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Bài giảng Electronic circuits - Chapter 2: BJT - Trịnh Xuân Dũng

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Dr. Dung Trinh  
HCMUT / 2017  
Electronic Circuits  
Chapter 2: BJT  
Dr. Dung Trinh  
1
Dr. Dung Trinh  
HCMUT / 2017  
Content  
Bipolar Junction Transistor  
BJT Modes of Operation  
BJT Circuits at DC  
Small Signal Models  
BJT Amplifier Configurations  
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Dr. Dung Trinh  
HCMUT / 2017  
Bipolar Junction Transistor  
• First Introduced in 1948 (Bell labs), consists of 2 pn junctions.  
• The choice in the design of both discrete and integrated circuit for 3 decades.  
• the MOSFET was undoubtedly the most widely used electronic device, and CMOS  
technology the technology of choice in the design of integrated circuits.  
• The BJT remains popular in discrete-circuit design.  
• The BJT is still the preferred device in very demanding analog circuit applications.  
• The BJT can be combined with MOSFETs to create innovative circuits.  
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Dr. Dung Trinh  
HCMUT / 2017  
Bipolar Junction Transistor (BJT)  
BJT consists of two pn  
junctions:  
- emitter-base  
(EBJ)  
junction  
- collector-base junction  
(CBJ)  
A simplified structure of npn transistor  
Depending on the bias  
condition (forward or reverse)  
of each of these junctions,  
differentmodes of operation  
of the BJT are obtained.  
A simplified structure of pnp transistor  
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HCMUT / 2017  
BJT Modes of Operation  
Regions of operation of a BJT transistor (example for a pnp BJT):  
푽푬푩  
Forward active region  
Saturation region  
(emitter-base FB, collector-base FB)  
(emitter-base FB, collector-base RB)  
푽푪푩  
Cut-off region  
(emitter-base RB, collector-base RB)  
Since BJT has three terminals, there are three possible amplifier types:  
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HCMUT / 2017  
Active mode (npn BJT)  
Emitter-base junction is Forward Biased, collector-base junction is Reversed Biased  
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HCMUT / 2017  
Active mode (npn BJT)  
- Forward bias of EBJ causes electrons to diffuse from emitter into base.  
- As base region is very thin, the majority of these electrons diffuse to the  
edge of the depletion region of CBJ, and then are swept to the collector by  
the electric field of the reverse-biased CBJ.  
- A small fraction of these electrons recombine with the holes in base region.  
- Holes are injected from base to emitter region.  
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HCMUT / 2017  
Active mode (npn BJT)  
푖퐶  
푖퐵 =  
훽
푖퐸 = 푖퐵 + 푖퐶 = 훽 + 1 푖퐵  
훽 + 1  
푖퐶  
훼
=
푖퐶 =  
훽
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Dr. Dung Trinh  
HCMUT / 2017  
Active mode (pnp BJT)  
Current flow in a pnp transistor biased  
to operate in the active mode.  
Large signal model for the pnp transistor  
operating in the active mode  
Current gain  
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Dr. Dung Trinh  
HCMUT / 2017  
Active mode (npn BJT)  
For modern npntransistors, 훽 is in the range 50 to 200, but it can be as high as  
1000 for special devices.  
Example 1:  
The transistor in the following circuit having 훽 = 100  
and exhibits a 푣퐵퐸 of 0.7푉 at 푖퐶 = 1 푚퐴. Design the  
circuit so that a current of 2푚퐴 flows through the  
collector and a voltage of +5 푉 appears at the collector.  
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Dr. Dung Trinh  
HCMUT / 2017  
Active mode (npn BJT)  
Example 2: In the circuit shown in figure a, the voltage at the emitter was measured  
and found to be – 0.7 푉. If 훽 = 50, find 퐼퐸, 퐼퐵, 퐼퐶 and 푉퐶.  
Example 3: In the circuit shown  
in figure b, measurement  
indicates 푉퐵 to be +1.0푉 and 푉퐸  
to be +1.7푉. What are 훼 and 훽  
for this transistor? What voltage  
푉퐶 do we expect at the collector?  
Figure a  
Figure b  
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HCMUT / 2017  
Transistor Characteristics  
푖퐶 - 푣퐵퐸 characteristic for an npn transistor.  
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HCMUT / 2017  
Transistor Characteristics – Early Effect  
푖퐶 - 푣퐶퐵 characteristic for an npn transistor.  
푽푨: Early Voltage.  
푽푨 ~ 10V – 100V.  
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HCMUT / 2017  
Transistor Characteristics – Early Effect  
In saturation region, it behaves as a closed switch with a small resistance 푅퐶퐸푠푎푡  
.
The saturation I-V curve can be approximated by a straight line intersecting the  
푣퐶퐸 axis at 푉퐶퐸표푓푓  
.
The  
output  
resistance  
looking into the collector is  
not infinite:  
푽푨 + 푽푪푬  
풓풐 =  
푰푪  
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HCMUT / 2017  
BJT Circuits at DC - npn  
• Step 1: assume the operation mode.  
• Step 2: use the conditions or model for circuit analysis.  
• Step 3: verify the solution.  
• Step 4: repeat the above steps with another assumption if necessary.  
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HCMUT / 2017  
BJT Circuits at DC - npn  
Example 4: Analyze the following circuit to  
determine all node voltages and branch currents.  
Assume that β is specified to be 100.  
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HCMUT / 2017  
BJT Circuits at DC - npn  
Example 5: Analyze the circuit in Example 1 to determine all node voltages and branch  
currents. The voltage at the base is now +0V.  
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Dr. Dung Trinh  
HCMUT / 2017  
BJT Circuits at DC - npn  
Example 6: Analyze the circuit in Example 1 to determine all node voltages and branch  
currents. The voltage at the base is now +6V.  
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HCMUT / 2017  
BJT Circuits at DC - pnp  
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Dr. Dung Trinh  
HCMUT / 2017  
BJT Circuits at DC  
Example 7: Analyze the following circuit to  
determine all node voltages and branch currents.  
Assume that β is specified to be 100.  
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