transistor biasing circuit explanation

 Transistor biasing

what is transistor?

     A transistor consists of two pn junction formed by sandwiching either p-type or n-type semiconductor pair of opposite types.

     There are two types of transistors names 

1. n-p-n transistor
2. p-n-p transistor

transistor biasing:

    The basic function of transistor is to do amplification.so first we give a weak signal to the base of transistor and amplified output is obtained in the collector circuit.

    The proper flow of zero signal collector current and the maintenance of proper collector-emitter voltage during the passage of signal is known as Transistor biasing.

faithful amplification :

    The process of raising the strength of a weak signal without any changes in its general shape .This process is known as faithful amplification.

    This process is revealed that it will function properly if its input circuit base emitter junction is forward biased and output circuit of collector base junction remains reversed biased.

    The following basic condition must be satisfied

1. proper zero signal collector current.
2. Minimum proper base-emitter voltage (Vbe)at any instant.
3. Minimum proper collector-emitter voltage (Vce) at any instant.   

     The basic purpose of transistor biasing is keep the base-emitter junction properly forward biased and collector base junction is properly reverse biased during the application of signal.this is can be achieved with a bias battery or associationg a circuit with a transistor.

Inherent variations of transistor:

     The inherent variations of transistor parameters may changes the operating point ,result in unfaithful amplification.It is therefore ,very important that biasing network be so changed that it should be to able to work with with all the transistors of one type whatever may be spread in β or Vbe.

    In other words ,the operating point should be independent of transistor parameter variations.

Essentials of transistor biasing:

1. It should ensure proper zero signal collector current.
2. It should ensure that Vce does not fall below 0.5v for Ge transistor and 1V for silicon transistor at any instant.
3. It should ensure the stabilization of operating point.

methods of transistor biasing:

     There are four methods available for transistor biasing.

They are,

1. Base resistor method or fixed bias method
2. Emitter bias method
3. Biasing with collector-feedback resistor resistor
4. voltage divider bias

Base resistor method or fixed bias method:

    In this method,a high resistance Rb (several hundred ohms)is connected between the base and +ve voltage supply for npn transistor and base negative end supply for pnp transistor.

Circuit analysis:

    +vcc = voltage supply

     Ib    = base current Ic = collector current

     Rb  = base resistor Rc = collector resistor

     Ie   =  emitter current Re = emitter resistor

    here the required zero signal base current is provided by Vcc and it flows through Rb.It is because now base is postive w.r.t emitter which means base-emitter junction is forward biased..The required value of zero signal collector current Ib can be made to flow by selecting the proper value of base resistor Rb.  

It is required to find the value of Rb so that required collector current flows in the zero signal conditions.

                     IB=(Ic/β)

             Vcc  = IBRB+Vbe

             IBRB= Vcc-Vbe

             RB   =(Vcc-Vbe)/IB

      since Vbe is generally quit small as compared to Vcc,the former can be neglected with little error.

      Now 

                     RB=Vcc/IB

Advantages of this method :

1.       This biasing circuit is very simple as only one resistance Rb is required.
2.       Biasing condition can easily be set and easy calculative.
3.       There is a no loading of the source by the biasing circuit since the no resistor is employed          across the base-emitter junction.

Emitter bias circuit:

      A circuit which is used to establish a stable operating point is the self biasing configuration.The current in the Re in the emitter lead causes voltage drop which is in the direction to reverse bias the emitter junction .since this junction is forward biased, base voltage is obtained from the supply voltage through the R1,R2 network.

     

    let us considered the circuit is this circuit differs from two important respects.first is it uses two seperate dc voltage across it Dc voltage sources .one is +v cc and other negative -Vee. normally two supply voltages are equal.

     for ex, if Vcc=+20V (dc),then Vee=-20V (dc)

     secondly ,there is a resistor Re in the emitter circuit.

circuit analysis:

 (i)collector current (Ic):

      Applying kirchoffs voltage law to the base-emitter circuit .

finally we get    Ic=(Vee-Vbe)/(Re+(Rb/β))

(ii)collector emitter voltage(Vce):

     applying kirchoffs voltage law to the collector side of the emitter bias circuit.

 finally we get      Vce=Vcc+Vee-Ic(Rc+Re)

biasing with collector feedback resistor :

   In this method one end of Rb is connected to the base and other end to the collector .here the required zero signal base current is determined not by Vcc.

advantages :

1.      It is a simple method its requires only one resistor Rb.
2.      This circuit provides some stablization of the operating point as

                        Vce=Vbe+Vcb