RF Amplifier

The purpose of this lab was to breadboard the RF amplifier circuit which is used to boost a weak AM signal received by the antenna prior to audio signal extraction by the detector. Our RF amplifier consisted of two stages: the common source (CS) amplifier which features high input impedance so most of the weak signal will appear across the input terminals and the common emitter (CE) which is used to increase gain.

We were first instructed to construct the common source amplifier shown in Figure 1 and measure and record the Q point and the gain. We were to run this 3 times using RL = 10 kOhm, 1 MOhm, and 1 kOhm. The results were recorded in Table 1.

Figure 1: Common Source Amplifier with AM input

Table 1: Measurements on CD Amp

Next, we were instructed to add the RFC shown in Figure 2. Once again, we were to use RL = 10 kOhm, 1 MOhm, and 1 kOhm and record the gain. The results can also be seen in Table 1.

Figure 2: CS Amplifier with RFC

I noticed that others were having difficulty with the input and output graph so I began to wonder if I had done part 1 and part 2 wrong. I realized that I was supposed to be using internal modulation. So, I started over and the appropriate results are recorded in Table 1.

Next, I added the common emitter amplifier shown in Figure 3. The Q point was found to be Q(non BJT) = (1.85 V, 3.99 mA). Next, I measured the gain 5 different times using RL = 1 MOhm, 100 kOhm, 10 kOhm, 1 kOhm, and 100 Ohms. The results are recorded in Table 2. This data is plotted in Figure 4.

Figure 3: Two-Stage RF Amplifier

Table 2: Gain of Two-Stage Amplifier

Figure 4: Voltage Gain vs. Load Resistance

Finally, I connected the RF amplifier to the input of my detector and tested for functionality. This produced a Vin of 80 mV, Vout of 240 mV, and a gain of 3 V/V. The output sounded great. It was strong and did not have much noise.

"It's so beautiful!"-Haley

This lab was very helpful in that it showed how the RF Amplifier works. It has been very neat to see how each component changes the output.