Choosing right W/L ratio

[navi_s]

Hello all,

I'm novice in Analog circuit design, and I have one (could be a stupid question)
So how to choose correct W/L ratio when design circuit?
At example if the calculation says that we need ratio of W/L=2, how to take a decision for the ratio. Should it be, 4u/2u or 8u/4u and etc.?

Thx

 

[viperpaki007]

Well that depends on the application. For example, if you are designing a switch from MOS transistor, which can switch realy fast then using a smaller gate length will be better. However, in opamp design, if you want higher gain you will like to have bigger gate length to ensure higher output resistance. There can be so many other examples and which value of W/L ratio should be chosen depends on the application.

 

[nitishn5]

The actual value of W and L would depend on a bunch of things depending on the application,

The Area i.e. W*L might be critical.
If you want less area and less capacitance, use the minimum L otherwise go for higher L.
If you want less mismatch between devices in a current mirror for example, you would need bigger area and hence larger L.
If you want to reduce the flicker noise which is inversely proportional to area, go for higher L.

If you want higher impedance i.e. ro/gds go for higher L.
If you want good Gm, faster speed, go for lower L.

IT is usually a trade-off between a bunch of things and depends a lot on your requirement.

 

[an_82]

Hello every one,
I have a question about a symbol.
We know that, complementary of a digital signal is shown by bar on that signal.
How about an analog signal. Suppose that we have a reference current of Iref, the output current is Iout=out1. Now we would like to have Iref-Iout. How can we display this value with respect to out1. May we mention by bar(out1 bar)?

Best regards,
an