Hybrid Transistor Model with Two Port Network

The hybrid transistor model with two port network is discussed here.

1. Hybrid Transistor Model With two port network Prepared by: Muhammad Ridwanul Hoque

2. Two Port Network • A pair of terminals through which a current may enter or leave a network is known as a port . • Two terminal devices or elements (such as resistors, capacitors, and inductors) results in one – port network . • A two – port network is an electrical network with two separate ports for input and output . • It has two terminal pairs acting as access points. The current entering one terminal of a pair leaves the other terminal in the pair.

3. Why to Study Two Port Network Such networks are useful in communication, control system, power systems and electronics. Knowing the parameters of a two – port network enables us to treat it as a “black box” when embedded within a larger network.

4. z -parameters/ Impedance Parameters Here, I 1 & I 2 are independent variables. A two-port network driven by voltage source V 1 = z 11 I 1 + z 12 I 2 V 2 = z 21 I 1 + z 22 I 2

5. Where, Z 11 = open – circuit input impedance . Z 12 = open – circuit transfer impedance from port 1 to port 2 . Z 21 = open – circuit transfer impedance from port 2 to port 1. Z 22 = open – circuit output impedance. Fig: Determination of the z parameters: ( a) finding z11 and z21 , (b) finding z12 and z22

6. Fig: General Equivalent Circuit of z-parameters

7. An ideal transformer has no z parameters . The defining equations for the two-port network are: V 1 = (1/ n) V 2 I 1 = − n I 2

8. y-parameters/ Admittance Parameters Fig; Determination of the y parameters: (a) finding y 11 and y 21, (b) finding y 12 and y 22.

9. I 1 = y 11 V 1 + y 12 V 2 I 2 = y 21 V 1 + y 22 V 2 Fig: General Equivalent Circuit y 11 = Short-circuit input admittance y 12 = Short-circuit transfer admittance from port 2 to port 1 y 21 = Short-circuit transfer admittance from port 1 to port 2 y 22 = Short-circuit output admittance

10. h-parameters/ Hybrid Parameters

11. Matrix Form : Here, I 1 & V 2 are independent variables.

12. When V 2 =0 When I 1 =0 Where, h 11 =short– circuit input impedance . h 12 = open – circuit reverse voltage gain. h 21 = short – circuit forward current gain. h 22 = open – circuit output admittance.

13. The h-parameter equivalent network of a two port network V 1 = h 11 I 1 + h 12 V 2 I 2 = h 21 I 1 + h 22 V 2

14. Why is the name hybrid parameters? The four parameters associated with this model are input impedance , current gain and output conductance . Since their units are completely different from each other, this set of parameters is called as hybrid parameters . This circuit is called hybrid equivalent because its input portion is a Thevenin equivalent , or voltage generator with series resistance, while output side is Norton equivalent , or current generator with shunt resistance. Thus it is a mixture or a hybrid .

15. What will be the case if v 1 & I 2 are taken as independent variables?

16. Transistor Hybrid Model

17. Table below shows the h- parameter nomenclature of a transistor : F irst letter i , r , f or o indicates the nature of parameter . The second letters b, e and c respectively indicate CB , CE and CC arrangement .

18. Common-emitter configuration: (a) graphical symbol; (b) hybrid equivalent circuit.

19. Common-base configuration: (a) graphical symbol; (b) hybrid equivalent circuit.

20. Fig: Effect of removing h re and h oe from the hybird equivalent circuit. Fig: Approximate hybrid equivalent model.

21. Merits of Hybrid Model

22. Limitations of h Parameters It is very difficult to get the exact values of h parameters for a particular transistor . It is because these parameters are subject to considerable variation—unit to unit variation, variation due to change in temperature and variation due to change in operating point. In predicting an amplifier performance, care must be taken to use h parameter values that are correct for the operating point being considered. The h- parameter approach gives correct answers for small ac signals only . It is because a transistor behaves as a linear device for small signals only.

23. THANK YOU