Bipolar Junction Transistors PPT physics

Bipolar junction transistor

1. Bipolar Junction Transistors Part I

2. Symbol : The arrow points from p side to n side . It represents the direction of Emitter current (I E ) .

3. Modes of operation of a Transistor: There are three modes of operation of a Transistor Active mode Cut-off mode Saturation mode

4. Transistor operation: One p–n junction of the transistor is reverse-biased whereas the other is forward-biased. Fig.: Biasing of a Transistor: (a) Forward bias (b) Reverse bias (a)Heavy flow of majority charge carriers (b) No majority charge carrier flow. Only minority carriers cross the depletion region. Majority carriers in (a) appear as minority carriers in (b) so they easily cross the depletion region.

5. The charge carriers that flow from emitter towards collector make the Emitter Current I E (order milliamperes ). During their motion, some carriers take the path of high resistance to the base terminal and contribute to Base Current I B . Its magnitude is very low i.e., of the order of microamperes . A large number of majority charge carriers diffuse through the base to the collector and form the Collector Current I C (order milliamperes ). Collector current I C comprises of both majority and minority charge carriers. Hence we can write Ic = Ic majority + Ico minority Ico minority is known as Leakage Current (I C current with emitter terminal open). It is temperature sensitive and affects the stability of the system. It is in the same direction as Ic majority and is of the order of micro/nanoamperes .

6. Applying Kirchoff’s Law in the figure, we get- I E = I B + I C Fig.: Majority and minority carrier flow of a pnp transistor.

7. Common Base Mode Fig.: Notation and symbol of CB mode of an n-p-n transistor

8. Input characteristics: I E vs V BE at constant V CB Curve is similar to forward biased p-n junction characteristic curve.

9. Output characteristics: I C vs V CB at constant I E

10. Different regions of operation in CB mode: Active region: B-E junction is forward-biased, whereas the C-B junction is reverse-biased. Cut-off region B-E junction and C-B junction both are reverse-biased. Saturation region B-E junction and C-B junction both are forward-biased.

11. Current Gain in CB Mode: It is defined as α = at constant V CB Since I C ~ I E , Hence α~1. Ideally α = 1, but we find conventionally that it is around 0.95-0.99  

12. Fig.: Notation and symbol of CE mode of an n-p-n transistor Common Emitter Mode

13. Input characteristics: I B vs V BE at constant V CE Curve is similar to forward biased p-n junction characteristic curve.

14. Output characteristics: I C vs V CE at constant I B

15. Current Gain in CE Mode: It is defined as β = at constant V CE Since I B is very small as compared to I C , hence β is very large.  

16. Since I E = I B + I C ; α = and β = Hence we can write; = + I c or, = + 1 and   Relation between α and β :

17. Cut-off, Active and Saturation region: 1. V BE < 0.7 V (Cut-off region) E-B junction is not properly forward biased. I B ≈ 0, I C ≈ 0, V CE = 3V Transistor is not conducting. 2. V BE ≈ 0.7 V, let I B = 10 μ A and β = 100 Then, I C = 1mA, Voltage drop =1V, V CE =2V. ( i ) 0 <I B <30 μ A, I C = β I B , 0 < V CE < 3V (Active region) (ii) I B > 30 μ A, I C = 3mA (max & constt .), V CE = 0V (Saturation region)

18. Fig.: Notation and symbol of CC mode of an n-p-n transistor Common Collector Mode

19. Input characteristics: I B vs V CB at constant V CE Curve is different from CB and CE configuration. Input V BC depends on V EC as, V BC = V CE – V BE V BE is knee voltage (0.7 for Si and 0.3 for Ge) At 60 μA , V EC ≈ V CC = 2V (say), then V BC = 2- 0.7 = 1.3V (for Si) Now, if V BC increases, V BE decreases, I B decreases (width of depletion region increases, effective base width decreases, I B decreases)  

20. Output characteristics: I E vs V CE at constant I B

21. Current Gain in CC Mode: It is defined as γ = at constant V CE Since I B is very small as compared to I E , hence γ is very large.  

22. Relation between α , β and γ : Since I E = I B + I C ; α = , β = and γ = or, ΔI C = α ΔI E Hence we can write; I E = I B + α I E I B = I E (1- α ) =  

23. Load line analysis: In absence of input signal in the base-emitter circuit, V CE is given as V CE = V CC – R L I C or I C = - Where, V CE is the e.m.f. of the source used in collector circuit, R L is the load resistance and I C is the collector current.   To find the collector current I C corresponding to a collector-emitter voltage V CE at a fixed base current I B , we consider the concept of load line.

24. Load line analysis contd … Graph plotted for I C versus V CE is a straight line with slope is -1/R L and intercept V CC /R L on I C -axis and intercept V CC on V CE -axis. Line AB is Load Line. The load line divides the output characteristics in the following three regions: (a) Cut-off region, (b) Saturated region, and (c) Active region.

25. Cut off region : The point R at which the load line AB meets the curve I B = 0, is called the cut off point . At the point R, I B = 0 and I C =I CEO (reverse leakage collector-emitter current) which is nearly zero, hence V CE ≈ V CC . Base-emitter junction is longer in the forward bias, hence the normal operation of transistor ceases. Saturation region : The point P at which the load line AB meets the curve at saturated base current (I B ) sat is called the saturation point . At the point P, I C = (I C ) sat ≈ V CC / R L and V CE = V knee . In the region left to AB line, the collector-emitter junction is no longer in the reverse bias, hence the normal working of transistor ceases. Load line analysis contd …

26. (c) Active region : The central part in which the collector current I C is nearly constant with increase in collector-emitter voltage V CE is called the active region. In this region, the emitter is forward biased and collector is reversed biased. It is the region where the transistor is operated to work as an amplifier. The reciprocal of the slope of the curve in this part gives the output dynamic resistance of transistor. The Q point lies almost in the centre of the curve as shown. It defines the operating conditions of the transistor. The transistor is operated at these values to get the maximum amplified output. Load line analysis contd …