Common Emitter Amplifier
- Consider the transistor amplifier circuit shown in Fig.
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| Input and output waveforms of CE amplifier |
- The emitter is common to both input and output. Hence this amplifier is known as a common emitter amplifier.
- The resistors R1, Ra form the voltage divider network. The voltage drop across Ra is used to bias the transistor.
- The coupling capacitor Cg is used to couple the input ac signal to the base of the transistor. It blocks any dc component present in the signal and passes only the ac signal.
- The coupling capacitor Cc is used to couple the output signal to the next stage.
- The bypass capacitor Cs is connected in parallel with emitter resistance Re to provide a low reactance path to the ac signal.
- If Cs is not connected, the amplified ac signal passing through Rg will cause a voltage drop across it.
- This will reduce the output voltage and reduce the gain of the amplifier.
- The input and output voltage waveforms are shown in Fig. We can find that the output is 180° out of phase with the input. This can be explained as follows.
- When the input is a positive-going signal, the base current increases. This increase in base current increases collector current.
- When collector current has increased the drop across the resistor Rc increases and thus reduces the level of the collector voltage.
- That is as the input voltage is increased in a positive direction, the output Vo goes in a negative direction. In the same way, when the input waveform goes in a negative direction, the base current decreases which decreases the voltage drop across Ro.
- This reduction increases the output voltage level.
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