This is the second part about BJT polarization. Are shown projects of circuits with fixed and emitter’s stable polarization.
If you aren’t from the field, I suggest to read the operation of BJT transistor and part 1 about this component’s polarization first.
Link to BJT operationClick here
Link to part 1 of BJT polarizationClick here
BJT fixed polarization
This is an example of fixed polarization circuit with NPN transistor. Emitter terminal is linked to ground.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/fixed-polarization-3-1.png)
Considering these information:
- Base current (I_{b}): 20 μA.
- Emitter current (I_{e}): 4 mA.
- Voltage between colector and emitter (V_{CE}): 7.2 V.
We have to find these information:
- Colector current (I_{c}).
- Beta (β).
- Supply voltage (V_{CC}).
- Base resistor (R_{b}).
Calculating I_{c}.
I_{e}=I_{c}+I_{b}
I_{c}=4000\mu -20\mu = 3980\mu =3.98mA
Finding out supply voltage V_{CC}.
I_{c}=\frac{V_{CC}-V_{CE}}{R_{c}}
3.98m=\frac{V_{CC}-7.2}{2.2k}
V_{CC}=15.956 V
Finding β and R_{b}.
I_{c}=\beta \cdot I_{b}
\beta= \frac{3.98m}{20\mu }=199
R_{b}= \frac{V_{CC}-V_{BE}}{I_{b}}
Considering V_{BE} as 0.7 V.
R_{b}= \frac{15.956-0.7}{20\mu }=762 k\Omega
Designing current sources with fixed polarization
This is a current source project with a NPN BD139 transistor, whose datasheet is in this link. Voltage supply is 6 Volts, while current colector must be 100 mA. Transistor must be in saturation state.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/current-source-design-1-1.png)
Looking at datasheet, you can see that in saturation state, voltage V_{CE} is 0.5 V.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/electrical-characteristics-Vce-1-1024x544.png)
Calculating R_{c}.
R_{c}=\frac{V_{CC}-V_{CE}}{I_{c}}=\frac{6-0.5}{0.1}=55\Omega
Collector’s resistor will dissipate sizeble power, therefore, must have a size bigger than 1/4 W resistors to avoid burn. The chosen commercial value is 47 Ω, because it was the closest I had with enough size to stand electric current.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/resistor-size-EN-1-768x1024.jpg)
To calculate base current I_{b}, current gain h_{FE} or β was considered 25.
I_{b}=\frac{I_{c}}{h_{FE}}=\frac{0.1}{25}=4mA
Determining value of R_{b}.
R_{b}=\frac{V_{CC}-V_{BE}}{I_{b}}=\frac{6-1}{4m}=1.25k\Omega
Commercial value of R_{b} is 1.2 kΩ.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/colector-current-in-Ic-1.png)
What if use a PNP transistor like BD136? Just invert DC source’s poles, emitter must be connected to positive pole.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/PNP-current-source-1-1.png)
This is another fixed polarization project, collector current was arbitrarily chosen as 150 mA. While gain, h_{FE} or \beta, is 40.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/datasheet-BD136-2-1024x542.png)
Calculating Rc.
Rc=\frac{V_{CC}-V_{CE}sat}{I_{C}}=\frac{6-0.5}{150m}=36\Omega
Finding Rb’s value. Parameters in the datasheet are negative, but since poles were exchanged, you can consider these values as positives.
I_{b}=\frac{0,15}{40}=3,75mA
R_{b}=\frac{5}{3,75m}=1,33k\Omega
The chosen commercial values for R_{c} and R_{b} are 30 Ω and 1,2 kΩ respectively. The value of I_{c} can vary between 149 and 155 mA.
Stable emitter polarization
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/circuit-sample-emitter-stability-1.png)
A resistor between emitter terminal and ground improves circuit’s stability. Mathematically demonstrating using two following circuits as examples.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/2-circuit-example-stability-1.png)
Calculating collector current I_{c} and colector-emitter voltage V_{CE} of T1 circuit, we have calculated values below. You can confirm considering base-emitter voltage V_{BE} as 0.7 V.
I_{c1}=2.925mA
V_{CE1}=8.089V
If T1’s β is 135, values are:
I_{c2}=4,38mA
V_{CE2}=4,174V
Calculating the percentage of parameters’ variation using the following equations.
%\Delta I_{c}=\left |\frac{I_{c}2-I_{c}1}{I_{c}1}\right |\cdot 100%
%\Delta V_{CE}=\left |\frac{V_{CE}2-V_{CE}1}{V_{CE}1}\right |\cdot 100%
Results are respectively 49.4% and 48.3%. Now, calculating the same parameters in T2 circuit. This equation can be used to find base current in emitter’s stable polarization circuit. R3 is emitter’s resistor. While V_{BE} is 0.7 V.
I_{b}=\frac{V_{CC}-V_{BE}}{R_{b2}+(\beta +1)\cdot R3}
Results are:
I_{c}=2.9 mA
V_{CE}=8.6465V
What if T2’s β is increased to 150?
I_{c}=3.9mA
V_{CE}=4.75V
Using percentage equations shown before.
%\Delta I_{c}=\left | \frac{3.9m-2.9m}{2.9m}\right |\cdot 100=34.4%
%\Delta V_{CE}=\left | \frac{4.75-8.647}{8.647}\right |\cdot 100=45%
T2 circuit is more stable, because collector current and collector-emitter voltage vary less when β increases 50%.
One more project
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/3rd-BJT-project-emitter-stability-1.png)
This is circuit project with a stabilizer resistor. Transistor used is BC547B, supply voltage V_{CC} is 6 V and collector current must be 20 mA. Looking at this graphic from datasheet, you can see that current base can be 0.15 mA when V_{CE} is 0.2 V.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/saturation-graphic-BC547B-1.png)
Using equation to find out values of R_{b} and R_{e}. β is:
\beta =\frac{20m}{0.15m}=133.3
Defining R_{b} and R_{e}.
I_{b}=\frac{V_{CC}-V_{BE}}{R_{b}+(\beta +1)\cdot R_{e}}
0.15m=\frac{6-0.7}{R_{b}+(133.3 +1)\cdot R_{e}}
0.15m\cdot R_{b}+20.145m\cdot R_{e}=5.3
The value chosen arbitrarily for R_{b} was 2.7 kΩ.
0.405+20.145m\cdot R_{e}=5.3
R_{e}=242\Omega
The commercial value of R_{e} is 220 Ω. Since emitter current is the sum of I_{b} and I_{c}, it’s value is 20.15 mA. Calculating emitter’s voltage.
V_{e}=220\cdot 20.15m=4.433V
Discovering voltage drop in R_{c}.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/finding-Rc-1.png)
V_{c}=6-(V_{CE}-V_{e})=6-(0,2+4,433)=1,367V
R_{c}=\frac{1,367}{20m}=0,06835k=68,35\Omega
Fortunately, I have a 68 Ω resistor. Finally, assembling the circuit in a protoboard and making measurements.
![](https://www.electricalelibrary.com/wp-content/uploads/2020/05/measuring-circuit-1-768x1024.jpg)
Current value I_{c} shown above aren’t exactly 20 mA. Because resistors have tolerance, causing a small variation in resistance.
Other parts about projects with BJT transistors will come.