Basic Amplifiers and Differential Amplifier 9/21/2018 CSE598A/EE597G Spring 2006 Basic Amplifiers and Differential Amplifier Insoo Kim, Kyusun Choi Mixed Signal CHIP Design Lab. Department of Computer Science & Engineering The Penn State University

Don’t let the computer think for you In today’s analog design, simulation of circuits is essential because the behavior of short-channel MOSFETs cannot be predicted accurately by hand calculations. Nonetheless, if the designer avoids a simple and intuitive analysis of the circuits and hence skips the task of gaining insight, then he/she cannot interpret the simulation results intelligently. For this reason, we say, “Don’t let the computer think for you.” - Behzad Razavi 9/21/2018

Contents Fundamentals Basic Amplifiers: Low Frequency Analysis Basic Amplifiers: High Frequency Analysis Differential Amplifier Feedback 9/21/2018

Fundamentals Definitions DC Operating Point & Load line Large Signal Analysis vs. Small Signal Analysis MOSFET intrinsic Capacitances

Definitions mb 9/21/2018

DC Operating Point & Load Line 9/21/2018

Large Signal Analysis vs. Small Signal Analysis 9/21/2018

Large Signal Analysis vs. Small Signal Analysis How convenient !! 9/21/2018

MOSFET Intrinsic Capacitances 9/21/2018

(cont’d) MOSFET Intrinsic Capacitances 9/21/2018

Basic Amplifiers: Low Frequency Analysis Single Stage Amplifiers Multi Stage Amplifiers

Single Stage Amplifiers: CS, CD, and CG Stage 9/21/2018

Common Source Stage : Voltage Gain 9/21/2018

Common Drain Stage: Output Resistance 9/21/2018

Common Gate Stage : Input Resistance 9/21/2018

Summary 9/21/2018

Quiz CD stage amplifier is suitable for output stage of OPAmp due to its low output impedance and large bandwidth. However, in CMOS analog IC, CS stage is more widely used for output stage OPAmp than CD stage. Why? 9/21/2018

Loads for basic amplifiers 9/21/2018

(cont’d) Loads for basic amplifiers Diode Connected Load 9/21/2018

(cont’d) Loads for basic amplifiers Source degeneration 9/21/2018

Cascode Stage Small Signal Analysis Rout 9/21/2018

Folded Cascode Stage 9/21/2018

(cont’d) Folded Cascode Stage What are the advantages of folded cascode amplifier? Disadvantages: Limited Output swing Large Voltage Headroom Large Power Consumption 9/21/2018

Basic Amplifiers: High Frequency Analysis Dominant Pole Approach

Frequency Analysis 9/21/2018

(cont’d) Frequency Analysis Bode Plot 9/21/2018

Dominant Pole Approach 9/21/2018

BW Estimation by Dominant Pole Approach 9/21/2018

Bandwidth Comparison 9/21/2018

Quiz Design an amplifier which satisfy following features using basic single-stage amplifiers. High gain Large Bandwidth High input impedance Low output impedance 9/21/2018

Differential Amplifier Single Stage Amplifiers Multi Stage Amplifiers

Why differential Amplifier? Single Ended Signal can be easily contaminated A Differential Signal can be cleaned up Power Supply noise can be reduced 9/21/2018

Differential Amplifier Analysis Classic Diff Amp 9/21/2018

(cont’d) Differential Amplifier Analysis 9/21/2018

Diff Amp with Current Mirror Load Common Mode Input Voltage Range VSS+VTN1+VDSAT5+VDSAT1 < VIC < VDD–|VDSAT3|–|VTP3|+| VTN1| 1. What is CM Input Voltage? 2. How do we prove this equation? 9/21/2018

(Std. Library) Design Exercise Design Flow Determine Specifications Power Consumption (ex. 1mW) Voltage Gain (ex. >30) Active Common Mode Input range (as large as possible) Others: slew rate, CMRR, PSRR, etc. Determine minimum channel length Determine channel width Determine W1,2 from voltage gain spec. Determine W5 & Bias Voltage from power consumption & CM min. Determine W3,4 from CM max. Determine Bias Level of current source tr. Check other specifications 9/21/2018

Feedback & Stability Voltage Amplifier Model Common Mode Feedback

Feedback & Stability 9/21/2018

Voltage Amplifier Model Models 9/21/2018

(cont’d) Voltage Amplifier Model 1st Order Model 9/21/2018

(cont’d) Voltage Amplifier Model 2nd Order Model 9/21/2018

(cont’d) Voltage Amplifier Model Time Response of the 2nd Order Model 9/21/2018

(cont’d) Voltage Amplifier Model 9/21/2018

Feedback Characteristics Gain desensitization Band width extension Noise Reduction Non-linearity Reduction w/o feedback w feedback 9/21/2018

Common Mode Feedback Why is CMFB circuit needed? Due to TR mismatch, TRs may not be in saturation region at operating point.  DM Gain decreases and CM gain increases Since output CM level is sensitive to device properties and mismatches, it cannot be stabilized by means of differential feedback. General Topology of CMFB Circuit 9/21/2018

(cont’d) Common Mode Feedback Examples of CMFB Folded cascode amplifier with CMFB Useful for low gain applications 9/21/2018

References Joongho Choi, “CMOS analog IC Design,” IDEC Lecture Note, Mar. 1999. B. Razavi, “Design of Analog CMOS Integrated Circuits,” McGraw-Hill, 2001. Hongjun Park, “CMOS Analog Integrated Circuits Design,” Sigma Press, 1999. 9/21/2018