A transistor operating in the active region has the capability to amplify weak signals.
Principal :
(i) Amplification is the process of increasing the signal ( increase in the amplitude.)
(ii) If a large amplification is required , the transistors are cascaded with coupling elements like resistors, capacitors, and transformers which is called as multistage amplifiers.
(iii) Here, the amplification of an electrical with a single stage transistor amplifier as shown in Figure.
Construction:
(i)Single stage indicates that the circuit consists of one transistor with the allied components. An NPN transistor is connected in the common emitter configuration.
(ii) The Q point or the operating point of the transistor is fixed so as to get the maximum signal swing at the output (neither towards saturation point nor towards cut-off ).
(iii) A load resistance `R_(c)` is connected in series with the collector circuit to measure the output voltage.
(iv) The capacitor `C_(E)` allows only the ac signal to pass through.
(v) The emitter bypass capacitor `C_(E)` provides a low reactance path to the amplified ac signal.
(vi) The coupling capacitor `C_(C)` is used to couple one stage of the amplifier with the next stage while constructing multistage amplifier`V_(S)`is the sinusoidal input signal source applied across the base-emitter.
(vii)The output is taken across the collector emitter.
Collector current `I_(C)=betaI_(B)[:'beta=(I_(C))/(I_(B))]`
(viii) Applying Kirchhoff's voltage law in the output loop , the collector -emitter voltage is given by
`V_(CE)=V_("CC")-I_(C)R_(C)`
Working of the amplifier:
(i) During the positive half cycle: Input signal `(V_(S))` increases the forward voltage across the emitter -base . As a result , the base current `(I_(B))` increases.
(ii) Consequently , the collector current `(I_(C))` increases `beta` times.
This increases the voltage `(V_(CE))` .
Therefore , the input signal in the positive direction produces an amplified signal in the negative direction at the output . Hence , the output signal is output signal is reversed by `180^(@)`
During the negative half cycle :
(i) Input signal `(V_(S))` decreases the forward voltage across the emitter - base.
(ii) As a result , base current `(I_(B))` decreases and in turn increases the collector current `(I_(C))` .
(iV) Thus, the input signal in the negative direction produces an amplified signal in the positive direction at the output .
(V) Therefore , `180^(@)` phase reversal is observed during the negative half cycle of the input signal .
