current mirrors and third generation current conveyors

11M.YıldızM.Yıldız

CURRENT MIRRORS AND CURRENT MIRRORS AND THIRD GENERATION THIRD GENERATION

CURRENT CONVEYORS CURRENT CONVEYORS

MERİH YILDIZMERİH YILDIZ

M.YıldızM.Yıldız 22

• IntroductionIntroduction• Current Mirrors Current Mirrors

– Simple current mirrorSimple current mirror (SCM)(SCM)– Wilson current mirrorWilson current mirror (WSM)(WSM)– Improved Wilson current mirrorImproved Wilson current mirror (IMCM)(IMCM)– Cascode current mirrorCascode current mirror (CCM)(CCM)– Improved active feedback cascode Improved active feedback cascode

current mirrorcurrent mirror (IAFCCM)(IAFCCM)– High Swing current mirrorHigh Swing current mirror (HSCM)(HSCM)

• Conclusion and Future workConclusion and Future work


IntroductionIntroduction• One of the most widely used analog building blocks One of the most widely used analog building blocks

is the current mirroris the current mirror• Current mirrors are commonly used inCurrent mirrors are commonly used in

– Signal processingSignal processing– analog circuits ( our target applications are current analog circuits ( our target applications are current

conveyors use CMs as current references and mirroring conveyors use CMs as current references and mirroring devices)devices)

So in these applications CM impact both So in these applications CM impact both Noise performanceNoise performance Frequency responseFrequency response


• Understanding the frequency behavior of CMs is important for designing current-Understanding the frequency behavior of CMs is important for designing current-mode circuitsmode circuits

• Analog signal processing circuits demandAnalog signal processing circuits demand– Higher bandwidths Higher bandwidths – better noise performancebetter noise performance

• Current Mode techniques are adequateCurrent Mode techniques are adequate– Parasitic capacitances can be minimized Parasitic capacitances can be minimized – And their effect in system will be lower than in the case of voltage-mode And their effect in system will be lower than in the case of voltage-mode

circuits.circuits. [Sedra Smith 1989, Elec.Lett.][Sedra Smith 1989, Elec.Lett.]

• Many references can be found considering CM’s accuracy and output impedance Many references can be found considering CM’s accuracy and output impedance or low-voltage applicationsor low-voltage applications

• Fewer papers consider the effect of noise and frequency in CMsFewer papers consider the effect of noise and frequency in CMs– Due to increasing usage at higher frequency, six most used CMs Due to increasing usage at higher frequency, six most used CMs

configurations are analyzed configurations are analyzed


CurrentCurrent Mirrors Mirrors

Simple CM Wilson CM Improved Wilson Simple CM Wilson CM Improved Wilson CMCM

Cascode CM IAFCCM High Swing Cascode CM IAFCCM High Swing CMCM


The CMs were chosen regarding several The CMs were chosen regarding several aspects, namely: accuracy, simplicity, low-aspects, namely: accuracy, simplicity, low-voltage operation and widespread usagevoltage operation and widespread usage • The configuration used as reference is the The configuration used as reference is the

simple current mirror (SCM) simple current mirror (SCM) • When higher accuracy is demanded common When higher accuracy is demanded common

circuits resort to Wilson and Cascode CMs.circuits resort to Wilson and Cascode CMs.• WCM and IWCM exhibit large output WCM and IWCM exhibit large output

impedance and high frequency response but impedance and high frequency response but are unsuitable for low-voltage operationare unsuitable for low-voltage operation

• Cascode CM is more suitable for low-voltage Cascode CM is more suitable for low-voltage operation and has high output impedanceoperation and has high output impedance


• The frequency behavior of these Six CM’s The frequency behavior of these Six CM’s configurations are comparedconfigurations are compared– This comparison is based on the two-port This comparison is based on the two-port

representation of the CM and comprises the representation of the CM and comprises the input impedance, the current gain, the output input impedance, the current gain, the output impedance and the reverse voltage gainimpedance and the reverse voltage gain

Analytical Analytical ExpressionExpressionCM two port equivalent (with CM two port equivalent (with

load)load)


• All transistors (except the mirroring All transistors (except the mirroring devices-transistors M2 in all the CMs) are devices-transistors M2 in all the CMs) are equalequal

