FET Current Mirrors
ECE 2204
Current Sources
• Ideal independent current sources are difficult to make
and are almost impossible to fabricate on an integrated
circuit.
• Instead, current mirrors are fabricated.
▫ These are circuits that contain two or more FETs, where the
drain of one of the FETs is connected to the rest of the
circuit.
▫ This FET is operating in the saturation/pinch-off mode.
Thus, it can be thought of as a dependent current source.
 The value of this dependent current source is determined by
the operating conditions of the other FETs in the current
mirror, not by the operating condition of the rest of the circuit
(at least over a certain range of currents).
2 Transistor
Current Mirror
Common Design in Current Mirrors
• At least two transistors
▫ Two transistors are wired in parallel
 shared VGS
▫ One transistor has the drain and gate tied together
 VDS > VGS – VTN; it is in saturation
Drain Current in M1
I D1 
I D1 
Kn 1
2
(V GS  V TN )
V DD  V DS 1
R1

2
V DD  V GS
R1
Bias Condition of M2
• M2 is assumed to also be in saturation.
▫ This depends in part on the rest of the circuit
above M2
 If the circuit above M2 requires too much current,
then M2 will be forced into the triode/nonsaturation
region. At which point, the current mirror circuit is
not functioning properly – which means someone
didn’t design their part of the circuit to the correct
specification.
Drain Current of M2
I D2 
I D2 
Kn 2
2
W
W
(V GS  V TN )
L 2
L 1
I D1 
2
W
W
L  2 V DD  V GS
L 1
R1
Current Mirror
with
Enhancement
Load
Modified
Wilson
Current
Mirror