Lab 5 - RF Amplifier and Radio Assembly

RF Amplifier

In this Lab the RF amplifier was simulated, built and tested. After the RF amplifier was thoroughly explored it was added to the radio design and the radio was tested as a unit.

The first part of the RF amplifier is a CS amplifier that is realized with a field effect transistor (FET). The CS is used because the input impedance is extremely high. This allows for the signal from the antenna to mostly go into the input instead of lost in the impedance of the antenna. However, the CS amplifier does not have extraordinary gain. This is made up for with a second stage CE amplifier.

The CS circuit shown in Figure 1 was constructed on the breadboard.

Figure 1: Common Source Amplifier Schematic

I calculated the Q point, and gain for different load resistances. ID was measured by placing a 10 Ohm resistor in series with RS and measuring the voltage across it. Then, an RFC was added in series with RD and the gain and calculated for varying load resistors. This data is recorded in Table 1.

I added the CE as the second stage. I measured the Q point of the npn BJT. IC was calculated by placing a 10 Ohm resistor in series with RC. The Q point was measured to be (1.85 V, 3.5 mA).

I then measured the gain of the two stage amplifier with different load resistors. Figure 2 shows the oscilloscope showing the input and output with a 100k Ohm load resistor. Table 2 shows this data.

Figure 2: CG / CE RF Amplifier Input and Output

This data is plotted in Figure 3

Figure 3: Plot of Gain vs Load Resistance for Two Stage CS / CE Amp

Radio Test and Improvement

Now that I have experimented with the RF amplifier, I added it to my radio. The complete radio schematic is shown in Figure 4.

Figure 4: Radio Schematic

When I ran the simulation I originally ran simulations on this radio, there was clipping on the output. I identified the clipping to be coming from the AB amplifier. In order to fix this I decreased the gain of the op amp by reducing the feedback resistance. This made the input to the AB amplifier less.

With the circuit shown above, the radio worked just as expected. Figure 5 shows the input and output of the radio.

Figure 5: Input and Output of Radio Circuit

From simulation I found that the quiescent power dissipation of the radio is 100.529 mW. If I were to power this radio from two nine volt batteries, the radio could remain on without an input signal for about 5 hours. (A rough estimate of 500 mAh [1] for the capacity of a nine volt battery was used). This is a reasonable amount I believe, however commercial portable AM radios have much longer battery life.

Another thought that I had to make the audio amplifier more powerful was to run the AB amplifier from -9 V and 9V instead of between 9 V and ground. In the simulation I got very impressive results doing this. The simulation was able to turn an input signal of around 1 mV pk / pk into a 3 V pk/pk output without any distortion (The feedback resistor on the op-amp was increased significantly). The draw back to this change is the power needed to drive the circuit increases. When I made this change in the lab, I began to smell something burning and abandoned the change immediately because my radio was working and I didn't want to break it. When I am including the antenna to the radio I may need to tweak the feed back resistance for the op-amp to optimize the output to the antenna input. Figure 6 is a picture of the completed circuit. In this picture the feedback resistor to the op-amp is increase to 127 kOhms.

Figure 6: Complete Radio Circuit

Conclusion

I was impressed with the final waveform that the radio was able to generate. There was no noticeable distortion on the audio output when it was kept within a reasonable voltage of around 1V - 2V. There was audible noise on the output when the radio was on but there was not input. I believe this is from the noisy power supplies. I believe this because when I remove the 1000uF capacitor, the noise increases. If the radio were to be powered with 9V batteries this noise could probably be decreased significantly.