9.14.2021 -- The purpose of this lab is to learn about the common source FET amplifier and use that to build and RF amplifier. This is needed to boost weak AM signals in order for the AM detector to read the intelligence signal of the in pout AM wave.
5.1 The Common Source Amplifier
5.1a The FET
In this lab, we used JFETs instead of MOSFETs because JFETs are not as sensitive to static, so they are much easier to use in the lab. FETs operate similar to BJTs, but the voltage between the gate and source terminals (VGS) are subject to change unlike the static voltage between the base and emitter terminals of a BJT (VBE). See Figure 5.2 for the answers to the section exercises. Note that Exercises 5.4-5.5 refer to the schematic in Figure 5.3.
Figure 5.1 field-effect transistor (FET)
Figure 5.2 Answers to Section 5.1 exercises
Figure 5.3 CS amplifier after adding an RFC
5.2 LTSpice Simulation of CS Amplifier
See Figure 5.4 for DC biasing network
Q(6.57395 V, 1.8241 mA)
See Figure 5.5 for simulation results
Vin(pk-pk) = 30 mV Vout(pk-pk) = 29.4 mV gain = 0.98 V/V The common emitter amplifier amplifies a sinusoidal input signal. The common source amplifier amplifies an input AM signal. However, this amplifier is not very good since the gain is very low. When looking at Figure 5.4 above, the output signal is almost the same amplitude as that of the input signal.
Vin(pk-pk) = 30 mV Vout(pk-pk) = 430 mV gain = 28.667 V/V
See Table 5.1 for the Gain vs. Resistance table and Figure 5.6 for the plotted data points.
Figure 5.4 My LTSpice Drawing of DC biasing network
Figure 5.5 Transient Analysis of CS Amplifier
RL | Vout(pk-pk) | Gain (V/V) | Gain (dB) |
1 MΩ | 10.5 | 350 | 50.8814 |
100 kΩ | 6 | 200 | 46.0206 |
10 kΩ | 430m | 28.667 | 29.1475 |
1 kΩ | 92m | 3.0667 | 9.7333 |
100 Ω | 9.3m | 0.31 | -10.1728 |
Table 5.1 Calculating Gain vs. Load resistance
Figure 5.6 Volage Gain vs. Load Resistance graph - data from Table 5.1 plotted in light blue, data from Table 5.2 plotted in dark blue
5.3 LTSpice Simulation: Adding CE Amp to the CS Amp
See Figure 5.7 for the constructed circuit
See Table 5.2 for the Gain vs. Load data
I estimated the impedance seen by the CS amp looking into the input of the CE amp to be about 250 Ω.
See Table 5.2 for the Gain vs. Load data
See Figure 5.6 for the plotted data
Figure 5.7 Two stage RF amplifier (CS amp + CE amp)
RL | Vout(pk-pk) | Gain (V/V) | Gain(dB) |
1 MΩ | 2.92 | 97.3333 | 39.7652 |
100 kΩ | 2.92 | 97.3333 | 39.7652 |
10 kΩ | 2.88 | 96 | 39.6454 |
1 kΩ | 2.48 | 82.6667 | 38.7652 |
100 Ω | 735m | 24.5 | 27.7833 |
Table 5.2 Calculating Gain vs. Load resistance for two-stage amplifier
5.4 LTSpice Simulation: Adding CE amp to the CS Amp
I am choosing to use the op amp audio amplifier and the CFP AM detector for the AM radio. The op amp amplifier had the best gain of a sinusoidal signal. The op amp wasn’t working properly in my simulations, so I will probably use the CC-CE amplifier for audio amplifier instead. The CFP detector produced the signal closest to the intelligence signal of the carrier wave. However, during lab 4, the CFP detector did not produce an intelligence signal. If the CFP detector still doesn’t work, then I will probably use the biased detector circuit instead because it had the second-best output signal.
See Figure 5.8 for full radio design.
The green signal indicates the carrier signal Vin, and the dark blue signal in this case indicates the amplified RF signal. I don’t know why the output signal is centered at a higher voltage rather than being centered at 0V. Regardless of this, the RF amp component exhibits a large gain. See Figure 5.9 for the simulation results.
The output of the RF amplifier is the input of the AM detector circuit. This time, the detected/output signal of the AM detector is graphed below the AM signal (for what reason? I’m still not sure). The output of the AM detector was a clean sine wave that matched the intelligence signal from the AM signal. See Figure 5.10 for simulation results.
In Figure 5.11, the audio amplifier exhibited a clipped output. Usually with a clipped output, there is a relatively large input signal to the amplifier. After lowering the Vin AM signal, I was able to produce a clean sine wave from the audio amplifier. This is shown in Figure 5.12.
Overall, I think AM radio design in Figure 5.8 is the best on to use for producing an audio signal from an AM signal. This radio circuit works best with small input signals. I may need to adjust some resistor values to allow a greater range of input voltages to be detected properly.
Figure 5.8 Full AM Radio Circuit
Figure 5.9 Transient Analysis of RF amplifier component
Figure 5.10 Transient Analysis of AM detector component
Figure 5.11 Transient Analysis of Audio Amplifier
Figure 5.12 Vout signal with smaller Vin signal – audio amplifier
5.5 RF Amplifier Construction
See Figure 5.13 for the circuit constructed and Table 5.3 for the Q-point and gain data.
See Figure 5.3 for the circuit constructed and Table 5.3 for the Q-point and gain data.
Q-point for CE amp - Q(4.09V, 2.5 mA)
See Table 5.4 for gain data and Figure 5.14 for the plotted data
connect RF amp to input of detector
When the full radio was connected to the input AM signal, it produced a clipped but clean oiutput wave and the speaker made a pretty loud noise. Overall, this was a success!
Figure 5.13 Circuit constructed for question 1
RL | 1 MΩ | 10 kΩ | 1 kΩ |
Q-point | Q(8.28 V, 576 μA)? | Q(8.28 V, 576 μA) | Q(8.28 V, 576 μA) |
Av (V/V) - no RFC | 0.4235 | 0.3571 | 0.30614 |
Av (V/V) - with RFC | 3.53614 | 3.94268 | 3.04469 |
Table 5.3 Measurements for CS Amp
RL | Gain (V/V) | Gain (dB) |
1 MΩ | 13.0576 | 22.3172 |
100 kΩ | 12.392 | 21.8628 |
10 kΩ | 12.7206 | 22.0901 |
1 kΩ | 11.7518 | 21.4021 |
100 | 5.25424 | 14.4102 |
Table 5.4 Gain of two-stage amp
Conclusion
In my opinion, this was one of the easier labs to complete because everything worked within the lab time! I decided that for my final radio design, I will be using the radio circuit from the lab manual in Figure 5.14. I wasn't able to get my original design working properly, but I'll probably rebuild my circuit design and test it again later if I have time in the lab. The AM radio in Figure 5.14 worked well, so I decided to switch the biased diode AM detector stage with the CFP. The CFP did not ouput any reading, so I won't be using it in my final design.
Figure 5.14 Full AM radio circuit (from lab manual)
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