GATE Electrical – Analog Electronics
Page 21 : Current Mirror Circuit
A Current Mirror is an analog circuit used to copy or mirror a reference current into another branch of a circuit.
It is widely used in integrated circuits and operational amplifiers.
Basic Idea
The current mirror uses two identical transistors:
- Q1 (Reference transistor)
- Q2 (Output transistor)
If both transistors are matched, the output current becomes approximately equal to the reference current.
Iout ≈ Iref
Working Principle
The base and collector of the reference transistor (Q1) are connected together. This forces the transistor to operate in the active region.
The same base voltage is applied to the second transistor (Q2).
Since both transistors have identical base-emitter voltage:
VBE1 = VBE2
Therefore the collector currents become equal.
Reference Current
The reference current is determined by resistor R:
Iref = (VCC − VBE) / R
This current is mirrored to the output transistor.
Advantages
- Provides constant current
- Useful for biasing circuits
- Simple design
- Widely used in integrated circuits
Applications
- Operational amplifiers
- Active loads
- Analog IC design
- Current sources
Important GATE Points
- Current mirror copies reference current.
- Requires matched transistors.
- Output current ≈ reference current.
- Commonly used in differential amplifier circuits.
Next Page → Widlar Current Source
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