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Describe the function of a transistor as an amplifier with the neat circuit diagram. Sketch the input and output wave form. |
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Answer» Solution :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 . |
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