1. Lecture 10 Bipolar Junction Transistor (BJT)

2. Outline Continue BJT Continue DC analysis
More examples
Introduction to AC signal analysis
BJT

3. Example (1)
A bipolar transistor having IS = 5×10-16 A is biased in the forward active region with VBE =750 mV. If the current gain (or β) varies from 50 to 200 due to manufacturing variations, calculate the minimum and maximum terminal currents of the device.
BJT

4. Solution
For a given VBE, the collector current remains independent of β
The base current varies from IC/200 to IC/50:
the emitter current experiences only a small variation because (β+ 1)/β is near unity for large β:
BJT

5. Example (2) Determine the dc level of IB and VC for the BJT circuit

6. Solution For the dc mode, the capacitor assumes
the open-circuit equivalence
and RB =R1+R2
BJT

7. Example (3)
Determine the dc bias voltage VCE and the current IC for the voltage-divider configuration shown in the figure. (use exact and approximate methods)
BJT

8. Solution
Using Exact Analysis Method
BJT

9. Solution (cont’d)
Return to the example:
BJT

10. Solution (cont’d)
BJT

11. Solution (cont’d) Using Approximate Analysis Method
The condition that will define whether the approximate approach can be applied
BJT

12. Solution (cont’d)
Return to the example:
Testing:
BJT

13. Solution (cont’d)
Now, compare between obtained results by the exact and approximate methods
BJT

14. Example (4)
Determine the voltage VCB and the current IB for the common-base configuration as shown in figure
BJT

15. Solution Applying Kirchhoff’s voltage law to the input circuit yields
Applying Kirchhoff’s voltage law to the output circuit gives
BJT

16. Example (5)
Given the device characteristics as shown in figure, determine VCC, RB, and RC for the fixed bias configuration
BJT

17. Solution
From the load line
BJT

18. BJT Transistor Modeling
A model is an equivalent circuit that represents the AC characteristics of the transistor
A model uses circuit elements that approximate the behavior of the transistor
There are two models commonly used in small signal AC analysis of a transistor:
re model
Hybrid equivalent model
BJT

19. BJT AC Analysis
BJT

20. The re Transistor Model
BJTs are basically current-controlled devices; therefore the re model uses a diode and a current source to duplicate the behavior of the transistor
Recall (from lecture 5): the ac resistance of a diode can be determined by the equation rac = 26 mV/ID, where ID is the dc current through the diode at the Q (quiescent) point
One disadvantage to this model is its sensitivity to the DC level. This model is designed for specific circuit conditions
BJT

21. Common-Base Configuration
Input impedance:
Output impedance:
Voltage gain:
Current gain:
Forward-biased junction RL
BJT

22. Example
For a common-base configuration, as shown in figure, with IE = 4 mA, α = 0.98, and an ac signal of 2 mV applied between the base and emitter terminals:
(a) Determine the input impedance.
(b) Calculate the voltage gain if a load of 0.56 kΩ is connected to the output terminals.
(c) Find the output impedance and current gain.
BJT

23. Solution
BJT

24. Lecture Summary Covered material Continue BJT
DC analysis
More examples
Introduction to AC signal analysis
Material to be covered next lecture
Continue BJT analysis with AC signal
BJT