EE 435 Lecture 12. OTA circuits. Cascaded Amplifiers. -- Stability Issues. -- Two-Stage Op Amp Design

Transcription

1 EE 435 Lecture 12 OTA circuits Cascaded Amplifiers -- Stability Issues -- Two-Stae Op Amp Desin

2 Review from last lecture: Current Mirror Op Amp W/O CMFB DD M : 1 1 : M M meq m1 Often termed an OTA I T I OUT m 1 : 1 Introduced by Wheatley and Whitliner in 1969 I OUT m IN 2

3 Review from last lecture: OTA Circuits OTA often used open loop Excellent Hih Frequency Performance Gain can be made prorammable with dc current Lare or very lare adjustment ranes possible m I OUT m K K I I ABC ABC for for BJT circuits MOS circuits I ABC 2 to 3 decades of adjustment for MOS 5 to 6 decades of adjustment for BJT 3

4 Review from last lecture: OTA Applications m R OUT m R IN m is controllable with I ABC oltae Controlled Amplifier Note: Technically current-controlled, control variable not shown here and on followin slides 4

5 Review from last lecture: OTA Applications R IN m R IN m R IN 1 m R IN 1 m oltae Controlled Resistances 5

6 Review from last lecture: OTA Applications m1 m1 m2 m2 OUT m1 m 2 Noninvertin oltae Controlled Amplifier in OUT m1 m 2 Invertin oltae Controlled Amplifier in Extremely lare ain adjustment is possible oltae Controlled Resistorless Amplifiers 6

7 Review from last lecture: OTA Applications m m C C sc sc m m OUT in OUT in Noninvertin oltae Controlled Interator Invertin oltae Controlled Interator oltae Controlled Interators 7

8 Review from last lecture: Comparison with Op Amp Based Interators C R R C 1 src OUT R 1 R 1 1 src OUT OTA-based interators require less components and sinificantly less for realizin the noninvertin interation function! 8

9 Review from last lecture: Properties of OTA-Based Circuits Can realize arbitrarily complex functions Circuits are often simpler than what can be obtained with Op Amp counterparts Inherently offer excellent hih frequency performance Can be controlled with a dc voltae or current Often used open-loop rather than in a feedback confiuration (circuit properties depend directly on m ) Other hih output impedance op amps can also serve as OTA Linearity is limited Sinal swin may be limited but can be ood too Circuit properties process and temperature dependent 9

10 Current-Mirror Op Amp offers stratey for m enhancement ery Simple Structure Has applications as an OTA But how ood are the properties of the CMOA? Is this a real clever solution? 10

11 Current Mirror Op Amp W/O CMFB DD DD M : 1 1 : M M 5 M 3 M 4 M 6 M 1 M 2 C L I T I T M 9 B1 M 7 M 8 1 : 1 And can use hiher output impedance current mirrors to decrease OEQ A OEQ O O6 O8 meq M m1 M O6 m1 O8 SR MI C L T 11

12 SR of Current Mirror Op Amp DD DD M 5 M 3 M 4 M 6 M 5 M 3 M 4 M 6 C L M 1 M 2 C L M 1 M 2 C L B2 M 9 B1 I T M 7 M 8 M 9 B1 I T M 7 M 8 SR MI 2C T L SR MI C L T 12

13 Fully Differential Current Mirror Op Amp with Improved Slew Rate DD M M : 1 1 : M M C L C L I T 1 : 1 1 : 1 Need CMFB circuit and requires modest circuit modification to provide CMFB insertion point 13

14 Fully Differential Current Mirror Op Amp with Improved Slew Rate This circuit was published because of the claim for improved SR (Fi 6.30 CMJ) DD M 5B M 5A M 3 M 4 M 6A M 6B M 1 M 2 I T M 9A B1 M 7 M 8A M 8B M 9B Need CMFB circuit and requires modest circuit modification to provide CMFB insertion point 14

