Single-Stage Integrated- Circuit Amplifiers

Single-Stage Integrated- Circuit Amplifiers

Outline Comparison between the MOS and the BJT From discrete circuit to integrated circuit - Philosophy, Biasing, etc. Frequency response The Common-Source and Common-Emitter amplifier with active loads The Common-Gate and Common-Base amplifier with active load The Source and Emitter Follower The CS and CE amplifier with source degeneration Current mirrors with improved performance Cascode amplifier and transistor pairings

6- Comparison between the MOS and the BJT DC characteristics Transconductor (i D -v GS v.s. i C -v BE )

Channel-length modulation (i D -v DS ) v.s. Early effect (i C v CE )

Low-frequency operation Input resistance Transconductance g m W 2 ( µ ncox ) L I D I g m V C T Output resistance Intrinsic gain

High-frequency operation Cutoff frequency f For C f T T gm 2π ( C + C gs gs >> C.5µ V 2πL OV 2 gd gd and C ) gs 2 WLC 3 ox f For C f T T g 2π ( C π >> C 2µ V 2πW π T 2 m + C µ µ ) and C π C de

Trends in IC technology Technology speed figure of merit v.s. Time: Estimated frequency performance based on scaling

Comparison From analogue or digital point of view? DC range of operation Small-signal output resistance ( r o ) Cutoff frequency ( f T ) BJT >5 decades of I C e VBE Slightly larger 00GHz MOS 2-3 decades of I D (V GS V T ) 2 Smaller for short channel 50GHz Switch implementation Poor Good Noise (thermal about the same) Less /f More /f Capacitor implementation Voltage dependent Reasonably good

6-2 From Discrete- to Integrated- circuit Components (The pictures are copied from http://40.4.23.4/htdocs/course/922_ee226002/introduction.pdf, S.H. Hsu)

Hardware products (The pictures are copied from http://40.4.23.4/htdocs/course/922_ee226002/introduction.pdf, S.H. Hsu)

Design topologies Active load to replace R R? Current biasing Direct-coupled Differential architecture to decouple DC signals (Ch7)

IC Biasing current mirror (amplifier) I o / I REF Minimum V o Effect of r o :

Current steering

BJT current mirror I o / I REF Minimum V o Effect of r o : I I O REF m m + + β

Electronics II 6-3 Frequency response In analysing the frequency response of circuits, capacitors /sc inductors sl T(s) V i (s) V o (s) 0 0 ) ( b s b s a s a s a s T n n n m m m m + + + + + + L L Zeros : Poles : ) ( ) )( ( ) ( ) )( ( ) ( 2 2 n m m P s P s P s Z s Z s Z s a s T L L Substituting sjw into T(s) The gain and phase response v.s. w

Bode plot T ( s) T (s) a pole T ( s) s + p 0 a zero T ( s) s + z 0 How about positive-valued poles/zeros?

Example T 0s s) 2 ( + s /0 )( + s /0 ( 5 )

Example- continued T 0s s) 2 ( + s /0 )( + s /0 ( 5 )

High-frequency response Frequency response of a direct-coupled amplifier F H ( s) ( + s / z ( + s / p )( + s / )( + s / z2) L( + s / zn) p ) L( + s / p ) 2 n We are mostly interested in 3-dB frequency, f H (or f 3dB )

Electronics II Methods to derive f H If a dominant pole exists / ) ( P H w s s F + If not, use the cond. F(w H ) ½ to derive + + + + + + 2 2 2 2 2 2 2 2 Zn Z Z Pn P P H w w w w w w w L L Open-circuit time constants i io i H C R w R io : resistance seen by C i when all other Cs are open-circuit and all sources equal zeros

Example 6.6

Miller s theorem one useful technique for analysing freq response

Example 6.7

6-4 Common-Source and Common-Emitter amplifers with acitive loads Common-source amplifier R R vo i A o

Common-source amplifier with active load

Large-signal characteristics Small-signal characteristics (in region III) R A R in v out

High-frequency response (i) Analysis by Miller s Theorem

(ii) Analysis by Open-circuit time constants

(iii) Exact analysis

Formulas for Common-Emitter amplifier with active load