17-Mar-2015: Time-varying Signals/A BJT Curve Tracer

Time-varying Signals

PURPOSE

The purpose of this lab was to examine the relationship between the input and output voltages (Vin and Vout) for time-varying signals.

PRE-LAB

Figure 1: Relationship between Vin and Vout

Figure 2: Graphs of Vin and Vout

Prior to commencing with our lab, we drew our predictions for the shapes of the Vin and Vout graphs based on the given criteria. We were able to do this by using the equation shown in Figure 1 and assuming that R1 and R2 were equal. The resulting graphs are shown in Figure 2.

PROCEDURES

Figure 3: Input sinusoidal graph

Figure 4: Output sinusoidal graph

Figure 5: Input triangular graph

Figure 6: Output triangular graph

Figure 7: Input square graph

Figure 8: Output square graph

Following the pre-lab, we implemented our set-up and captured the resulting input and output voltage graphs using the WaveGen instrument. We repeated this process for three different graph shapes: sinusoidal, triangular, and square. These graphs are displayed above in Figures 3 through 8 (the input graph is shown first and then the output for each case).

CONCLUSION

According to the results, this experiment was successful. This can be seen by observing the frequency and amplitude of the Vout graphs. For example, the output square graph has a frequency of 1.00001 kHz and an amplitude of 996.8 mV. These values are considerably close to the expected values of 1 kHz and 1 V. In fact, the percent error for these values are only .001 and .32 percent, respectively.

A BJT Curve Tracer

PURPOSE

The purpose of this experiment was to observe the relationship between the base-emitter voltage (Vbe) and collector current (Ic) by utilizing the WaveGen instrument.

PROCEDURES

We began this experiment by setting up our circuit as shown in Figure 1. This set-up consisted of a 2N3904 NPN transistor, a 100 Ω resistor, a 100 kΩ, and some wires to connect the elements. Before proceeding with the experiment, we measured the actual resistances of these resistors as shown in Figures 2 and 3.

Figure 4: Stair-step function graph

Figure 5: Triangular function graph

Next, we generated two curves: a stair-step wave and a triangular wave. These graphs are shown in Figures 4 and 5. 

Figure 6: Collector current (Ic) vs base-emitter voltage (Vbe)

Figure 7: Scaled graph of Ic vs Vbe

After plotting the previous graphs, we constructed another with collector current with respect to base-emitter voltage of the transistor. This graph is shown in Figure 6. Then, we fit the graph to a different scale to get a better view of what was happening. The resulting graph is illustrated in Figure 7 above.

CONCLUSION

This experiment gave us a better understanding of the relationship between the base-emitter voltage and collector current. This allowed us achieve a deeper understanding of how BJT transistors function.