Q. Write a short note on the frequency response of a R-C coupled amplifier.
At low frequencies (<50Hz), the reactance of coupling capacitor Cc is quite high and hence very small part of signal will pass from one stage ot the next stage. Moreover, Ce cannot shunt the emitter resistance Re effectively because of its large reactance at low frequencies. These two factors cause a falling of voltage gain at low frequencies.
At high frequencies (>20KHz), the reactance of Cc is very small and it behaves as a short circuit. This increases the loading effect of next stage serves to reduce the voltage gain. Moreover, at high frequency, capacitive reactance of base-emitter junction is low which increases the base current. This reduces the current amplification factor b. Due to these reasons, the voltage gain drops off at high frequency.
At mid-frequencies (50Hz to 20KHz), the voltage gain of the amplifier is constant. The effect of coupling capacitor in this frequency range is such so as to maintain a uniform voltage gain. Thus, as the frequency increases in this range, reactance of Cc decreases which tends to increase the gain. However, at the same time, lower reactance means higher loading of first stage and hence lower gain. These factors almost cancel each other, resulting in a uniform gain at mid frequency.