INTRODUCTION
The basic amplifying action of a transistor is produced by transferring the current from a low resistance input circuit to the high resistance output circuit. Hence the basic function of transistor is to do amplification. The weak signal with applied at the base of the transistor and the amplified output is obtained across the load connected in the collector circuit. The transistor can be used as an amplifier in the modes namely common base,common emitter and common collecter modes. The common base amplifiers are not frequently used. The amplifier are used to provide voltage gain without current gain and for impendence matching in high frequency application. The common emitter amplifiers provide large current gain, and large voltage gain. The common collecter amplifiers have very high input resistance and very low output resistance. These amplifiers do not provide any voltage gain. The common collector amplifiers are seldom used for amplification.They are used for impedance matching.The process of increasing the magnitude of a weak signal without any change in its shape,is known as faithful amplification. The faithful amplification can be achieved if the base -emitter junction is forward biased and the collector -base junction is reverse biased during both the positive half cycles and the negative half cycles of the signal. The proper flow of zero signal collecter current and the maintenance of proper collecter emitter voltage during the passage of signal is known as transistor biasing.The basic purpose of transistor biasing is the proper functioning of a transistor. The circuit which provides transistor biasing is known as biasing circuit. The process off making the operating point independent of temperature changes or variation in transister parameters is known as stabilisation. A good biasing circuit alway ensures the stabilisation of the operating point
FATHFUL AMPLICATION
Faithful amplification is the process of a weak signal without any change in its shape .
In order to achive faithful amplification the transistor amplifier must satisfy the following three basic condition.
(I) Proper zero signal collecter current
(II) Proper base-emitter voltage at any instant.
(III) Proper collecter -emmitter voltage at any instant.
The first two conditions are satisfied when base -emittter junction is properly forward biased. The last condition is satisfied when base collecter junction is properly reverse biased. If these conditiond are fulfilled then the transistor works in the active region of the output characteristicks between saturation and cutoff regions.Proper zero signal current :
Consider an NP-transister amplifier in common emitter mode with a D. C. source V¢¢ in the collector -emitter circuit and without aD.C
source in the base-emitter circuit.
During the positive half cycles of the signal, the base becomes positive and become negative and hence emitter junction becomes forward biased. Therfour the base current flows and a amplified collecter current flows in the output circuit. During the negative half cycles of the signal base becomes negative with respect to the emitter and hence the base-emitter junction becomes reverse biased. Therefore no current flows due to negative half cycles of the signal. Hence the positives half cycle of the signal are amplified in the collector circuit but the negative half cycles of the signal are cut-off. This is unfathefuly amplification and is shown.