# Homework Help: MOS Amplifier Design

1. Dec 28, 2012

### xortan

1. The problem statement, all variables and given/known data
I need to design a MOS voltage amplifier with a gain of 50.

2. Relevant equations
tox = 5 nm
u = 500 cm2/V-s
Cox = 6.903 mF/m
λ = 0.1
Vdd = 2.5V

3. The attempt at a solution

I am getting confused on just about everything...

I am to design a voltage amplifier with a gain of 50 V/V that is connected between a 100k source and a 2k load. We can use more than one stage so I was going to use one common-source with a gain of 10, second stage is a common-source with gain of 5 and the last was gonna be a source follower with a gain of 1.

I started out by picking a Vgs of 0.7 and a Vdsat of 0.2...don't know why it was just kinda arbitrary I guess.

I calculated my Id based off this equation Id = 1/2 * Cox * u * (W/L) * Vdsat2
I varied my W while keeping L constant. From my Id I was able to calculate my gm by using the equation 2*Id / Vdsat.

I know the equation for the gain of a common source amplifier is $\frac{Rg}{Rg + Rsig}$*$\frac{Rd}{1/gm + Rs}$ so I made my Rs 100 times larger than 1/gm and made my Rd 11 * (1/gm + Rs).

However when I enter the Rs and Rd values in LTSpice I am getting a drain current which is over 5 orders of magnitude away and its just really frustrating.

Please someone help! This thing is due in like 5 days and I've spent like 15 hours working on this and I can't even design a basic common-source amplifier. If you can provide links or write down some steps to follow in a design procedure that'd be great.