Electronics Engineering (ELEX) Board Practice Exam

For the low-frequency response of a BJT amplifier, under what condition is the maximum gain achieved?

1. R_E = 0 Ω

2. R_E = ∞ Ω

3. R_E = 10 Ω

4. R_E = 5 Ω

The correct answer is: R_E = 0 Ω

In a BJT amplifier, particularly in the common-emitter configuration, the emitter resistor (R_E) plays a significant role in defining the amplifier's performance, especially regarding gain and stability. The maximum gain is typically achieved when there is no emitter resistor present, meaning R_E equals 0 Ω. When R_E is at zero, the amplifier operates in a manner that allows the full voltage swing at the collector to effectively influence the output, resulting in higher gain. This condition maximizes the voltage gain since the negative feedback introduced by an emitter resistor is absent, thereby enabling a larger proportion of the input signal to be amplified without loss. In scenarios where R_E has a finite value, such as 10 Ω or 5 Ω, the negative feedback increases, leading to reduced gain. An infinitely large R_E would effectively mean that the amplifier's gain could drop significantly, as it would prevent any significant voltage swing, thus reducing the overall output. Thus, achieving maximum gain is contingent upon the emitter resistor being absent completely.