Purpose: To get some experience with bipolar junction transistors (BJT's), specifically the 2N3904 general purpose NPN transistor to use small currents to control larger currents, and create a constant current source.
Procedure:
1) Build the circuit as shown in Fig. (2) and measure the current into the lamp, Ic, and then measure and compare it with the current into the base of the transistor, Ib. Measure the Vce in both the on and off states.
2) Create a graph of Ic vs Ib.
3)Build the constant current source shown in Fig. (1). Measure the collector current as a function of the variable load resistance, and explain what is happening. Estimate beta for your transistor.
Data:
1) After building the circuit in Fig.(2), the base current and collector current were measured, along with the Vce whilst on and off.
Ic = 0.136 A
Ib = 4.167mA
Vce on = 1.25V
Vce=10V
Current through the collector to emitter is controlled by the base current supply.
2)Graphs are represented by Fig. (3) and (4), and were created in LabVIEW as shown in Fig. (5) and (6).
3) The circuit from Fig. (1) was built and data was recorded for a series of different resistance values. Since there is a constant current on the base pin of the transistor, the collector current remains constant. Variable resistance is negligible as long as there is sufficient voltage at the source (within the collector pin connection). Data is represented by Fig. (7), and here an estimated value for beta.
- \(\beta\ =\ \frac{I_c}{I_b}\ ,\ \ \beta\ =\ 183.57\)