RC Coupled Amplifier

Why is it called as RC Coupled Amplifier?

• The output of the first stage is connected to input of the second stage by coupling capacitor and shunt resistor therefore it is called as RC coupled amplifier. 
• The output of the amplifier is inverted if there is odd number of stages like 1, 3, or 5. 
• The output of the amplifier is same as that of input of amplifier for even number of stages like 2, 4 or 6.

• Figure shows a circuit for RC coupled transistor amplifier. 
• The resistor R1, R2 and RE provides biasing and stabilization circuit. 
• The voltage gain of the each stage reduces without emitter bypass capacitor CE, it provides low reactance path to the signal.
• The coupling capacitor at the output stage blocks DC signal but allows AC signal for the next stage. It also prevents shifting of operating point of transistor.

Working

• When AC signal is applied at the input of the transistor, the amplified output is available across collector load resistor RC.
• This amplified signal is given to base of next stage through coupling capacitor. 
• The second stage further amplify signal. The overall gain of the amplifier increases by cascading of amplifier stages.

Gain of the amplifier

• The total gain of the amplifier is less than the product of individual gain of all amplifiers because the effective load resistance of first stage is reduced due to shunting effect of input of second stage and so on.
• Let us consider the case of three stages RC coupled transistor amplifier. 
• The gain of first stage reduces due to loading of second stage, the gain of second stage reduces due to loading of third stage but the gain of third stage unaltered due to no loading effect.

Summary for three stage amplifier

• First stage – Gain reduces
• Second stage – Gain reduces
• Third stage – Gain unaltered

Frequency Response

• The analysis of frequency response for RC coupled transistor amplifier is done at low frequencies ( < 50 Hz ) , mid frequencies ( 50 Hz to 20 kHz ) and high frequencies                   ( > 20 kHz ).

Low frequencies ( < 50 Hz )

The voltage gain reduces particularly at low frequency. This is due to

• The reactance of coupling capacitance becomes very high particularly at low frequencies therefore very small part of signal pass through first stage to next stage.

      Capacitive Reactance X_C = 1 / ωC

• The emitter reactance X_E becomes very large at low frequencies therefore it cannot shunt emitter resistance R_E effectively.

Mid frequencies ( 50 Hz to 20 kHz )

• The voltage of the RC coupled amplifier is almost constant in this range. 
• The coupling capacitor maintains constant voltage in this range of frequencies. 
• As the frequency increases, the coupling capacitor reactance C_C decreases which result in large of input signal passes from first stage to next stage therefore the gain of amplifier increases.
• However the lower coupling capacitor reactance increases loading of next stage which result in decrease in voltage gain.
•  The gain of amplifier remains constants considering these two combined effects.

Higher frequencies ( > 20 kHz )

The voltage gain reduces at higher frequencies due to

• The coupling capacitance reactance X_C becomes very small and behaves as short circuit at higher frequencies. 
• This will increases the loading of the next stage and therefore the gain of the amplifier decrease. 
• The capacitive reactance of base – emitter junction becomes low at high frequencies which increase the base current. This will reduce the current amplification factor β.

      β= I_C / I_B

Advantages

• Excellent frequency response
• The gain is almost constant over audio frequency range ( 50 Hz to 20 kHz )
• Low cost due to absence of heavy component like inductor circuit
• Circuit becomes compact due to compact size of capacitors and resistors

Disadvantages

• The output impedance of each stage is very low therefore it has low voltage gain and low power gain.
• Produce noise as it becomes older
• As the output impedance of the RC coupled amplifier is in terms of few hundred whereas the input impedance of speaker is tens of ohm therefore it has poor impedance matching property.

Applications

• It has excellent audio fidelity over wide range of frequencies therefore they are used for voltage amplification. 
• They are used for initial stage of amplifier. As it has poor impedance matching, it is not used at the last stage of amplification.

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