ELEC 3030 Lab 5: RF Amplifier

In this week's lab activity, I studied the RF amplifier, the section of the AM radio responsible for boosting the weak AM signal received by the antenna prior to the audio signal extraction. To construct this RF amp, I used two stages: the common source (CS) amp and the common emitter (CE) amp (for boosting gain). After analyzing this circuit in simulations in LTspice, I built the circuit in lab, added it to the rest of my AM radio, and tested it.

First, I constructed the first part of the two-stage RF amplifier circuit, namely the common source amp, shown in Figure 1 and measured the Q-point at varying values of the load resistance, 10 kilo-Ohms, 1 kilo-Ohms, and 1 mega-Ohms. These results are recorded in Table 1 (values measured at a time scale of 40 us). Next, I added the RFC (RF choke) to the circuit (Figure 2) and recorded the new gain levels in Table 1.

Figure 1: Two-Stage RF Amplifier Circuit

Table 1: Measured values from the common source amp

Next, I built the common emitter amp, added it to the common source amp shown in Figure 2, and recorded the Q point for its NPN BJT as Q(1.881 V, 3.64 mA). Then I measured the gain of this circuit at various values of load resistance and recorded these measurements in Table 2, plotting these values in the graph in Figure 3.

Figure 2: Two-Stage RF Amplifier Circuit

Table 2: Gain of Two-Stage RF Amplifier at Varying Load Resistance Values

Figure 3: Bode Plot for Varying Values of Load Resistance

Finally, after these tests, I connected the RF amp to the rest of my radio circuit, specifically connecting it to the input of the AM detector portion (Figure 4). In Figure 5, the full AM radio circuit (no power or input applied) is featured, each portion having its own breadboard. The video below displays the circuit's response to an amplitude-modulated input of 1.23 MHz carrier frequency at 1 kHz modulated frequency.

Figure 4: Full AM Radio Circuit Schematic

Figure 5: Full AM Radio Circuit

After placing the last piece of the audio portion of the radio circuit, I was very excited to hear the sharp, crisp hum from the speakers, and I am excited to add the final missing piece soon so that this will be a fully functioning AM radio. Furthermore, after witnessing not only how incredible this engineering feat is but also witnessing how fragile the circuit is, I am contemplating many improvements to the flimsy, unreliable design which I am using right now. I also now have planned to build a permanent AM radio as a fun project, using some commercial parts.