ECEN474: (Analog) VLSI Circuit Design Fall 2012

Transcription

1 ECEN474: (Analo) VLSI Circuit Desin Fall 0 Lecture 9: Output Staes Sam Palermo Analo & Mixed-Sinal Center Texas A&M University

2 Announcements & Aenda Exam 3 is on Friday Nov. 30 Output Staes Source Follower (Class A) Push-Pull (Class B) Push-Pull w/ Small Quiescent Current (Class AB)

3 OpAmps and OTAs OpAmp OTA Hih voltae ain Hih input impedance Voltae source output (low impedance) Hih voltae ain Hih input impedance Current source output (hih impedance) 3

4 Three-Stae OpAmp Av > Av >> Av = V i- V i - V o Differential input stae Amplifies differential input Sets specs such as G m, CMRR, and slew rate Second ain stae Provides additional ain Often used to provide Miller compensation Output stae Power Amplifier Input Stae Lare current ain and near unity voltae ain Small output impedance Gain Stae Output Stae 4

5 Buffered OTA = Operational Voltae Amplifier (OPAMP) ELEN-474 VDD SIMPLE MACROMODEL: M M i 0 i0 i0 i 0 M 3 R C CM i 0 M 4 v 0 v v - m (v -v - ) R v y C( m3 r o3 )CM r 0 A A 3 (v y ) v out v d -v d M M IB IB IB3 VSS A VDC ω p GBW ω p ω A A A VDC 3 GBW Internal pole = ( R ) m3 r m o3 0.5 C m M A nd Stae & Buffer A Rout = r0 / m4 Notice that load capacitors must satisfy the followin condition, otherwise phase marin is not ood enouh GBW< r 0 C L 3 Jose Silva-Martinez -5- Texas A&M University

6 Source Follower (Class A) Output Stae 6 m o mb o m out L m L m L o mb o m m dc R R R A = = c) (Optimisti Voltae ain close to Low output resistance DC level shift of V GS Class A output stae transistors conduct current over an entire input cycle [Gray]

7 Source Follower (Class A) Transfer Characteristic V o = V i V T I µ C n Q ox V R o L ( W L) [Gray] Maximum V o If V in V DD, then Maximum V o = V DD -V GS Output transistors remain in saturation up to V o = V DD -V DSAT if V in swins up to V DD V T Minimum V o For small R L (heavy load), M ets cutoff and V o -I Q R L For lare R L (liht load), M will o into triode reion at V DD V DSAT 7

8 Source Follower (Class A) Power Efficiency Assumin a sinusoidal output voltae Vo = Vm sin( ωt) The power delivered to the load at the sinal frequency ω is Vm Vm Pac = = RL RL The averae power consumed by the source follower is Pav = ( IQ )( VDD ) ( IQ )( VDD ) = IQVDD The output stae power efficiency is Pac Vm Peff = Pav 4RLIQVDD Maximum power efficiency is achieved when the output VDD amplitude approaches VDD and IQ is desined to be RL Max Peff = or 5% (not that ood!) 4 8

9 Super Buffer Output Stae [Silva] R out m ( A ) v 9

10 Push-Pull Source Follower (Class B) Output Stae Class B output staes improve power efficiency by operatin at zero quiescent current However, if - V TP V in V TN, then no output sinal 0

11 Push-Pull Source Follower (Class B) Crossover Distortion [Sedra]

12 Push-Pull Source Follower (Class B) Power Efficiency Assumin a sinusoidal output voltae Vo = Vm sin( ωt) The power delivered to the load at the sinal frequency ω is Vm Vm Pac = = RL RL The current consumed by the push - pull stae from the two supplies Vm consists of half - sine wave of peak amplitude R L Vm The averae current will be πrl V = m Vm VmVDD P ( ) ( ) = π π av VDD VDD RL RL πrl The output stae power efficiency is Pac πvm Peff = Pav 4V DD Maximum power efficiency is achieved when the output amplitude approaches VDD π Max Peff = or 78.5% (much better!) 4

13 Push-Pull w/ Small Quiescent Current (Class AB) Output Stae [Gray] Power efficiency of the Class-B output stae is reat, but the crossover distortion is a major issue Solution to the crossover distortion is to bias the transistors into conduction at a low quiescent current Level-shift transistors M4 and M5 are sized such that VGS and VGS are slihtly larer than their threshold voltaes 3

14 Push-Pull w/ Small Quiescent Current (Class AB) Output Swin Rane A drawback of the CMOS Class AB output stae is the limited output swin rane Maximum V o set by M source follower V o V DD - V DSAT3 -V GS Minimum Vo set by M source follower V o -V SS V DSAT6 V SG 4

15 Next Time Bandap Reference Circuits 5