CSE 598 AEE 597 G Spring 2006 Basic

CSE 598 A/EE 597 G 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 10/30/2020 Insoo Kim

Contents n Fundamentals n Basic Amplifiers: Low Frequency Analysis n Basic Amplifiers: High Frequency Analysis n Differential Amplifier n Feedback 10/30/2020 Insoo Kim

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

Definitions mb 10/30/2020 Insoo Kim

DC Operating Point & Load Line 10/30/2020 Insoo Kim

Large Signal Analysis vs. Small Signal Analysis n Large Signal Analysis 10/30/2020 Insoo Kim

Large Signal Analysis vs. Small Signal Analysis n Small Signal Analysis How convenient !! 10/30/2020 Insoo Kim

MOSFET Intrinsic Capacitances 10/30/2020 Insoo Kim

(cont’d) MOSFET Intrinsic Capacitances 10/30/2020 Insoo Kim

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

Single Stage Amplifiers: CS, CD, and CG Stage 10/30/2020 Insoo Kim

Common Source Stage : Voltage Gain 10/30/2020 Insoo Kim

Common Drain Stage: Output Resistance 10/30/2020 Insoo Kim

Common Gate Stage : Input Resistance 10/30/2020 Insoo Kim

Summary 10/30/2020 Insoo Kim

Quiz n 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? 10/30/2020 Insoo Kim

Loads for basic amplifiers 10/30/2020 Insoo Kim

(cont’d) Loads for basic amplifiers n Diode Connected Load 10/30/2020 Insoo Kim

(cont’d) Loads for basic amplifiers n Source degeneration 10/30/2020 Insoo Kim

Cascode Stage n Small Signal Analysis n Rout 10/30/2020 Insoo Kim

Folded Cascode Stage 10/30/2020 Insoo Kim

(cont’d) Folded Cascode Stage n What are the advantages of folded cascode amplifier? n Disadvantages: Ø Ø Ø Limited Output swing Large Voltage Headroom Large Power Consumption 10/30/2020 Insoo Kim

Basic Amplifiers: High Frequency Analysis q q Frequency Analysis Dominant Pole Approach

Frequency Analysis 10/30/2020 Insoo Kim

(cont’d) Frequency Analysis n Bode Plot 10/30/2020 Insoo Kim

Dominant Pole Approach 10/30/2020 Insoo Kim

BW Estimation by Dominant Pole Approach 10/30/2020 Insoo Kim

Bandwidth Comparison 10/30/2020 Insoo Kim

Quiz n Design an amplifier which satisfy following features using basic single-stage amplifiers. Ø Ø High gain Large Bandwidth High input impedance Low output impedance 10/30/2020 Insoo Kim

Differential Amplifier q q Single Stage Amplifiers Multi Stage Amplifiers

Why differential Amplifier? n Single Ended Signal can be easily contaminated n A Differential Signal can be cleaned up n Power Supply noise can be reduced 10/30/2020 Insoo Kim

Differential Amplifier Analysis n Classic Diff Amp 10/30/2020 Insoo Kim

(cont’d) Differential Amplifier Analysis 10/30/2020 Insoo Kim

Diff Amp with Current Mirror Load Common Mode Input Voltage Range VSS+VTN 1+VDSAT 5+VDSAT 1 < VIC < VDD–|VDSAT 3|–|VTP 3|+| VTN 1| 1. What is CM Input Voltage? 2. How do we prove this equation? 10/30/2020 Insoo Kim

(Std. Library) Design Exercise n Design Flow Ø Determine Specifications - Ø Ø Determine minimum channel length Determine channel width - Ø Ø Power Consumption (ex. 1 m. W) Voltage Gain (ex. >30) Active Common Mode Input range (as large as possible) Others: slew rate, CMRR, PSRR, etc. Determine W 1, 2 from voltage gain spec. Determine W 5 & Bias Voltage from power consumption & CM min. Determine W 3, 4 from CM max. Determine Bias Level of current source tr. Check other specifications 10/30/2020 Insoo Kim

Feedback q q q Feedback & Stability Voltage Amplifier Model Common Mode Feedback

Feedback & Stability 10/30/2020 Insoo Kim

Voltage Amplifier Model n Models 10/30/2020 Insoo Kim

(cont’d) Voltage Amplifier Model n 1 st Order Model 10/30/2020 Insoo Kim

(cont’d) Voltage Amplifier Model n 2 nd Order Model 10/30/2020 Insoo Kim

(cont’d) Voltage Amplifier Model n Time Response of the 2 nd Order Model 10/30/2020 Insoo Kim

(cont’d) Voltage Amplifier Model 10/30/2020 Insoo Kim

Feedback Characteristics n n Gain desensitization Band width extension 10/30/2020 n Noise Reduction n Non-linearity Reduction (a) w/o feedback (b) w feedback Insoo Kim

Common Mode Feedback n Why is CMFB circuit needed? Ø Ø n 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 10/30/2020 Insoo Kim

(cont’d) Common Mode Feedback n Examples of CMFB Folded cascode amplifier with CMFB 10/30/2020 Useful for low gain applications Insoo Kim

References n n n Joongho Choi, “CMOS analog IC Design, ” IDEC Lecture Note, Mar. 1999. B. Razavi, “Design of Analog CMOS Integrated Circuits, ” Mc. Graw-Hill, 2001. Hongjun Park, “CMOS Analog Integrated Circuits Design, ” Sigma Press, 1999. 10/30/2020 Insoo Kim