RC Coupled BJT Amplifier (Bipolar Junction Transistor)Full Working Procedure - Knowledge Gyan

What is an Amplifier?

 An amplifier is an electronic circuit which can produce an enlarged but exact undistorted version of small a.c voltage or current waveforms . In many electronic devices such as radio,TV,tape recorder,public address system etc.

We require amplifiers.For example the small voltage(few microvolts) induced in the antenna of a radio set due to radio waves coming from a distant broadcasting station requires amplification before it can be fed to the loudspeaker.

Today We will discuss about RC coupled BJT amplifier,stay tuned with us to know full working procedure,Circuit Diagram and its application.

What is RC coupled BJT amplifier?

RC coupled BJT amplifier Is a cascaded or multistage amplifier a coupling network is used to connect the output of one stage to the input of the next stage.The choice of the coupling  element depends on the field of application of the amplifier.Thus we have RC coupled , transformer coupled, direct coupled etc.

For AF applications RC coupled amplifiers are probably the most widely used type of amplifiers because such amplifiers are stable,relatively less expensive and have better frequency response characteristics.

  Working Principle

The output V01 of stage-1 is coupled to the input Vi2 of stage -2 via a coupling capacitor Cc.It blocks d.c components of output voltage V01 from reaching the input of stage-2. The capacitor of the coupling capacitor Cc is chosen small such that it allows easy flow to the a.c components.The emitter resistor Re, the  resistors R1 and R2 and the power supply Vcc are used to establish the d.c bias .The bypass capacitors CE prevent loss of amplification due to negative feedback through RE.

   Frequency Response :

The variation of the voltage gain of an amplifier with the frequency of the input,signal is often referred to as the frequency response of the amplifier.A typical frequency response characteristic of an RC coupled amplifier is shown. 

The gain remains practically constant over a wide range of frequencies in the "Mid frequency range", but the gain decreases both in the low and high frequency ranges.The frequency response is primarily determined by the coupling capacitors and various junction capacitance of the transistor.

What is Mid-frequency range of RC Coupled BJT amplifier?

 In the mid-frequency range the reactances of the capacitors Cc and CE are so small as to constitute a virtual short circuit for the a.c components.The junction capacitances are very small and hence their reactances are very large.As they appear effectively in parallel with associated resistances these reactances need not be considered in the mid-frequency range.

The output voltage,

 Vo =-hfeIb . (RL||hie)....(1) 

Vi = hieIb.........(2)

 The voltage gain in mid-frequency range,

 AVM=Vo/Vi = -hfeRL/hie+RL 

The gain is independent of the frequency of the input-signal.

What is Low frequency Range of RC Coupled BJT amplifier? 

At low frequency the effect of various junction capacitors and CE remains negligible while the reactance of the coupling capacitor Cc becomes appreciable. The a.c voltage drop across Cc causes reduction in voltage gain.The approximate h-parameter equivalent circuit of an intermediate CE stage in this frequency range.

Therefore,low frequency voltage gain, 

AVL=AVM/1-j.f1/f 

AVL/AVM=1/(1+f12/f)^1/2 

At f=f1 AVL=AVM/2^1/2 and the power deliverder to a fixed load is reduced by a factor1/2.For this a f1 is called lower half power frequency or lower 3dB frequency.

What is High Frequency Range of RC Coupled BJT amplifier?

 At high frequency range the capacitors may be assumed to act as a virtual short a.c. components and their effect can be neglected. But the effect of base-emitter(Cbe) and the base collector(Cbc) junction capacitances can be neglected as they tend to shunt the load resistancee. The effective a.c. load becomes smaller and hence the voltage gain decreases with the increase in frequency.

At High frequency Range,

 

Cd=Cbe+(1+hfe/hie . RL)Cbc 

This equations that Cbe and Cbc are in effect equivalent to a single capacitor Cd connected across the input resistance hie. This equations shows that the base-collector junction capacitance Cbc, Which provides feedback from the output to the input,has the effect of a large capacitance at the input.This amplification of capacitive effect is known as Miller Effect.

Bandwidth of RC Coupled BJT amplifier 

The frequency range between the upper and the lower half power points i.e, f2-f1 is called the Bandwidth of the amplifier.It may be considered as a useful frequency range in which the amplifier can signal without much distortion.

Applications

1. This type of amplifier has excellent audio fidelity over a wide range of frequency.

2. It is widely used as voltage amplifiers.

3. Due to poor impedance matching, RC coupling is rarely used in final stages.

This is the main application of  this type of amplifier.

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