• Transistors M2 have an aspect ratio of Transistors M2 have an aspect ratio of aa • Major influence over the CM frequency Major influence over the CM frequency

response are response are – the transconductance of the transistors (the transconductance of the transistors (ggmm),),– the gate to source capacitance (the gate to source capacitance (CCgsgs))– the output conductance (the output conductance (ggoo))– the configured aspect ratio of transistors M2 (the configured aspect ratio of transistors M2 (aa))


Frequency response of several Frequency response of several current mirrors configurationcurrent mirrors configuration


Maximizing bandwidth in CM Maximizing bandwidth in CM relies on three design relies on three design strategiesstrategies• Raising the quiescent current Raising the quiescent current • decreasing both W and Ldecreasing both W and L• Decreasing Decreasing aa• Special care must be taken if current Special care must be taken if current

accuracy is also a design goal becauseaccuracy is also a design goal because– Decreasing L and raising IDecreasing L and raising IDD also promotes also promotes

on the output impedance, thus affecting on the output impedance, thus affecting current accuracycurrent accuracy


Load EffectLoad Effect• Connecting a load to the output of a CM Connecting a load to the output of a CM

affects the frequency response of the affects the frequency response of the system, but not the maximum bandwidth system, but not the maximum bandwidth [Luis Nero Alves, Rui L. Aguar,2002][Luis Nero Alves, Rui L. Aguar,2002]

• Typical load impedance consisting of Typical load impedance consisting of admittance Gadmittance GLL in parallel with capacitance in parallel with capacitance CCLL, introduces in the frequency response , introduces in the frequency response one zero and one pole, as expressedone zero and one pole, as expressed


• Depending on the magnitude of the Depending on the magnitude of the GGLL and Cand CLL this zero and pole can this zero and pole can appear inside the system appear inside the system bandwidth causing a “kind of step” bandwidth causing a “kind of step” behaviorbehavior

• Besides this middle frequency, the Besides this middle frequency, the upper cutoff frequency remains upper cutoff frequency remains unchanged unchanged

Frequency response AFrequency response Aii(s) (middle (s) (middle frequencies)frequencies)


Simulation ResultsSimulation Results• To test the six proposed CMs we used a reference To test the six proposed CMs we used a reference

input current source. input current source. • A capacitor CA capacitor CLL in parallel with a resistance Rin parallel with a resistance RLL was was

used as loadused as load• The CM’s output port also tied to a fixed voltage The CM’s output port also tied to a fixed voltage

VVDCDC sufficient to grant the saturation condition in sufficient to grant the saturation condition in all the output transistorsall the output transistors

• Default value for W/L ratio is 20Default value for W/L ratio is 20m/2m/2m, except m, except transistors M2 that were designed with ratio transistors M2 that were designed with ratio a a (a=1 as default)(a=1 as default)

• The default current IThe default current IDD was 20 was 20


Simulation Results are graphically summarized in Simulation Results are graphically summarized in figuresfigures

Bandwidth versus W Bandwidth versus L

Bandwidth versus ID Bandwidth versus a


Improved Active Feedback Improved Active Feedback Cascode Current Mirror Cascode Current Mirror (IAFCCM)(IAFCCM)• IAFCCM was IAFCCM was

proposed toproposed to– Increase the output Increase the output

voltage swingvoltage swing– Output impedance Output impedance

and matching and matching accuracyaccuracy

IAFCCMIAFCCM

79297 dsdsdsmmo rrrggR ggmm: Transconductance of the : Transconductance of the transistorstransistorsrrdsds: Output impedance of the : Output impedance of the transistorstransistors


Simulation ResultsSimulation Results

Output and Input CurrentsOutput and Input Currents

Output voltage SwingOutput voltage Swing


ConclusionConclusion• Theoretical demonstrations pointed Theoretical demonstrations pointed

that CMs frequency response is that CMs frequency response is dominated by the relationship between dominated by the relationship between ggmm, C, Cgsgs and g and goo in their transistors in their transistors

• Simulation results showed good Simulation results showed good agreement with the analysis carried outagreement with the analysis carried out

• High performance Third generation current High performance Third generation current conveyors will be investigated by using these six conveyors will be investigated by using these six CMsCMs

Future WorkFuture Work


THANK YOUTHANK YOUMerih YILDIZMerih YILDIZ

Doğuş University-İSTANBUL-TURKEYDoğuş University-İ[email protected]@dogus.edu.tr

current mirrors and third generation current conveyors

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