1. Microelectronics, BSc course
Bipolar transistors 3

2. Characteristics of the ideal BJT
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

3. Common base setup input output Also called grounded base setup
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

4. Common base setup Input characteristic: Output characteristic: IE
normal active IE
saturation
normal active
closed
saturation
closed
inverse active
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

5. The process of amplification
common base setup in
out
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

6. Common emitter setup Also called as grounded emitter setup 13-10-2015
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

7. Common emitter setup B : common emitter, large signal current gain
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

8. Common emitter setup No current flows.
increasing
is constant
charge increases
charge is constant
No current flows.
Part of the base current is spent on accumulating the base chrage. UBE voltage increases, the emitter current starts to flow. The other part of the base current is spent on recombination with some part of the emitter current.
The charge in the base is not increased any longer. Any increase of the base current recombines with a given part of the emitter current, thus, the emitter current will also increase.
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

9. Common emitter setup Input characteristic: Output characteristic:
saturation
saturation
inverse active
normal active
normal active
closed
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

10. Threshold of saturation:X
saturation
UCE
Threshold of saturation:
UBC = 0
UBE = UCE
saturation
inverse active
normal active
normal active
closed
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

11. Characteristics of real BJTs: secondary effects
Parasitic CB diode
Series resistances
Early effect
Operating point dependence of the gain
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

12. Effect of the parasitic CB diode
inner transistor
parasitic junction
No emitter region opposite to it, thus, in inverse oparation the electrons injected from the collector into the base will be lost: inverse active current gain is worsened.
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

13. Effect of series resistances
Base contact
RBB'
The "inner base" – good approximation: RBB' B' B
Where is it exactly? E C
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

14. Effect of series resistances
Collector contact
n+ emitter
reduction of RCC' in case of discrete transistors: epitaxial structure (like in case of diodes)
collector
p base
chip carrier (collector lead)
ICRCC' adds to UCE  characteristics can be only on the right hand side of the 1/RCC' line
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

15. The Early effect
Backlash: The output voltage influences the input characteristic
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

16. The Early effect
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

17. The Early effect The Early voltage out 13-10-2015
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

18. Early effect at common base setup
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

19. Early effect: the backlash
~exp(UBE/UT)
CB setup
CE setup
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

20. The Early effect Problem
What is the output resistance of the transistor in common emitter setup is the Early voltage is 80V and the collector current in the operating point is 5mA?
out
out
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

21. Op.p. dependence of current gain
Voltage dependence: due to the Early effect
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET

22. Op.p. dependence of current gain
Voltage dependence: due to the Early effect
Current dependence:
high level
Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET