CMOS Analog IC Review Part I

Index                                    
1. Introduction                                            
2. MOS Device Physics                     
   2.2. MOS I/V Characteristics              
   2.3. Second-Order Effects                 
   2.4. MOS Device Models                    
3. Single-Stage Amplifiers
   3.3. Common-Source Stage
   3.4. Source Follower
   3.5. Common-Gate Stage
   3.6. Cascode Stage
4. Differential Amplifiers
   4.2. Basic Differential Pair
5. Current Mirrors
6. Frequency Response of Amplifiers
7. Noise

1. Introduction

This is a review on analog IC design mostly based on the book Design of Analog CMOS Integrated Circuits by Behzad Razavi, hence the index follows the same order as the chapters in the book, and most figures and text screenshots are directly from the book. Some contents are omitted for a concise review.

2. MOS Device Physics

2.2 MOS I/V Characteristics

Below is a figure showing the basic structure of a NMOS on a p-substrate.

Suppose source is grounded. As gate voltage V_GS increases, holes in the p-sub are repelled from gate area to create a depletion region. When V_GS reaches the threshold voltage V_TH and a positive voltage is applied on VDS, electrons flow from source to the drain and form a channel. In an NMOS,

When V_DS ≤ V_GS – V_TH, the channel spans the full length between source and drain. The MOSFET is in triode region, and the drain-to-source current I_D is given by:

I_D = μ_nC_ox(W/L)[(V_GS – V_TH)V_DS – V_DS²/2]

The drain-to-source resistance R_DS, or R_on, is given by:

R_on = 1/[μ_nC_ox(W/L)(V_GS – V_TH)]

Whereas for V_DS > V_GS – V_TH, the channel is pinched off and does not span the full length. The MOSFET enters saturation region, and I_D is given by:

I_D = (1/2)μ_nC_ox(W/L’)(V_GS – V_TH)²

Here, L’ typically remains close to L, it can be observed that I_D is relatively independent of VDS.

Transconductance (usually defined in the saturation region) is defined as

g_m = ∂I_D/∂V_GS = μ_nC_ox(W/L)(V_GS – V_TH) = √[2μ_nC_ox(W/L)I_D] = 2I_D/(V_GS – V_TH)