Week 5 | Elec3030

5.0 RF Amplifier

OBJECTIVES

GOALS

The objective of this lab is to learn about and construct a two-stage RF amplifier and then incorporate that knowledge alongside the material learned from previous labs to construct a fully functional AM radio.

Chapter 5 Goals

Introduce the common source FET amplifier and perform LTspice simulation
Add a CE amplifier; LTspice simulation
Breadboard the RF amplifier circuit

OBJECTIVES

GOALS

The objective of this lab is to introduce the function of a two-stage RF amplifier and include it in the final schematic of the AM radio before troubleshooting to obtain a fully functional radio.

Chapter 5 Goals

Introduce the common source FET amplifier and perform LTspice simulation
Add a CE amp; LTspice simulation
Breadboard the RF amplifier circuit

5.1 The Common Source Amplifier

The RF amplifier used in this lab will consist of two stages, a common source (CS) amplifier based on the field effect transistor (FET) and a common emitter (CE) amplifier. The CS amplifier features high input impedance meaning that most of the weak signal will appear across the input terminals. Because of this, the gain will be small and will need to be increased with the second stage CE amplifier.

The FET's saturation operating region is ideal for biasing this radio because the drain current can be very small (~mA) and output a sufficiently large voltage close to the DC voltage supply value. These biasing conditions will output a small transconductance and return a gain less than 1 V/V; therefore, requiring the addition of an ideal RF Choke (RFC) to increase the small gain.

5.2 CS Amplifier (LTspice)

First, the circuit in Figure 1 is constructed in LTspice and a DC operating point simulation is ran to determine the Q-point for the FET and get the results displayed in Figure 2. In comparison to the hand calculations for this FET, the simulation's Q-point of (6.576 V, 1.82 mA) was very close to the estimated (6.67 V, 1.75 mA).

Because the simulated Q-point will produce a small gain, coupling and bypass capacitors, an AM source, and a load resistor are added to create the CS amplifier shown in Figure 3. A transient analysis is performed for 4 ms and the peak-to-peak signal is determined to be 15 mV at the input and 15 mV at the output based on the results shown in Figure 4. This equates to a gain of about 1 V/V.

To increase the gain, an RFC is added between the drain resistor and the drain of the FET and another transient analysis is performed to get the results shown in Figure 5. The peak-to-peak at the input still remained at 15 mV while the peak-to-peak at the output increased to about 450 mV, producing a new gain of 30 V/V.

Since the gain of the CS amplifier is proportional to the resistance seen by the drain, the load resistance was varied to observe the change in gain. The circuit was re-simulated with different load resistances and the results were displayed in Table 1 while the relationship was plotted in the graph shown in Figure 6.

5.3 Adding a CE Amp to the CS Amp (LTspice)

As mentioned before, adding a CE amplifier to the output of the CS amplifier as the second stage of the RF amplifier will increase the gain. To test that, the circuit of the complete RF amplifier shown in Figure 7 is constructed and a transient analysis is ran to get the results displayed in Figure 8.

The peak-to-peak at the input is measured to be 15 mV and the peak-to-peak at the output is around 1.56 V. This produces a gain of about 104 V/V, or 40.34 dB, a much larger value than the gain calculated without the CE amplifier. To observe the relationship between load resistance and the voltage gain with the addition of the CE amplifier, the gain was calculated for varying load resistances and displayed in Table 2. The data in Table 2 is superimposed with the data from Table 1 and displayed in Figure 9.

Based on this gain and the plot in Figure 6, the impedance seen by the CS amplifier looking into the input of the CE amplifier is estimated to be about 10.3 kohm because that is where the two lines intersect.

5.4 Full AM Radio (LTspice)

With the goal of this lab being to build a fully functional radio, LTspice will be used to simulate the full AM radio before breadboarding it. Using the most ideal components from the previous labs, the radio was connected as follows: RF Amplifier - AM Detector - Audio Amplifier - 10 ohm load.

For the AM detector, the biased diode detector and Complementary Feedback Pair (CFP) detector are more ideal options than a simple diode detector because of the slower RC time constant. Because both detectors work well at lower amplitudes and produced sufficient gains around 1 V/V or more, the biased diode detector was chosen because of it's simpler design, higher gain, and ability to follow the carrier signal. The CE-CC amplifier was chosen as the audio amplifier because of its ideal unity gain and high input impedance.

After constructing the full radio schematic shown in Figure 10, an AM wave was input to the RF amplifier and the simulation was ran beginning at 10 msec and ending at 20 msec. The transient analysis results are displayed in Figure 11.

