Electronics Engineering (ELEX) Board Practice Exam

In the principle of current mirror, what happens when the gate-source potentials of two identical MOS transistors are equal?

1. The channel currents are unequal

2. The channel currents are equal

3. The channel currents are zero

4. The channel currents fluctuate wildly

The correct answer is: The channel currents are equal

When the gate-source potentials of two identical MOS transistors are equal, it ensures that both transistors are subjected to the same electrical conditions. In a current mirror configuration, the gate-source voltage (Vgs) applied to the transistors controls the operation of the devices. The proportionality of the gate voltage to the drain current means that if both MOS transistors have the same Vgs, they will generate the same drain current, assuming they operate in the saturation region. In saturation, the current flowing through each transistor is a function of the gate-source voltage, which is determined by the characteristics of the MOSFET itself (like threshold voltage and transconductance). Because the gate-source potentials are identical, the transistors will have the same level of channel modulation and thus conduct the same amount of current. This creates a mirror effect where the current through one transistor is reflected through the other, leading to equal channel currents as long as the transistors are operating under similar conditions. This principle is fundamental in analog circuit design, particularly in biasing and current steering applications where precise control of current is necessary.