JEE Main & Advanced Physics Semiconducting Devices Transistor as an Amplifier

A device which increases the amplitude of the input signal is called amplifier.

The transistor can be used as an amplifier in the following three configuration

(i) CB amplifier 

(ii) CE amplifier 

(iii) CC amplifier

(1) NPN transistor as CB amplifier

(i) \[{{i}_{e}}={{i}_{b}}+{{i}_{C}};\] \[{{i}_{b}}=5%\] of \[{{i}_{e}}\] and \[{{i}_{C}}=\text{ }95%\] of \[{{i}_{e}}\]

(ii) \[{{V}_{EE}}<{{V}_{CC}}\]

(iii) Net collector voltage \[{{V}_{CB}}={{V}_{CC}}-{{i}_{C}}{{R}_{l}}\]

When the input signal (signal to be amplified) is fed to the emitter base circuit, it will change the emitter voltage and hence emitter current. This in turn will change the collector current \[({{i}_{c}})\]. This will vary the collector voltage \[{{V}_{CB}}\]. This variation of \[{{V}_{CB}}\] will appear as an amplified output. (iv) Input and output signals are in same phase

(2) NPN transistor as CE amplifier 

(i) \[{{i}_{e}}={{i}_{b}}+{{i}_{C}};\] \[{{i}_{b}}=5%\] of \[{{i}_{e}}\] and \[{{i}_{c}}=95%\] of \[{{i}_{e}}\]

(ii) \[{{V}_{CC}}>{{V}_{BB}}\]

(iii) Net collector voltage \[{{V}_{CE}}={{V}_{CC}}-{{i}_{c}}{{R}_{l}}\]

(iv) Input and output signals are \[{{180}^{o}}\] out of phase.