CSE 577 Spring 2011 Basic Amplifiers and Differential Amplifier, 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
Contents Fundamentals Basic Amplifiers: Low Frequency Analysis Basic Amplifiers: High Frequency Analysis Differential Amplifier Feedback
Fundamentals Definitions DC Operating Point & Load line Large Signal Analysis vs. Small Signal Analysis MOSFET intrinsic Capacitances
Definitions mb
DC Operating Point & Load Line
Large Signal Analysis vs. Small Signal Analysis Large Signal Analysis
Large Signal Analysis vs. Small Signal Analysis Small Signal Analysis How convenient!!
MOSFET Intrinsic Capacitances
(cont d) MOSFET Intrinsic Capacitances
Basic Amplifiers: Low Frequency Analysis Single Stage Amplifiers Multi Stage Amplifiers
Single Stage Amplifiers: CS, CD, and CG Stage
Common Source Stage : Voltage Gain
Common Drain Stage: Output Resistance
Common Gate Stage : Input Resistance
Summary
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?
Loads for basic amplifiers
(cont d) Loads for basic amplifiers Diode Connected Load V I X X = 1 gm+ gmb+ r o g m 1 + g 1 mb 1 g m = g m 1 + g mb r o R X A v = g m1 1 g m 2 ( W / L) ( W / L) 1 2
(cont d) Loads for basic amplifiers Source degeneration G m g 1+ g m m R S R out = R r o S [( g [ R S m2 ( g + g m2 mb2 + g ) r o mb2 + 1] + r ) + 1] o
Cascode Stage Small Signal Analysis V A out v = ( Rout RD ) gm 1Vin = g ( R R ) m 1 out D Rout R out = r o1 r o2 [( g [ r o1 m2 ( g + g m2 mb2 + ) r g o2 mb2 + 1] + r ) + 1] o2
Folded Cascode Stage A= g R m1 o R = R R ω D t o = = o2c ω = Aω [ gm2cro2c( ro2 ro7) ] [ gm4cro4cro3] 1/ ( C R ) D L SR= 2I/C L o = g o4c m1 /C L
(cont d) Folded Cascode Stage What are the advantages of folded cascode amplifier? Disadvantages: Limited Output swing Large Voltage Headroom Large Power Consumption
Basic Amplifiers: High Frequency Analysis Frequency Analysis Dominant Pole Approach
Frequency Analysis
(cont d) Frequency Analysis Bode Plot
Dominant Pole Approach
BW Estimation by Dominant Pole Approach
Bandwidth Comparison
Quiz Design an amplifier which satisfy following features using basic single-stage amplifiers. High gain Large Bandwidth High input impedance Low output impedance
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
Differential Amplifier Analysis Classic Diff Amp
(cont d) Differential Amplifier Analysis
Diff Amp with Current Mirror Load G R A m out v = = g r g m2,4 o2 m2,4 r o4 ( r o2 r o4 ) CMRR= (2g r m1 o5 = CMRR( R ) g m3 ( r o1 r load) g o3 m3 ) ( r Common Mode Input Voltage Range V SS +V TN1 +V DSAT5 +V DSAT1 < V IC < V DD V DSAT3 V TP3 + V TN1 o1 r o3 ) 1. What is CM Input Voltage? 2. How do we prove this equation?
(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 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
Feedback Feedback & Stability Voltage Amplifier Model Common Mode Feedback
Feedback & Stability
Voltage Amplifier Model Models
(cont d) Voltage Amplifier Model 1 st Order Model
(cont d) Voltage Amplifier Model 2 nd Order Model
(cont d) Voltage Amplifier Model Time Response of the 2 nd Order Model
(cont d) Voltage Amplifier Model
Feedback Characteristics Gain desensitization A f da da A f x x f f o s A = 1+ βa da = (1+ βa) 2 1 da = 1+ βa A Band width extension AM A(s) = 1+ s/ ω A f H A(s) (s) = 1+ βa(s) AM /(1+ βam ) = 1+ s / ω (1+ βa H M ) Noise Reduction S = N V o S N V V s n A1A2 = Vs 1+ βa A Vs = V n A 2 1 2 + V n A1 1+ βa A Non-linearity Reduction (a) (b) w/o feedback w feedback 1 2
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
(cont d) Common Mode Feedback Examples of CMFB Folded cascode amplifier with CMFB Useful for low gain applications A v = g ( ro 1,2 ro 3, 4 R m1,2 F )
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.