15 Fully Differential Current Mirror Op Amp with Improved Slew Rate M 5B M 5A DD M 3 M 4 M 6A M 6B SR MI C L T M 1 M 2 SR CMOp Amp MI 2C L T M 9A B1 I T M 7 M 8A Improved a factor of 2! but M 8B M 9B Need CMFB circuit and requires modest circuit modification to provide CMFB insertion point 15

16 Fully Differential Current Mirror Op Amp with Improved Slew Rate M 5B M 8B M 5A M 9A B1 DD M 3 M 4 M 1 M 2 I T M 7 M 6A M 8A M 9B M 6B MIT SR CL MIT SR CMOp Amp 2C Improved a factor of 2! but I 1 P P SR SR CMOp Amp CMOp Amp I L DD 1 T 2 DD T M M P M C 2 1 M DD L P M C 1 2M DD L SR actually about the same for improved SR circuit and basic OTA 16

17 Comparison of Current-Mirror Op Amps with Previous Structures Does the simple mirror ain really provide an almost free ain enhancement? DD A O M 2 O 6 m1 O8 M 5 M 3 M 4 M 6 M 1 M 2 M WL 6 WL B2 M 9 B1 I T M 7 M 8 Ask the apple comparison question! 17

18 Comparison of Current-Mirror Op Amps with Previous Structures Does the simple mirror ain really provide an almost free really lare ain enhancement? DD M 5 M 3 M 4 M 6 M 1 M 2 A O M 2 O 6 m1 O8 B2 M 9 B1 I T M 7 M 8 M WL 6 WL Are we comparin Apples with Apples? In the small-sinal parameter domain? In the practical parameter domain? Does it matter if we are makin a comparison? 18

19 Reference Op Amp Consider sinle-ended output performance : A(s) sc L 2 m1 O1 O3 DD B1 M 3 M 4 A O 1 2 O1 m1 O3 A 0 λ 1 1 λ 3 1 EB1 C L M 1 M 2 C L C m1 GB 2 L GB P 2 C DD L 1 EB1 B2 I T M 9 IT SR 2 C L SR 2 P DD C L 19

20 Comparison of Current-Mirror Op Amps with Previous Structures Does the simple mirror ain really provide an almost free ain enhancement? I IN M 4 M 6 I OUT WL 6 M WL 4 M 4 6 m6 m 4 A O A O M 2 O 6 m 6 m 4 O 6 m1 O8 2 m1 O8 Gain Enhancement Potential Less Apparent but still Improved by m6 / m4 ratio 20

21 Comparison of Current-Mirror Op Amps with Previous Structures Does the simple mirror ain really provide an almost free ain enhancement? A O M 2 O 6 m1 O8 B2 DD M 5 M 3 M 4 M 6 M 9 B1 M 1 M 2 I T M 7 M 8 Consider how the ain appears in the practical parameter domain A 0 EB1 1 2 IT 2 M 2 λ λ I IT λ λ 2λ M6 M8 D8Q EB1 M6 M8 EB1 EB1 λ M6 IT M 2 λ M8 M 2 This is exactly the same as was obtained for the simple differential amplifier! For a iven EB1, there is NO ain enhancement!

22 Comparison of Current-Mirror Op Amps with Previous Structures How does the GB power efficiency compare with previous amplifiers? M : 1 DD 1 : M GB P C meq L m1 M 2 C L MIT 2 C I 1 M DD T EB1 L M 1 M 2 I T GB MIT 2 C EB1 L 2 EB1 P DD C L M 1 M GB for Telescopic Cascode and Ref Op Amp! GB efficiency decreased for small M!! 22

23 Comparison of Current-Mirror Op Amps M : 1 with Previous Structures How does the SR compare with previous amplifiers? DD M 1 M 2 1 : M I T SR P SR Ref Op Amp SR DD I T P 2 C DD L IT 2CL MI 2 C SR Improved by factor of M! but 1 M M 1 M P SR Ref OpAmp 2 C DD L SR Really Less than for Ref Op Amp!! L T 23

