Experiment: Determining BJT Current Gain $\beta$ and Verifying the Equivalent Circuit

1.

Set up the circuit in Fig. 2.3. Let R_E=500, R_C=500, R_B=200K. By measuring the voltages V_C, V_B, and V_E, and solving the appropriate loop equations, determine I_C, I_B, and I_E, respectively. Verify that $I_C \approx I_E$.From the ratio $\frac{I_C}{I_B}$, calculate $\beta$.Note that the BJT is in the forward active region. A NPN BJT is in forward active when V_C > V_B > V_E.

2.

Change R_E to three different values. Make a table of R_E, I_B, I_C, I_E, V_BE and $\beta$ for each value of R_E. Note that the values you choose for R_E cannot be anything, but must be chosen so that your BJT is still operating in forward active for each choice of R_E. Your data should verify that $\beta$ is fairly constant and V_BE is approximately 0.7V (one diode drop).

3.

Set R_E back to its original value of 500. Now change R_C to three different values. (Again, R_C values must be chosen to ensure you're still in forward active.) Measure I_B, I_C, I_E, and make a table to verify that I_C is approximately independent R_C. In other words, I_C should not change and the collector acts as a good current source as shown in the equivalent circuit.

4.

From your data, construct an equivalent circuit for the BJT by putting in an average value for $\beta$ and an average value for V_BE into the circuit in Fig. 2.3.

  

Figure 2.3: Circuit for Determining $\beta$