RC Coupled Amplifier: What is it? (Working Principle & Applications)

What is a RC Coupled Amplifier?

A Resistance Capacitance (RC) Coupled Amplifier is basically a multi-stage amplifier circuit extensively used in electronic circuits. Here the individual stages of the amplifier are connected together using a resistor–capacitor combination due to which it bears its name as RC Coupled.

Figure 1 shows such a two-stage amplifier whose individual stages are nothing but the common emitter amplifiers. Hence the design of individual stages of the RC coupled amplifiers is similar to that in the case of common emitter amplifiers in which the resistors R₁ and R₂ form the biasing network while the emitter resistor RE form the stabilization network.

Here the C_E is also called bypass capacitor which passes only AC while restricting DC, which causes only DC voltage to drop across R_E while the entire AC voltage will be coupled to the next stage.

Further, the coupling capacitor C_C also increases the stability of the network as it blocks the DC while offers a low resistance path to the AC signals, thereby preventing the DC bias conditions of one stage affecting the other. In addition, in this circuit, the voltage drop across the collector-emitter terminal is chosen to be 50% of the supply voltage V_CC inorder to ensure appropriate biasing point.

In this kind of amplifier, the input signal applied at the base of the transistor in stage 1 (Q₁) is amplified and appears at its collector terminal with a phase-shift of 180^o.

The AC component of the signal passes through the coupling capacitor C_C to the second stage, entering as an input at the base of the second transistor, Q₂.

This is further amplified and is passed-on as an output of the second stage and is available at the collector terminal of Q₂ after being shift by 180^o in its phase.

This means that the output of the second stage will be 360^o out-of-phase with respect to the input, which inturn indicates that the phase of the input signal and the phase of the output signal obtained at stage II will be identical.

Further it is to be noted that the cascading of individual amplifier stages increases the gain of the overall circuit as the net gain will be the product of the gain offered by the individual stages. However in real scenario, the net gain will be slightly less than this, due to the loading effect.

In addition, it is important to note that by following the pattern exhibited by Figure 1, one can cascade any number of common emitter amplifiers but by keeping in mind that when the number of stages are even, the output will be in-phase with the input while if the number of stages are odd, then the output and the input will be out-of-phase.

The frequency response of an RC coupled amplifier—depicted in Figure 2 as a gain versus frequency curve—shows a stable gain across a broad mid-frequency range, with significant drops at both the low and high ends.

At low frequencies, the high reactance of the coupling capacitor C_C allows only a fraction of the signal to pass between stages. Similarly, the emitter capacitor C_E also exhibits high reactance, failing to adequately shunt the emitter resistor R_E, thereby reducing the voltage gain.

Conversely, at high frequencies, the low reactance of C_C mimics a short circuit, increasing the loading effect on the subsequent stage and reducing the voltage gain. Moreover, the capacitive reactance at the base-emitter junction is also low.

This increased base current reduces the voltage gain by lowering the current amplification factor β. In the mid-frequency range, though, decreasing reactance of C_C would normally increase the gain, but this is offset by a rise in the loading effect, keeping the amplifier’s gain uniform throughout.

Advantages of RC Coupled Amplifier

The advantages of a RC coupled amplifier include:

1. Cheap, economical and compact as it uses only resistors and capacitors.
2. Offers a constant gain over a wide frequency band.

Disadvantages of RC Coupled Amplifier

The disadvantages of a RC coupled amplifier include:

1. Unsuitable for low-frequency amplification.
2. Low voltage and power gain as the effective load resistance (and hence the gain) is reduced due to the fact that the input of each stage presents a low resistance to its next stage.
3. Moisture-sensitive, making them noisy as time elapses.
4. Poor impedance matching as it has the output impedance several times larger than the device at its end-terminal (for example, a speaker in the case of a public address system).
5. Narrow bandwidth when compared to JFET amplifier.

Applications of RC Coupled Amplifier

The applications of a RC coupled amplifier include:

1. RF Communications.
2. Optical Fiber Communications.
3. Public address systems as pre-amplifiers.
4. Controllers.
5. Radio or TV Receivers as small signal amplifiers.