PART 5: RF Amplifier
INTRODUCTION
In this lab, the student will be working with RF Amplifiers. The goals of this lab are to construct the common source FET amplifier, adding an RF Choke (RFC), and a two-stage CS+CE amp. The student will be measuring the Q point and gain for different load resistor values. The student will conclude the lab by constructing their full radio.
5.5 RF Amplifier Construction
To begin, the student will constructing the common source amplifier using the FET, shown in Figure 5.1. Starting with a load resistance of 10kΩ, the Q point and gain of the circuit are calculated and recorded in Table 5.1. The function generator was set to 10mVpp at 1230 kHz with a 1 kHz frequency at 50% modulation. With the DC power source on, the probes were used to measure across the resistor Rd to find the Q point. The gain was found from the input and output voltage waveforms on the oscilloscope, shown in Figure 5.2. These steps were then repeated, swapping out the load resistance for a 1kΩ (Figure 5.3) and 1MΩ resistor (Figure 5.4).
FIGURE 5.1: Common source amplifier with AM input
FIGURE 5.2: Common Source input/output voltages with 10kΩ
FIGURE 5.3: Common Source input/output voltages with 1kΩ
FIGURE 5.4: Common Source input/output voltages with 1MΩ
Then, the student added the RF choke to the circuit, shown in Figure 5.5, and measured the new gain for the three load resistor values: 1kΩ (Figure 5.6), 10kΩ (Figure 5.7), and 1MΩ (Figure 5.8). The student observed that the Q point did not change as the load resistor changed, these results were calculated and recorded in Table 5.1.
FIGURE 5.5: CS amplifier after adding an RFC
FIGURE 5.6: CS amplifier with RFC and 1kΩ
FIGURE 5.7: CS amplifier with RFC and 10kΩ
FIGURE 5.8: CS amplifier with RFC and 1MΩ
Next, the student added the common emitter amplifier to the circuit from Figure 5.9 to create a two stage RF amplifier, shown in Figure 5.10. To find the Q point, the voltage was measured at the collector, emitter, and across the Rc resistor. Using the oscilloscope, the output voltage was measured with the cursors on the waveform (Figure 5.11). The voltage was used to calculate the gain, recorded in Table 5.2. These steps were then repeated using different load resistor values: 1MΩ (Figure 5.12), 100kΩ (Figure 5.13), 1kΩ (Figure 5.14), and 100Ω (Figure 5.15). The data was recorded in Table 5.2 and then plotted in Figure 5.16.
FIGURE 5.9: Two stage RF amplifier LTspice circuit
FIGURE 5.10: Two stage RF amplifier breadboarded circuit
FIGURE 5.11: Two stage RF amplifier with 10kΩ
FIGURE 5.12: Two stage RF amplifier with 1MΩ
FIGURE 5.13: Two stage RF amplifier with 100kΩ
FIGURE 5.14: Two stage RF amplifier with 1kΩ
FIGURE 5.15: Two stage RF amplifier with 100Ω
FIGURE 5.16: Bode plot of Table 5.2 Data
To conclude this lab, the student connected the output of the RF amplifier to the input of the detector to create a full radio, shown in Figures 5.17 and 5.18. The radio was tested with the appropriate input signal to make sure that it was working correctly.
FIGURE 5.17: Full Radio LTspice Circuit
FIGURE 5.18: Full Radio Breadboarded Circuit
CONCLUSION
In conclusion to this lab, the student was able to successfully work with RF amplifiers. The goals to be introduced the common source FET amplifier, RF choke, and two-stage amplifier were accomplished. The student feels in this lab a lot of beneficial knowledge was gained in determining how to construct the ideal radio. Having a working radio will make the upcoming antenna lab easier to test.