GATE Electrical – Analog Electronics
Page 22 : Widlar Current Source
The Widlar Current Source is a modification of the basic current mirror that allows the generation of very small output currents without using large resistor values.
It is widely used in integrated circuits and operational amplifiers.
Basic Idea
In a simple current mirror, the output current is nearly equal to the reference current. However, sometimes we require a much smaller current.
The Widlar current source solves this problem by adding an emitter resistor to the output transistor.
Circuit Structure
- Two matched transistors (Q1 and Q2)
- Reference resistor R
- Emitter resistor RE in output transistor
The resistor in the emitter of Q2 reduces the output current.
Working Principle
Since the bases of both transistors are connected together:
VBE1 = VBE2 + IE2 RE
Because of this voltage drop across RE, the collector current of Q2 becomes smaller than the reference current.
Widlar Current Equation
The relationship between reference current and output current is:
Iref / Iout = e^(Iout RE / VT)
Where:
- VT = thermal voltage ≈ 25 mV
Advantages
- Generates very small currents
- Suitable for IC design
- Requires smaller resistors
Applications
- Operational amplifier bias circuits
- Integrated circuit current sources
- Analog IC design
Important GATE Points
- Widlar source produces smaller current than reference current.
- Emitter resistor reduces output current.
- Widely used in integrated circuits.
Next Page → Wilson Current Mirror
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