Transistor as an Amplifier

Transistor as an Amplifier

• The function of the transistor is to raise the strength of input signal in this sense it works as an amplifier.  
• Let us consider that the Common Base ( CB ) transistor configuration is given. 
• The input or weak ( AC ) signal is given at the base – emitter ( BE ) junction and output is taken from collector – base ( CB ) junction.

The following condition must be fulfilled for faithfully amplification

• Input circuit always be forward biased
• Output circuit always be reverse biased

Bias Voltage

• The DC voltage V_BE is applied at the input in addition to AC signal for faithful amplification. 
• This DC voltage is known as bias voltage and its function is to always keep input circuit forward biased regardless the polarity of the AC input signal.

Effect of Input resistance and Output resistance

• When there is a small change in input signal voltage, small change in emitter current because the input resistance is very small. 
• The collector current flows in the high load resistance R_L will produces a large voltage drop across it. 
• Therefore the weak signal at the input is amplified in the collector or output circuit.

Example

Input signal = 200 mV

Input resistance = 20 ohm

Load resistance = 1 k ohm

Input current ( Emitter current )

                        = 200 m V / 20

                       = 10 m Amp

As the current amplification factor α close to unity.

               α = I_C / I_E

               I_C ≈ I_E  ( As α = 1 )

Collector current = 10 m Amp

Output voltage = Collector current × Resistance R_L

                         = 10 m × 1 k ohm

                         = 10 volt

Finally we can say that

Input voltage = 200 mV

Output voltage = 10 V

Amplification = Output voltage / Input voltage

                       = 10 V / 200 mV

                       = 50

The output voltage is 50 times that of input voltage.

Why the DC input voltage is applied at the input of the transistor?

• The input circuit becomes reverse biased during negative half cycle of the alternating supply. 
• The input circuit must be forward biased for the faithful amplification of the input signal. 
• Therefore the DC voltage is applied at the input side to keep the input circuit always forward biased.

What do you mean by bias voltage?

• The DC input voltage in the amplifier circuit in addition to the AC signal is known as bias voltage.

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