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CURRENT MIRRORS AND THIRD
GENERATION CURRENT
CONVEYORS
MERİH 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 CM’s 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
Cascode CM
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Wilson CM
IAFCCM
Improved Wilson CM
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 CM’s
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
CM two port equivalent (with load)
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Analytical Expression
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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
–
–
–
–
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the
the
the
the
transconductance of the transistors (gm),
gate to source capacitance (Cgs)
output conductance (go)
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 CM’s 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 (a=1 as
default)
The default current ID was 20mA
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Simulation Results are graphically summarized in figures
Bandwidth versus W
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Bandwidth versus ID
Bandwidth versus L
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
Ro  g m7 g m9 rds 2 rds9 rds7
gm: Transconductance of the transistors
rds: Output impedance of the transistors
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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
Doğuş University-İSTANBUL-TURKEY
[email protected]
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