Based on the results, the output is steady and should produce a loud sound at the load. For further troubleshooting, the input and output of each section of the AM radio are observed to ensure proper function throughout the entire process.

The RF amplifier's purpose is to boost a weak AM signal received by the antenna so that it can be detected by the AM detector. Based on the results shown in Figure 12, the AM signal is amplified after going through the two-stage CS-CE amplifier and begins to clip at the height of the input signal, meaning the amplifier is successfully amplifying. The input of the radio is represented by the orange signal and the output before entering the AM detector is represented by the blue signal.

Then, the signal flows through the AM detector, which extracts the amplified audio signal and amplifies it again before passing it to the speaker. The results shown in Figure 13 depict that the signal was detected and amplified to around 2 V peak-to-peak. The input of the AM detector is represented by the orange signal and the output before flowing through the audio amplifier is represented by the thinner blue line.

Finally, the signal reaches the audio amplifier to be amplified once more to drive the speaker. The results in Figure 14 show how the audio amplifier produces a final steady and strong signal that will drive the speaker and produce the output shown in Figure 11. The input of the audio amplifier is represented by the orange signal and the output at the load resistor is represented by the blue line.

5.5 RF Amplifier Construction

First, the CS amplifier displayed in Figure 3 is constructed and the Q-point and gain are recorded into Table 3 for various load resistances to, again, observe the relationship between load resistance and voltage gain. The screenshots shown in Figures 15, 16, and 17 display the gain for each load resistance and the more ideal results come from the 10 kohm resistor.

However, it is known that the addition of an RFC, as determined using LTspice, will produce a larger gain, so an RFC was added between the drain resistor and the drain of the FET and produced the results shown in Table 3. To observe the physical difference in gain, the screenshot for the gain with a 10 kohm load resistor is displayed in Figure 18.

Then, the CE amp is added to construct the complete RF amplifier shown in Figure 7 and the Q-point at the BJT was measured at (1.80 V, 0.02 A). Different load resistances were switched out and the gain was measured and recorded in Table 4 and this data was plotted in the graph shown in Figure 23. In comparison to the voltage gain observed with the CS amplifier, the screenshots of the voltage gain for the two-stage amplifier are shown in Figures 19, 20, 21, and 22.

The RF amplifier was finally connected to the input of the AM detector and the full functionality of the full AM radio was tested.

Figure 1: DC Biasing Network of FET

Figure 2: DC Operating Point of FET

Figure 3: CS Amplifier Schematic

Figure 4: Transient of CS Amplifier w/o RFC

Figure 5: Transient of CS Amplifier w/ RFC

Table 1: CS Amplifier Gain with Different Load Resistances

Figure 6: Plot of Load Resistance vs Voltage Gain (Table 1)

Figure 7: Two Stage RF Amplifier

Figure 8: Transient of RF Amplifier

Table 2: RF Amplifier Gain with Different Load Resistances

Figure 9: Plot of Load Resistance vs Voltage Gain (Table 1 and 2)

Figure 10: Full AM Radio Schematic

Figure 11: Transient of Input vs Output of Full AM Radio

Figure 12: Transient of RF Amplifier in AM Radio

Figure 13: Transient of AM Detector in AM Radio

Figure 14: Transient of Audio Amplifier in AM Radio

Figure 15: Vin-Vout of CS Amplifier w/ RL = 1 kohm

Figure 16: Vin-Vout of CS Amplifier w/ RL = 10 kohm

Figure 17: Vin-Vout of CS Amplifier w/ RL = 1 megaohm

Figure 18: Vin-Vout of CS Amplifier (w/ RFC) w/ RL = 10 kohm

Table 3: Actual CS Amplifier Gain with Different Load Resistances

Figure 18: Vin-Vout of RF Amplifier (w/ RFC) w/ RL = 100 ohm

Figure 18: Vin-Vout of RF Amplifier (w/ RFC) w/ RL = 1 kohm

Figure 18: Vin-Vout of RF Amplifier (w/ RFC) w/ RL = 10 kohm

Figure 18: Vin-Vout of RF Amplifier (w/ RFC) w/ RL = 1 megaohm

Table 4: Actual RF Amplifier Gain with Different Load Resistances

Figure 23: Plot of Load Resistance vs Voltage Gain (Table 4)

Reflective Writing

For this lab, I prepared for breadboarding the full AM radio by first, constructing it in LTspice, and then running a lot of trial-and-error. To be specific, I originally wasn't sure about which AM detector to use, but observing the output of the entire radio with a biased diode detector vs a complementary feedback pair detector helped me come to that final conclusion. I also put a little extra time toward cleaning up the breadboard with my final AM radio and the output sounded much cleaner.