RC Coupled AmplifierPublished on 24/2/2012 & updated on 28/8/2018
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 CE is also called bypass capacitor which passes only AC while restricting DC, which causes only DC voltage to drop across RE while the entire AC voltage will be coupled to the next stage.
Further, the coupling capacitor CC 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 VCC 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 180o. The AC component of this signal is coupled to the second stage of the RC coupled amplifier through the coupling capacitor CC and thus appears 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 180o in its phase. This means that the output of the second stage will be 360o 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 a RC coupled amplifier (a curve of amplifier's gain v/s frequency), shown by Figure 2, indicates that the gain of the amplifier is constant over a wide range of mid-frequencies while it decreases considerably both at low and high frequencies. This is because, at low frequencies, the reactance of coupling capacitor CC is high which causes a small part of the signal to couple from one stage to the other. Moreover for the same case, even the reactance of the emitter capacitor CE will be high due to which it fails to shunt the emitter resistor RE effectively which inturn reduces the voltage gain. On the other hand, at high frequencies, the reactance of CC will be low which causes it to behave like a short circuit. This results in an increase in the loading effect of the next stage and thus reduces the voltage gain. In addition to this, for this case, the capacitive reactance of the base-emitter junction will be low. This results in a reduced voltage gain as it causes the base current to increase which inturn decreases the current amplification factor β. However, in mid-frequency range, as the frequency increases, the reactance of CC goes on decreasing which would lead to the increase in gain if not compensated by the fact that the reduction in reactance leads to an increase in the loading effect. Due to this reason, the gain of the amplifier remains uniform/constant throughout the mid-frequency band.
Advantages of RC Coupled Amplifier
- Cheap, economical and compact as it uses only resistors and capacitors.
- Offers a constant gain over a wide frequency band.
Disadvantages of RC Coupled Amplifier
- Unsuitable for low-frequency amplification.
- 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.
- Moisture-sensitive, making them noisy as time elapses.
- 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).
- Narrow bandwidth when compared to JFET amplifier.
Applications of RC Coupled Amplifier
- RF Communications.
- Optical Fiber Communications.
- Public address systems as pre-amplifiers.
- Radio or TV Receivers as small signal amplifiers.
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