JNTU B.Tech III INDUSTRIAL ELECTRONICS Previous Question Paper (November/December - 2020 - R16)

A Darlington Emitter Follower consists of two bipolar junction transistors connected such that the emitter of the first transistor drives the base of the second. This configuration provides an extremely high input impedance and a very high current gain, which is the product of the individual gains of the two transistors (β_total ≈ β₁ × β₂).

In the circuit, the collector of both transistors is connected to the supply voltage, and the output is taken from the emitter of the second transistor. A bias resistor is connected to the base of the first transistor. Because the input signal is applied to the base of the first transistor and the output is taken from the emitter of the second, the circuit acts as a voltage follower with a voltage gain close to unity. The main advantage is the significant reduction in loading effect on the preceding stage due to the high input impedance.

Choppers are static power electronic devices that convert fixed DC input voltage to a variable DC output voltage. They are classified based on the direction of current and voltage flow into five types: Class A (First quadrant), Class B (Second quadrant), Class C (Two-quadrant type-A), Class D (Two-quadrant type-B), and Class E (Four-quadrant).

For a step-down chopper, the average output voltage (V_o) is derived by considering the duty cycle (α), which is the ratio of the ON time (T_ON) to the total chopping period (T = T_ON + T_OFF).

Since the duty cycle is defined as α = T_ON / T, the expression becomes:

Where V_s is the input DC voltage and α ranges from 0 to 1.