Jue Shi Presentation

7/25/2019 Jue Shi Presentation

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Design Techniques for Cascoded CMOS Op Amps with Improved PSRR and Common-Mode Input Range! DA"ID # RI#$%R A$D MI&%S A' COP%&A$D

presented ! (ue Shi

Agenda

O)ective* pro+ems this paper tr! to so+veSo+utions to the three pro+emsSma++ signa+ mode+s ana+!sis and simu+ationTest circuit and resu+tsConc+usion

Introduction

The PSRR is defined asthe ratio of signal path gain to the gain of power supply to Vout transfer function.

It simplifies to the ratio of the change in supply voltage to the equivalent (differential) input voltage it produces in the op-amp, often

expressed in decibels

Objective:

This paper presents two circuits that overcome the PSRR pro+ems of the ear+ier amp+ifier'one for virtua+ ground app+ications such as switched-capacitor integrators,and the other for uffer app+ications requiring wide common-mode input range'

Sma++ signa+ ana+!sis is deve+oped for the open +oop and PSRR responses of the two amp+ifiers'

In addition, design guide+ines are suggested and test resu+ts are presented'

A test circuit inc+uding these amp+ifiers has een faricated and demonstrates the improved performance'

Problem 1: poor ac PSRR

Intemaf+! compensated two-stage CMOS op amp suffers frompoor ac power supp+! re)ectionto one of the power rai+s'

Cause The output drive transistor at moderate frequencies ecomes .diode connected. with its drain ac shortedto its gate ! the compensating capacitor, which coup+es the supp+! signa+ to the output'


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The artical ignores the gm1/gm2 dependent-sources saying"g02 connects to the source of the input pair which is an open-circuit for common-mode signalssuch as power supply noise and must be excluded."

The model in the article basically removed the circuit on the left f the 2.

The !ero at the low freuency of the dominant pole prematurely degrades the #$%%.The explanation is that as freuency rises& the impedance of the compensation capacitor 'c becomes low and the gateand drain of () begin to trac* one another.The transistor is current-source biased by (+and must maintain a relatively constant gate-source driveconsistent with

the bias current.This reuirement forces the gate of () to trac* , fluctuations which are in turn transmitted by 'c tothe drain& which is the output of the amplifier.

Solution to problem 1'ascoding techniue greatly improves high-freuency reection& as shown in ig. 2.

(/( decouples the gate of the driver transistor from the compensation capacitor.

detailed small-signal model for the cascoded op amp presented in this paper simplifies the design of these op amps. small-signal #$%% model demonstrates the improvement in #$%% of the new op amps over the earlier one in ig. 1.


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DC gain

compared to the original #$%%


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The expression for the zero is very similar to that for the noncascoded op amp. They differ only in thevalue of thecapacitance: Cc for the previous op amp, and Cz for the cascode op amp.Cc is the compensation capacitor whereas C2 is only the drain to gate capacitance of asaturated transistor ( M!.C2


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A recent paper [7] presented an n-channel input circuit similar to the circuit of Fig. 3, and emphasized its widecommon-mode input range.That circuit, however, suffers from a peculiar adversit due to its !iasing, which can !e discussed in the conte"t of thep-channel input circuit of the present paper.

#f the gates of transistors $%& and $%% are driven ! a fixed voltage,as in [7], then for large positive common-modeinputs, as e"ists with a voltagefollower for e"ample,the output will a!ruptl spi'e up to the positive suppl voltage. The sources of the inputtransistorsreduce the drain-source voltage $T, for large positive common-mode inputs, and the !ias current is reducedsu!stantiall.

For large positive common-mode inputs, the input pair current is thus too small to !e significant withrespect to the fi"ed !ias current of $%& and $%%, and the gate voltage of the output driver $( is therefore a!ruptlpulled down, causing the output to pull high.

Solution to problem 3This pro!lem can easil !e avoided ! using the novel !iasing techni)ue for the n-channel currentsources *$%+ $%3 shown in Fig. 3. A current mirroringapproach is used here where! the!ias currentthrough$%& and $%% trac's the current through the input source-coupled pair. For e"cessive positive common-mode inputs,the use of $%3with its gate driven ! the plus input, will force the current of $lz to follow that of $T. This imposesthe same reduced drain-source voltage on $lz as is on $T, and this current is mirrored to the n-channel transistors.ifference-mode input components are normall held ver small ! feed!ac', and thus do not modif this !ehaviorsignificantl. Through the use of this method there is no penalt when the common-mode input limit is e"ceededother than soft clipping.

Simulation Results


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Also included in the figure is a PSRR frequency response curve for a noncascoded version of thesame op amp. A major improvement in high frequency PSRR ( >30 d! is evident for the cascoded" op amps.

Test Results


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In all other respects similar test results were obtained for both amplifiers asexpected and are outlined for the circuit of Fig. 2, in Table I. ?? A photograph of the experimental VDD PSRR forthecircuit of Fig. 3 showing the excellent high-frequenc re!ection " #$% d& at '(( )*+ appears in Fig. ''.

CONCLUSIONS

'.// impro0ement for both circuits.2.The circuit sol0es an output spi)e phenomena which arose in a pre0iousl reported op amp.3.The small signal model is 0alidated b simulation and test results.$.1esign techniques for frequenc compensation are presented.

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