What happens if I keep increasing W/L ration of MOSFET

[dkdlek93]

What happens when I huge W/L mosfet in the single stage amp?

I think if I keep raising w/l, the total gain will be increased and the GBW will be reduced due to parasitic Cgs. Am i right? Also, I want to know what changes will occur if I actually use a huge w/l except for the size problem.

--- Updated Aug 2, 2021 ---

I mean when I use huge W/L. There was typo

 

[crutschow]

If you use the MOSFET as a switch, its on-resistance will also go down as the W/L ratio goes up.

 

[arthorios]

It really depends on what you want to achieve by using a large W/L.

You can end with a large input stage when you want lower 1/f noise. But you might also want to get more transconductance for a given current, which also leads to a large aspect ratio (depending on the current).

As you already mentioned, you will pay for it with a large input capacitance. This might limit the closed loop BW, and can potentially cause stability problems.

 

[andre_luis]

What happens when I huge W/L mosfet in the single stage amp

Although it's not clear what you meant by "single stage amplifier", you should know that an amplifier in the practical sense does not consist of just one MOSFET, but of several ones, each one playing a specific role at each stage of the circuit, so unless you specify exactly in what context this W/L ratio applies, the question might be considered somewhat vague.

 

[dkdlek93]

Although it's not clear what you meant by "single stage amplifier"

I meant single stage CS amplifier.

Does the intrinsic gain "gmro" continue to goes up if I keep increasing W/L in a single-stage CS amplifier? Is there any limit?

 

[Dominik Przyborowski]

Transconductance is limited to Id/nVt, so you will not be increasing your gain (MOS cannot be better than BJT) . What you can achieve is high intrinsic capacitance and thus lower bandwidth. There are other effects, too.

 

[junhao]

With the W/L increasing, the Cgs will increase, and you should consider the ft of the transistor. The ft will decrease due to the large W/L.