Lab 5 - RF Amplifier

The primary goal of lab 5 was to become familiar with the analysis and design of an RF amplifier. RF amplifiers are used to boost a weak AM signal received by the antenna prior to the audio signal extraction by the detector. We utilized a two stage configuration as shown in Figure 1 below. The first stage is a common source (CS) amplifier based on the field effect transistor. The CS amplifier has high input impedance so most of the weak signal will appear across the input terminals. To increase gain, a common emitter amp is added as a second stage.

Figure 1: Diagram of two-stage RF Amplifier

The input signal from the antenna must be amplified so that it can be properly processed in order to produce a quality sound on the output. I began this lab by first creating a common source amplifier shown in Figure 2 and measuring the Q-point and gain for the amplifier, both with and without the RFC inductor. Results are given in Figure 4.

Figure 2: CS amp with RF choke

The gain of the CS amp is proportional to the resistance seen by the drain, which is the parallel combination of the 330 and 10k ohm resistors. An RF choke is placed between the 330 ohm resistor and the drain terminal of the transistor to increase gain. An ideal RFC will appear as an open circuit to the RF signal. An actual RFC has a very high impedance.

I then added a common emitter (CE) amplifier as shown in Figure 3 in order to create a two stage RF amplifier. The results of my measurements of the gain of this amplifier for various load resistances is included in Figure 4.

Figure 3: Two stage RF amplifier

Figure 4: Measurement tables on CS amp and gain of two-stage amp

Shown below is the logarithmic graph comparing the voltage gain of the amplifier as load resistance increases:

Figure 5: Plot of gain vs load resistance for two-stage RF amp (CS + CE)

After completing the testing of the two stage amplifier, I connected the two stage RF amplifier, AM detector, and the LM386 amplifier together to form my AM radio circuit. To test its performance I connected the speaker to the output and used the function generator to produce an AM signal similar to what would be picked up by an antenna. The resulting sound output was relatively clear and distortion free. Overall, I am impressed by the performance of my radio, but I have plans to reduce some of the noise I heard by eliminating some long wires that may be negatively impacting the performance of the radio.