 

Hi,

I am not sure if your M=0 causes the problem. Try replacing it with M=1. Doubling the size makes M=2.

Hope this helps.
 

I am not sure if your M=0 causes the problem. Try replacing it with M=1. Doubling the size makes M=2.
Click to expand...

--> W1/L1=3u/3u, M=1 and W3/L3=6u/3u, M=1 ---> Io= ~80uA

--> W1/L1=3u/3u, M=1 and W3/L3=3u/3u, M=2 ---> Io= ~50uA

Still the same. It looks that the discrepancy came from setting M?

sj[/quote]
 

M=2 is not the same as doubling the width of the transistor (and I am not sure about the idea of using M=0 - unity is more normal).

M=2 assumes two identical transistors are put in parallel - you should be able to confirm that in a simulation. However ift here is a width reduction in the process so 3um drawn ends up as 2.5um, using two in parallel looks like 5um wide wheras drawing 6um wide would result in a 5.5um wide transistor. I hope that makes sense!

What process are you using?

Keith.
 

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    sjamil02

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Hi Keith,

Thanks. Yes, you're right. I confimed it in my simulation.

I am using 0.5um technology (level=7) for academic purpose only.
Given in model file, Wint=7.56E-7 and Lint=2.67E-7

Weff=Wdrawn-(2*Wint) and Leff=Ldrawn-(2*Lint)

For Wdrawn/Ldrawn =3um/3um=1 --> Weff/Leff=3um-(2*Wint)/3um-(2*Lint)=0.6


Thanks for your encouraging feedback. I just press helped me button.

cheers
sj
 

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