1. Three Regions

2. Three Regions
Active Region: The collector-base junction is reversed biased, while the base-collector junction
Cutoff Region: The collector-base and base-emitter both junction are reversed biased.
Saturation Region: The collector-base and base-emitter both junction are forward biased.

3. saturation region
Suppose VBB and VCE are set to produce a current IB > 0 and VCE=0V. Then both BE and BC junctions are forward biased. The saturation region   corresponds to the case where both junctions are forward biased

4. cutoff region
Figure shows collector curves for different values of base current. When IB=0 A, the transistor is in its cutoff region  , and only a small leakage current flows as IC.

5. active region
As VCE increases and reaches about 0.7 V, the BC junction becomes reverse biased (before this, the transistor is in saturation). When VCE exceeds 0.7 V, the transistor goes into the active region,   or linear region   corresponding to the BE junction forward biased and BC reverse biased. This is the normal mode of operation of the transistor, e.g. as in amplifiers. In the active region the collector characteristic is almost flat, meaning that IC is almost constant but VCE can vary. The value of IC is determined by IB:
If VCE increases too much, the transistor goes into the breakdown region. This should be avoided.

6. Identify the Regions

7. Problem: Using the figure opposite:
1. Determine Collector current at IB=100uA, and VCE=10V
2. Determine IC at the same base current but at VCE of 2V.

8. DC Model of a Transistor
Alpha  describes the relation between collector and emitter currents; i.e.;
The value of the alpha is normally ranges from 0.90 to 0.998

9. Parameters of a Transistor
Beta: gives relation between Collector Current and Base current; i.e.
Typical values of Beta ranges from 50 to 400

10. Basic Three configurations
1. Common Emitter Common Collector Common Base

11. Common Emitter

12. Common Base in Forward

13. Common Base in Reverse

14. Common Collector

15. Common Emitter
In an emitter follower circuit with the collector connected to +V and a load connected between the emitter and ground, the voltage applied to the base minus the base emitter forward voltage drop (~0.6 V) will appear across the load (i.e., 5 V base = 4.4 V emitter). The only caveat is that the voltage source at the base must be able to supply about 5% of the load current without appreciable voltage drop. This is a non-inverting voltage follower circuit.
NPN C B E

16. Common Emitter N
Collector
Base P N
Emitter

17. Common Base
A popular circuit for use at high frequencies is the cascade amplifier, as shown below:-

18. Common Base
The Circuit diagram and its equivalent circuit model for the CB amplifier are shown in Fig Since the sum of the currents leaving the emitter junction is 0, we have the following expression