24 Comparison of Current-Mirror Op Amps with Previous Structures DD How does the Current Mirror Op Amp really compare with previous amplifiers or with reference amplifier? M : 1 1 : M Perceived improvements may appear to be very sinificant M 1 M 2 Actual performance is not as ood in almost every respect! I T But performance is comparable to other circuits and the circuit structure is really simple Widely used architecture as well but maybe more for OTA applications 24

25 meq Gain Enhancement Stratey I B MQC m1 M m is increased by the mirror ain! 1 : M C L Foldin is required to establish the correct bias current direction Consider usin the quarter circuit itself to form the op amp M 1 Consider this quarter circuit Could have done this for other quarter circuits as well but there is a particularly important reason we are followin this approach with this quarter circuit What is it? Output conductance of QC: OQC 25

26 Current-Mirror Op Amps Another Perspective! DD M 5 M 3 M 4 M 6 M 1 M 2 B2 M 9 B1 I T M 7 M 8 Differential Half-Circuit

27 Current-Mirror Op Amps Another Perspective! M 4 M 6 M 2 M 8 Differential Half-Circuit Cascade of n-channel common source amplifier with p-channel common-source amplifier!

28 Current-Mirror Op Amps Another Perspective! M 2 M 4 M 6 M O O m m m A O O m m m O O m O M A Differential Half-Circuit Cascade of n-channel common source amplifier with p-channel common-source amplifier! From Current Mirror Analysis :

29 Comparison of Different Circuit Desins An objective comparison of different desin approaches is often a critical part of the desin process Different objective functions or different comparison approaches often lead to different conclusions Textbooks and the technical literature do not always identify the most appropriate objective functions Critical to identify metrics that capture the important characteristics of a desin when makin comparisons but this is often a challenin task? 31

30 Current Mirror Op Amp Summary Current-mirror op amp offers no improvement in performance over the reference op amp Current-mirror op amp can be viewed as a cascade of two common-source amplifiers, one with a low ain and the other with a larer ain Current-mirror op amp is useful as an open-loop prorammable transconductance amplifier (OTA) Current-mirror op amp will work in feedback applications as well but performance would often be better with alternative Op Amp architectures

31 Stability Sometimes circuits that have been desined to operate as amplifiers do not amplify a sinal but rather oscillate when no input sinal is present ( in =0 or I in =0A) or latch up Circuits that are desined to operate as amplifiers but instead either oscillate or latch up are said to be unstable The stability of any circuit is determined by the location of the poles We will discuss stability with more rior later It will be shown that if the poles of an open-loop amplifier are widely separated on the neative real axis, then the feedback amplifier built usin the open-loop amplifier will be stable And, it will be shown that if the poles of an open-loop amplifier are not widely separated on the neative real axis, then the feedback amplifier built usin the open-loop amplifier will be unstable

32 Poles of an Amplifier The poles of an amplifier are the roots of the denominator of the transfer function Each enery storae element (capacitor or inductor) introduces an additional pole (except when capacitor or inductor loops exist) The poles of an amplifier can often be approximated by independently considerin the impedance facin each capacitor and assumin all other capacitors are either open circuits or short circuits

33 Poles of an Amplifier The dead network of a circuit is obtained by settin all independent sources to zero The poles of a circuit are absolute: That is, they are independent of where the excitation is applied or where the response is taken provided the dead networks are the same! Stability is absolute: That is, a circuit is either stable or unstable irrespective of where the input is applied or the response is taken provided the dead networks are the same

34 Current-Mirror Op Amps Another Perspective! Differential Half-Circuit Are there stability issues or concerns for a FB amplifier usin this Op Amp? Small M 4 M 6 M 2 C 1 s-plane M 8 C 2 Im -p 2 -p 1 p 2 p 1 O6 C 2 C O8 m 4 1 p p 1 2 Lare Lare Small It will be shown later that FB amplifiers with a lare pole spread do not have stability problems No stability problems for current mirror op amp provided C 2 is sufficiently lare! Re

35 End of Lecture 12