Prove that in a common-emitter amplifier, the output and input differ in phase by 180. In a transistor, the change of base current by 30 HA produces change of 0-02 V in the base-emitter voltage and a change of 4 m. in the collector current. Calculate the current amplification factor and the load resistance used, if the voltage gain of the amplifier is 400.
The circuit of a common-emitter amplifier using an n-p-n transistor is shown below :
In a common emitter amplifier circuit, the input signal voltage and output collector voltage are in opposite phase. i.e 180° out of phase. Thus the phase difference between the input signal and output voltage is 180°.
The common emitter transistor amplifier is the only configuration that gives an inversion, 180°, between the input and output signals.
The reason for this can be seen from the fact that as the input voltage rises, so the current increases through the base circuit. In turn this increases the current thought the collector circuit, i.e. it tends to turn the transistor on. This results in the voltage between the collector and emitter terminals falling.
In this way an increase in voltage between the base and emitter has resulted in a fall in voltage between the collector and emitter terminals, in other words the phase of the two signals has been inverted.
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