After setting up this circuit I will measure and record Vbe, Vce, Ib and Ic whilst varying my base resistor.
Vs= 5V, Rc= 470Ω, BC547 NPN Transistor, Rb= (2.2KΩ, 46.7kΩ, 218KΩ, 270KΩ, 333KΩ)
- Rb= 2.2kΩ results in Vbe= 781mV, Vce= 34.7mV, Ib= 188.4µA, Ic= 9.32mA
- Rb= 46.7K results in Vbe= 704mV, Vce= 113.7mV, Ib= 93.2µA, Ic= 9.16mA
- Rb= 218K results in Vbe= 684mV, Vce= 1.84V, Ib= 20µA, Ic= 5.44mA
- Rb= 270K results in Vbe= 682mV, Vce= 2.37V, Ib= 16.2µA, Ic= 4.42mA
- Rb= 333K results in Vbe= 679mV, Vce= 2.75V, Ib= 13.2µA, Ic= 3.64mA
For Vbe, the readings slowly decrease from 781mV down to 679mV due to the resistors providing greater resistance. Therefore the greater current restriction (leading to voltage restriction) from the higher ohm rated resistors I used is the cause of the steady drop in Vbe that I recorded.
In this experiment the current for Ib is rated only in micro amps (µA) because it is only the current running throught the transistor switching circuit although it is still taken into account when using the formula Ie= Ic+Ib. This formula also shows that Ic and Ib will increase as the other does. If the current increases then Ib, Ic and Ie will all increase together, resulting in the steady increase in Ib as the resistor values are lowered.
When talking of Ic, my readings decrease steadily from 9.32mA to 3.64mA because of the sized resistors I am placing into the circuit before the collector. The bigger ohm resistor I place into the circuit means the current will be reduced further. With a 2K2Ω resistor, I obtain an Ic reading of 9.32mA, wereas with a 383KΩ resistor, my current has been restricted further and only get an Ic reading of 3.64mA.
Using Ic and Vce, I plotted them on the graph to create a load line. Then I used Vbe on the Vce scale to plot the values of Ib.
The Beta (Hfe) when relating to the graph above equals: B= Ic/Ib
- At 113.7mV B= 9.16/0.0932
- At 1840mV B= 5.44/0.020
- At 2750mV B= 3.64/0.0132
The load line graph I have plotted shows me my transistor in its saturated, active and cut-off regions. My two readings of Ic: 9.32mA, Vce: 34.7mV and Ic: 9.16mA, Vce: 113.7mV are both situated in the saturated region, showing me that at these points Ic is very high, Vce is very low and a large amount of collector current can flow.
My reading of Ic: 5.44mA, Vce: 1840mV is located in the active amplification region of the graph, resulting in a balance Ic and Vce with some collector current being able to flow.
My final two readings of Ic: 4.42mA, Vce: 2370mV and Ic: 3.64mA and Vce: 2750mV both reside in the cut-off region, meaning Vce is very high whilst Ic sits very low. At this region of the graph no collector current can flow.
