Transistor Biasing: Common Emitter, Base, Collector

[amaresh92]

can anyone please tell me when and why we required different configuration of transistor i.e. common emitter or common base or common collector?
advanced thanks.

 

[vk6kro]

Common emitter amplifiers provide excellent gain but fairly low input impedance and some distortion.
It is possible to trade off some of the gain for better input impedance and lower distortion by introducing an unbypassed emitter resistor.
At high frequencies they also experience Miller Effect where the base-emitter capacitance is magnified by the gain of the transistor and appears as a large capacitance across the input of the amplifier. This can severely limit the high frequency response of the amplifier.

Common collector circuits ( also called emitter followers) are mainly used as buffers. They provide good isolation between input and output. They have a high input impedance and low distortion and good frequency response.
However, they have a voltage gain of less than 1, so you get less voltage out than you put in.
The voltage you do get out is at very low impedance, though, and this can be an advantage if the amplifier is being used as the output device of an instrument.

Common base amplifiers are rarely used now. They give high gain and good frequency response, but they have a very low input impedance which makes them dofficult to drive. Their input is almost like a short circuit.
There are some oscillator circuits that use a grounded or bypassed base and these work very well at UHF.

 

[skeptic2]

Some uses for common base amplifiers are non-inverting amplifiers with gain and level shifters. I draw common base amplifiers the same as common emitter amplifiers with fixed bias and an emitter resistor. The base may also have a substantial capacitor on it. Instead of inputting the signal at the base, the signal is input at the emitter. The non-inverted output appears on the collector. I have used that as a level shifter to convert TTL to 15 V CMOS.

Common base amplifiers are also used for RF because they too have good input - output isolation and the input impedance can easily be matched to 50 ohms.

 

[amaresh92]

as i am concerned about the cb configuration it amplify or increases the input voltage.can anyone tell me how does it happening?
thanks

 

[sophiecentaur]

In very general terms, the choice will relate to considerations like - do you want it to invert? what input impedance do you need? what output impedance do you need ? How much voltage gain do you want?
Each of the configurations give different combinations of the above. But the transistor itself is still functioning in exactly the same way. i.e. the current into its base is controlling the current flowing from c to e.

In a cb config, a small variation of Vbc will cause a large variation in collector current - which will cause a large voltage swing across the collector load. The input impedance in this case is quite low.

 

[amaresh92]

[

In a cb config, a small variation of Vbc will cause a large variation in collector current - which will cause a large voltage swing across the collector load. The input impedance in this case is quite low.[/QUOTE]

can you please tell me what is meant by input impedance? is it different from ac impedance?
thanks

 

[sophiecentaur]

Try
http://en.wikipedia.org/wiki/Input_impedance" ?