The two-transistor current source, also called a current mirror, is the basic building block in the design of INTEGRATED CIRCUIT CURRENT SOURCES.
Figure shows the circuit Diagram of Current Source Using Transistors
It consist of two matched or identical transistors Q1 and Q2, operating at the same temperature; with their base terminals and emitter terminals connected together.
The base-emitter voltage, Vbe is therefore the same in the two transistors.
Transistor Q1 is connected as a diode; consequently, when the supply voltage is applied, the base-emitter junction of transistor Q1 is forward biased and a reference current Iref is established.
Although there is a specific relationship between Iref and Vbe, the same Vbe is applied to the Transistor Q2, hence it develops the current similar the collector current of Q1.
So the name CURRENT MIRROR.
OPERATION OF CURRENT MIRROR
1. Q1 acts like a diode and a current is established through it.
2. Q1 will develop a voltage drop Vbe, in response to the current.
3. The base-emitter of Q2 is in parallel with Q1
4. Q2's collector current will be established in response to its Vbe.
5. Since the two transistors are matched, Q2's collector current will be approximately equal to Q1's collector current.
Q2's collector current is said to "MIRROR" the current through Q1.
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