CURRENT MIRRORS AND THIRD GENERATION CURRENT CONVEYORS

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CURRENT MIRRORS AND THIRD GENERATION CURRENT
CONVEYORS

• MERIH YILDIZ

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• Introduction
• Current Mirrors
• Simple current mirror (SCM)
• Wilson current mirror (WSM)
• Improved Wilson current mirror (IMCM)
• Cascode current mirror (CCM)
• Improved active feedback cascode current mirror
  (IAFCCM)
• High Swing current mirror (HSCM)
• Conclusion and Future work

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Introduction

• One of the most widely used analog building
  blocks is the current mirror
• Current mirrors are commonly used in
• Signal processing
• analog circuits ( our target applications are
  current conveyors use CMs as current references
  and mirroring devices)
• So in these applications CM impact both
• Noise performance
• Frequency response

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• Understanding the frequency behavior of CMs is
  important for designing current-mode circuits
• Analog signal processing circuits demand
• Higher bandwidths
• better noise performance
• Current Mode techniques are adequate
• Parasitic capacitances can be minimized
• And their effect in system will be lower than in
  the case of voltage-mode circuits. Sedra Smith
  1989, Elec.Lett.
• Many references can be found considering CMs
  accuracy and output impedance or low-voltage
  applications
• Fewer papers consider the effect of noise and
  frequency in CMs
• Due to increasing usage at higher frequency, six
  most used CMs configurations are analyzed

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Current Mirrors
Simple CM Wilson CM
Improved Wilson CM
Cascode CM IAFCCM
High Swing CM
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The CMs were chosen regarding several aspects,
namely accuracy, simplicity, low-voltage
operation and widespread usage

• The configuration used as reference is the simple
  current mirror (SCM)
• When higher accuracy is demanded common circuits
  resort to Wilson and Cascode CMs.
• WCM and IWCM exhibit large output impedance and
  high frequency response but are unsuitable for
  low-voltage operation
• Cascode CM is more suitable for low-voltage
  operation and has high output impedance

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• The frequency behavior of these Six CMs
  configurations are compared
• This comparison is based on the two-port
  representation of the CM and comprises the input
  impedance, the current gain, the output impedance
  and the reverse voltage gain

Analytical Expression
CM two port equivalent (with load)
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• All transistors (except the mirroring
  devices-transistors M2 in all the CMs) are equal
• Transistors M2 have an aspect ratio of a
• Major influence over the CM frequency response
  are
• the transconductance of the transistors (gm),
• the gate to source capacitance (Cgs)
• the output conductance (go)
• the configured aspect ratio of transistors M2 (a)

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Frequency response of several current mirrors
configuration
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Maximizing bandwidth in CM relies on three
design strategies

• Raising the quiescent current
• decreasing both W and L
• Decreasing a
• Special care must be taken if current accuracy is
  also a design goal because
• Decreasing L and raising ID also promotes on the
  output impedance, thus affecting current accuracy

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Load Effect

• Connecting a load to the output of a CM affects
  the frequency response of the system, but not the
  maximum bandwidth Luis Nero Alves, Rui L.
  Aguar,2002
• Typical load impedance consisting of admittance
  GL in parallel with capacitance CL, introduces in
  the frequency response one zero and one pole, as
  expressed

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• Depending on the magnitude of the GL and CL this
  zero and pole can appear inside the system
  bandwidth causing a kind of step behavior
• Besides this middle frequency, the upper cutoff
  frequency remains unchanged

Frequency response Ai(s) (middle frequencies)
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Simulation Results

• To test the six proposed CMs we used a reference
  input current source.
• A capacitor CL in parallel with a resistance RL
  was used as load
• The CMs output port also tied to a fixed voltage
  VDC sufficient to grant the saturation condition
  in all the output transistors
• Default value for W/L ratio is 20mm/2mm, except
  transistors M2 that were designed with ratio a
  (a1 as default)
• The default current ID was 20mA

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Simulation Results are graphically summarized in
figures
Bandwidth versus W
Bandwidth versus L
Bandwidth versus ID
Bandwidth versus a
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Improved Active Feedback Cascode Current Mirror
(IAFCCM)

• IAFCCM was proposed to
• Increase the output voltage swing
• Output impedance and matching accuracy

gm Transconductance of the transistorsrds
Output impedance of the transistors
IAFCCM
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Simulation Results
Output and Input Currents
Output voltage Swing
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Conclusion

• Theoretical demonstrations pointed that CMs
  frequency response is dominated by the
  relationship between gm, Cgs and go in their
  transistors
• Simulation results showed good agreement with the
  analysis carried out

• Future Work

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

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• THANK YOU
• Merih YILDIZ
• Dogus University-ISTANBUL-TURKEY
• myildiz_at_dogus.edu.tr