Saturday, February 09, 2013

Short Notes on Bipolar transistors

NPN Transistor

Placing P-type semiconductor between two N-type semiconductor is NPN transistor

It operates by a small current flow from the emitter to base.current will not  flow from the emitter to the collector until a small voltage (at least 0.7 volts) is applied to the base.

Collector current is much larger than the base current.


 PNP Transistor

 Placing N-type semiconductor between two P-type semiconductor is PNP transistor.

Its operation is same as NPN but reversed.The voltage relations also reversed.So to turn on the device both the collector and base must be negative.

PNP symbol


 Transistors are used as amplifiers to increase the input signals in TV ,stereo and others applications.It is often called as linear electronics since it contains direct relation with the input and output signal.

If the base is given with the power,it gets biased.It switch on the transistor.An increase or decrease in the input signals makes an increase and decrease in the output signal but with the signal inverse.Frequency remains the same.


The measure of amplification is the gain.

For example, if the input signal has an amplitude of 0.2 volts, and the output signal has an amplitude of 10 volts, then:
Power gain =current gain x voltage gain.


Transistors can also be used for switching in case of digital circuits.Digital circuits needs switch on and off.

When the voltage is below 0.6v no current flows through the circuit.when transistor is not conducting ,it is said to be cut-off.

When the base voltage is about 0.85 volts, sufficient base current flows to turn the transistor fully on. The collector voltage drops to approximately half a volt because of the voltage drop across the collector resistor. A transistor which is conducting the maximum current is said to be in saturation.

Transistor Application






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