RC Coupled Amplifier
Rc coupled amplifier is a two stage CE amplifier using NDN transistor is shown in the above picture. It is a most popular type of coupling because it is cheap and provide excellent operation
Rc (Rc1 and R2) is the collector load resistor. R1,R2 and Rg provide biasing and stabilization. Ce prevents the loss of amplification due to negative feedback.
The capacitors Ci and Co are used to block the DC signal and transmit only AC signal. The capacitor Cc is used to connect the output (collector) of first stage to the input (base) of second stage.
When an AC signal is applied to the base of the first transistor (Q1) it appears in the amplified form across its collector load register Rc1. This amplified signal is given to the base of next stage through pling capacitor Cc..
The second stage does further amplification of the signal. The first stage produces 180° phase shift and the second stage produces another 180° phase shift. So the total phase shift is 360°. Hence the output signal is in phase with the applied input signal.
The overall gain of this amplifier is equal to the product of the gain of two stages. The influence of the capacitors used in this circuit, reduces the overall gain of the amplifier at low and high frequency ranges.
The gain is high and constant, only at mid frequency ranges.
An amplifier should operate irrespective of the frequency of the input signal. but practically an amplifier does not magnify all frequency range of input signal with an equal amount
Practically, by using the reactive components, the gain will be decreased at low and high frequency ranges and constant only at mid frequency ranges. This behaviour of the amplifier is briefly explained below.
1.Low Frequency Range
‘At low frequencies, the reactance of coupling capacitor ce is quit high and hence very low amount of signal will pass from first stage to the second stage. So gain will be reduced. Similarly the reactance of the bypassing capacitor C is very high, hence it cannot shunt the emitter resistor R effectively.
These two factors reduce the voltage gain of the amplifier at low frequency range.
High Frequency Range
At high frequencies the reactance of the coupling capacitor is very low. Hence it behaves as a short circuit. This increases the loading effect of next stage.
Similarly at high frequencies the reactance of bypassing capacitor Ce is very low which increases the base current. This reduces the current amplification factor B. These two factors reduce the voltage gain of the amplifier at high frequency range.
Mid Frequency Range
At mid frequencies the voltage gain of the amplifier is constant. At this range the effect of coupling capacitor is to maintain a uniform voltage gain.
In this range as the frequency increases, the reactance of C, decreases which tends to increase the voltage gain.
However at the same time, lower reactance means higher loading of the first stage tends to decrease the Voltage Gain these two factors almost cancel each other resulting in a uniform gain at mid frequency range
RC Coupled Amplifier Frequency Response
The frequency response characteristics of a two stage RC coupled amplifier is shown in the below picture
The gain is reduced at low and high frequency ranges and constant only at mid frequency range. From the response curve we will calculate lower cut off frequency (fL),upper cut off frequency (fu) and Bandwidth (BW)
Lower Cut Off Frequency
The frequency at which the voltage gain of the amplifier is 70.7% of maximum gain on lower side of the frequency range is called lower cut off frequency
Upper Cut Off Frequency
The frequency at which the voltage gain of the amplifier is 70.7% of maximum gain on the higher side of the frequency range is called upper cut off frequency
The range of frequency over which the gain is equal to or greater than 70.7% of maximum gain is known as bandwidth
It is also defined as the frequency difference between the upper cut off frequency and lower cut off frequency
Advantages Of RC Coupled Amplifier
- Frequency response is excellent
- Cost is low
- Non linear distortion is low
- It is compact, light and small
Disadvantages Of RC Coupled Amplifier
- Low voltage gain and power gain
- Noise is produced with age, particularly in moist climates
- Impedance matching is poor
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