Question

Which of the following is not correct with respect to Darlington amplifier?

• A. High input impedance
• B. Two cascaded emitter followers
• C. Overall voltage gain is less than unity
• D. Overall leakage current is less

Hint:

Darlington Pair: High current gain (\(\beta \approx \beta_1\beta_2\)), high input impedance, low output impedance.
Voltage gain is slightly less than 1 (when used as emitter follower).
Disadvantages include higher \(V_{BE(on)}\) (approx \(2 \times 0.7V\)), slower switching speed, and increased leakage currents.

Answer 1

The Correct Option is D

A Darlington pair (Darlington amplifier) consists of two bipolar junction transistors connected in such a way that the current amplified by the first transistor is further amplified by the second one. The collectors are typically connected together, and the emitter of the first transistor is connected to the base of the second. Properties of a Darlington pair (usually configured as an emitter follower): (a) High input impedance: True. The input impedance is approximately \(\beta_1 \beta_2 R_E\), which is very high. (b) Two cascaded emitter followers: True. It is effectively a cascade of two emitter follower stages. (c) Overall voltage gain is less than unity: True. Like a single emitter follower, the voltage gain of a Darlington emitter follower is close to unity but slightly less than 1 (typically 0.98-0.99). (d) Overall leakage current is less: This statement is NOT CORRECT. The overall leakage current (e.g., \(I_{CEO}\)) of a Darlington pair is generally *higher* than that of a single transistor. The leakage current of the first transistor is amplified by the second transistor. \(I_{CBO}\) of the first transistor becomes base current for the second, effectively increasing the overall collector leakage current. Therefore, the incorrect statement is (d). \[ \boxed{\text{Overall leakage current is less}} \]