ECEN474/704: (Analog) VLSI Circuit Design Spring 2018

Transcription

1 ECEN474/704: (Analo) VLSI Circuit Desin Sprin 08 Lecture 6: Output Staes Sam Palermo Analo & Mixed-Sinal Center Texas A&M University

2 Announcements Project eport Due May it to me by 5PM Exam 3 is on May 3 3PM-5PM Closed book w/ one standard note sheet 8.5 x front & back Brin your calculator Covers material throuh Output Staes Lecture Previous years exam 3s are posted on the website for reference

3 Aenda Output Staes Source Follower (Class A) Push-Pull (Class B) Push-Pull w/ Small Quiescent Current (Class AB) 3

4 OpAmps and OTAs OpAmp OTA Hih voltae ain Hih input impedance Voltae source output (low impedance) Hih voltae ain As lon as it s drivin a hih impedance load (capacitor) Hih input impedance Current source output (hih impedance) 4

5 Three-Stae OpAmp ~ i- i+ o Differential input stae Amplifies differential input Sets specs such as G m, CM, and slew rate Second ain stae Provides additional ain Often used to provide Miller compensation Output stae Power Amplifier Lare current ain and near unity voltae ain Small output impedance 5

6 Buffered OTA = Operational Voltae Amplifier (OPAMP) ELEN-474 M M M 3 i0 A VDC i 0 i 0 VDD i0 i0 C CM v d -v d M M VSS Jose Silva-Martinez -6- M 4 IB IB IB3 p GBW p v 0 v + v - m (v + -v - ) SIMPLE MACOMODEL: A A A VDC 3 GBW m 3 r v y Internal pole C+(+ m3 r o3 )CM m o C m M A 3 r 0 A A 3 (v y ) nd Stae & Buffer A Texas A&M University v out out r0 / m 4 Notice that load capacitors must satisfy the followin condition, otherwise phase marin is not ood enouh GBW r 0 C L

7 Source Follower (Class A) Output Stae 7 m o mb o m out L m L m L o mb o m m dc 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]

8 Source Follower (Class A) Transfer Characteristic V o V i V T I C n Q ox V o L W L [Gray] Maximum V o is set by M saturation condition 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 depends on L For small L (heavy load), M ets cutoff and V o -I Q L For lare L (liht load), M will o into triode reion at V DD +V DSAT 8

9 Source Follower (Class A) Power Efficiency Assumin a sinusoidal output voltae Vo Vm sint The power delivered to the load at the sinal frequency is Vm Vm Pac L L The averae power consumed by the source follower is Pav IQ VDD I Q V DD IQVDD The output stae power efficiency is Pac Vm Peff Pav 4LIQVDD Maximum power efficiency is achieved when the output VDD amplitude approaches VDD and IQ is desined to be L Max Peff or 5% (not that ood!) 4 9

10 Super Buffer Output Stae [Silva] out m A v Employs a ain-boostin technique where the effective transcoductance of M is boosted by -A v, where A v is the ain from the source-to-ate, which should be neative for stability esults in an output resistance reduction by a /(+A v ) factor 0

11 Push-Pull Source Follower (Class B) Output Stae Class B output staes have only one transistor conductin current for each half cycle M durin positive half M durin neative half esults in improved power efficiency by operatin at zero quiescent current However, if - V TP V in V TN, then no output sinal

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

13 Push-Pull Source Follower (Class B) Power Efficiency Assumin a sinusoidal output voltae Vo Vm sint The power delivered to the load at the sinal frequency is Vm Vm Pac L L The current consumed by the push - pull stae from the two supplies Vm consists of half - sine wave of peak amplitude L Vm The averae current will be L V m Vm VmVDD P av VDD VDD L L L The output stae power efficiency is Pac Vm Peff Pav 4V DD Maximum power efficiency is achieved when the output amplitude approaches VDD Max Peff or 78.5% (much better!) 4 3

14 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 V GS and V SG are slihtly larer than their threshold voltaes 4

15 Push-Pull w/ Small Quiescent Current (Class AB) Output Swin ane [Gray] 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 5

16 Next Time Bandap eference Circuits 6