Small Signal Amplifiers.

Basic Transistor circuit:

*Q-point of above circuit:

      Is simply the values of Ic and Vce that are present in the transistor circuit.So if you are ever asked to find the Q point of a transistor,solve for Ic and Vce values for given circuit.

  

* the first step in the DC analysis of any transistor circuit is to solve for one of the unknown currents,Ic ,Ie and  

Ib.

* Recommended way of solving for one of the currents is to write kirchoff's voltage law loop.

Vbe=.0.7 ,  Ic=Voltage gain*Ib   ,Ie=Ie+Ic

* The operating point Q can be anywhere on the load line that is intersected by a collector characteristic.Note that this is the same as the diode method where one uses device characteristics in combination with circuit equations.

* Importantly , Vce determination allows one to verify if the initial assumption of active bias is correct.

And also we can write

                                  Ic=(Vcc-Vce)/Rc

                                  Ic=-(1/Rc)*Vce+ Vce/Rc

   

Graphically view of the load line.

 

This load line can be get for given circuit, by substituting Ic=0 and get  Vce,and by substituting Vce=0 and get Ic value and finaly we can mark calculated Ic and Vce value on the load line.It is the Q point of the given circuit.

Transistor as a switch

* The two extreme possible operating points Q(sat) and Q(cut off) show how the transistor can act as a switch.Simply When it is in saturation region switch is on and when it is cut off it is in off mode.When it is no current flowing.because of Ic=0 .

* The operating point Qsat corresponds to the maximum value of Ic.

* As Ic increase from zero,the voltage drop cross Rc increae,and so Vce must decrease until the BE junction becomes forward biased and transistor saturates.The saturations value of Vce is denoted Vce(sat)

 and is typically 0.2v

* The saturation value of the collector current is,

             

                        Ic =Vcc-Vce(sat)

                                      Rc

* In order to achieve saturation,VB1 must be must be large enough to ensure that Ib is greater than   

                     

                                       Ic(sat)

                                       β

                 which means that VB1 must satisfy

                             

                                                           VB1 >  Ic(sat)*Rb  +0.7v

                                                             β

* In saturation mode,the transistor is on, allowing,maximum current to flow.

* At the other extreme, Q(cutoff) the transistor is cutoff.This requires Ib=0, and hence Ic=0.The cutoff value of Vce is.

                    Vce(cutoff)= VB2

                                    VB1<  0

                              

          of course ,in this mode the transisor is off.switch is also off.

Amplifier analalysis using h- parameter model:

* The term small signal amplifier refers to the use of signals that take up a relatively small percentage of an amplifier's operational range. That is ,signals that use only a small portion of the load line.

*Device operating on small signals are found in almost every electronic system, such as pre amplifiers and operational amplifiers, for example ,and they are therefore of major importance.

*the small signal approximations are made  in the form of approximate model that take the the form of two-port systems.

*Bipolar junction transistors (BJTs) in in the forward active region and in the inverse-active region are approximately linear in nature. That is under small signal conditions.

*Once  that ac equivalent circuit has been determined, this model can replace the graphical symbol of transistor. Then , the basic method of ac circuit analysis can be used to determine the response of the amplifiers.

*The industrial and educational institutions rely heavily on the hybrid parameters(or h-parameters) equivalent circuit. The hybrid equivalent circuit continues to be very popular in the analysis of small-signal ac response of the BJT amplifier. This h-parameter realization is a common representation for small signal, low frequency BJT operation

AC Small Signal Model

       This is a small signal amplifier having two port.It  is activate and powered from same ac source.It is shown in below.

We can equate this circuit for following model.There is some parameters.we called the h parameters.we've mentioned it below.

These h parameters expressed from partial derivation of voltages and currents.

*  hix ,hie  - is the input resistance.(impedence) and appers in series at the input.

*  hrx , hre-  is the reverse ratio.And is multiplied by the output voltagen to produce an equivelent                 voltage source in series with the input.

*  hfx , hfe-is the forward current gain and is multipied by the input current to produce an equivalent  

                           current source in the output.

                        

*   hox ,hoe-  is the output conductance is and appears across the output terminals.