TWO STAGE RC COUPLED AMPLIFIERS

TWO STAGE RC COUPLED AMPLIFIERSBy,Madan H RDepartment of ECE,RVCE, BengaluruIn this, resistances and capacitances are used to couple one transistor stage to the otherthe signal developed across RC of the first stage is coupled to the base of the second stage through the capacitor CCThe coupling capacitor blocks the DC voltagereaching the second stageSome loss of the signal voltage always occurs due to the drop across the coupling capacitorThis loss will be happen when the frequency of the input signal is low and This is the main drawback of this methodBut, if we are interested in amplifying a.c signals of f>10Hz, this type of coupling is the best solutionThe most convenient and the least expensive way to build a multi-stage amplifierWhen a.c signal is applied to the base of the first transistor, it appears in the amplified form across its collector load RCThe amplified signal across RC base of the next stage, further amplified by the second stage, and the overall gain is considerably increased.

Gain and frequency responseHere, total gain less than the product of the gains of individual stagesBecause, when the second stage is made to follow the first stage, the effective load resistance of the first stage is reduced due to the shunting effect of the input resistance of the second stage. This reduces the total gainIt is clear that voltage gain drops off at low (20KHz) frequencies whereas it is uniform over mid-frequency range (50Hz to 20KHZ)

At low frequencies,the reactance of coupling capacitor CC is quite high and hence very small part of the signal will pass from one stage to the next stageThe CE cannot shunt the emitter resistance RE effectively because of its large reactance at low frequenciesThese two factors cause a falling of voltage gain at low frequencies.At high frequencies,the reactance of coupling capacitor CC is small and behaves as short circuit.This reduces the voltage gain.At high frequency, capacitive reactance of base emitter junction is low which increases the base currentThis reduces the current amplification factor. Due to this the voltage drops off at high frequency.

At mid frequencies,the voltage gain of the amplifier is constant The effect of coupling in this frequency range is such as to maintain a uniform voltage gain.As the frequency increase in this range, reactance of CC decreases which tends to increase the gainLower reactance means higher loading of first stage and hence lower gainThese factors almost cancel each other, resulting in a uniform gain at mid frequencyThe RC-coupled amplifiers have excellent audio fidelity over a wide range of frequency

they are widely used as voltage amplifiers. They are not used at final stage, because of its poor impedance matching.

In RC-coupled amplifiers:The various stages are DC isolated. This feature facilitates the biasing of individual stages.The various stages can be similar. Hence the design of the amplifier is simplified.The coupling capacitors influence the responses of the amplifier.A great number of biasing resistors is necessary.

Advantages, disadvantages and applicationsExcellent frequency response, Low cost due to RC coupling and Circuit is very simpler

Has low voltage and power gain, Behaves noisy with age and poor impedance matching

Used as voltage amplifier in public addressing systems.ProblemsCalculate suitable resistor value for the CE amplifier shown, given RL = 120K, VCC = 24V