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RF Amplifier
Objectives:
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The purpose of the RF Amplifier lab is to introduce the common source FET amplifier.
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The amplifier is breadboarded and tested.
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The second stage CE amp is added to the circuitry.
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The RF amplifier is tested with the Audio Amplifier.
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The pre-lab:
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The prelab of this lab is much like the in-lab procedure except LTspice is used to simulate the results.
RF Amplifier Construction
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The RF amplifier lab is designed to boost the performance of the audio amplifier. The amplifier boosts the weak AM signal received by the antenna prior to audio signal extraction by the detector. The RF amp consists of two stages, the first being the Common Source amp, which is based on the field effect transistor (FET). This amp features high input impedance so the weak signal appears across the input terminals; however, this amp suffers from weak gain. To compensate for this, the second-stage common emitter (CE) amp is added.
The RF Amplifier lab begins with the Common Source amplifier. The Q-point is measured with load resistances of 10 kΩ, 1 kΩ, and 1 MΩ, respectively. The Q-point was consistent with all three load resistances, resulting in:
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Q-point: ( 1.27 mA, 7.4 V )
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Vd = 8.67 V
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Vs = 1.27 V
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Vds = (8.67 - 1. 27) V = 7.4 V
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Id = 0.42 V / 330 Ω = 1.27 mA
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Gain was also measured with each resistor and the results were as follows:
Following, an RF Choke (RFC) was added to the circuit between the 330Ω resistor and the drain of the FET. An Ideal RFC appears as an open circuit to the RF signal; therefore an actual RFC has very high impedance. Gain was again measured and recorded as follows:
The Common Emitter amplifier was then added to the circuit. The CE amplifier adds additional gain to the RF amplifier. The Q-point was measured for the npn bjt and found to be:
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Q-point: ( 2.89 mA, 1.77 V )
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Vd = 4.71 V
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Vs = 2.94 V
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Vds = (4.71 - 2.94 ) V = 1.77 V
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Id = 2.89 V / 1 kΩ = 2.89 mA
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As before, gain was measured with various resistors. The results are shown as follows:
Lastly, the RF amplifier was connected to the AM detector circuit and tested with the audio amplifier. Complete circuitry shown below:
Load Resistance
1 MΩ
10 kΩ
1 kΩ
Gain ( V / V )
(Output/ Input)
Output: 42.4 mV
Input:
66.0 mV
=0.64
Output: 36 mV
Input:
64 mV
=0.56
Output:
32.8 mV
Input:
66.0 mV
=0.50
Figure 1: CS amplifier breadboarded
Load Resistance
1 MΩ
10 kΩ
1 kΩ
Gain ( V / V )
(Output/ Input)
Output: 218 mV
Input:
54 mV
=4.03
Output: 133 mV
Input:
59 mV
=2.25
Output:
97 mV
Input:
57 mV
=1.7
Table 2: Measurements on CS amp ( RFC added )
Figure 2: CS amplifier breadboarded, RF Choke included
Load Resistance
1 MΩ
100 kΩ
10 kΩ
1 kΩ
100 Ω
Gain ( V / V )
(Output/ Input)
Output: 1.86 V
Input:
74 mV
=25
Output: 1.78 V
Input:
71 mV
=25
Output:
1.68 V
Input:
70 mV
=24
Output:
1.54 V
Input:
76 mV
=20
Output:
736 mV
Input:
76 mV
=9.81
Gain (dB)
dB=20*log(V/V)
=27.9
=27.9
=27.6
=26
=19.83
Table 3: Gain of two-stage amp
Figure 3: Two-stage RF Amp
Figure 4: Plot of Gain(dB) for two-stage RF Amp
Figure 5: Simple AM Radio
Figure 6: Simple AM Radio (Top View)
This lab was very informative in revealing the purpose of the RF amplifier and how it assist in overall radio functionality. This lab is based off of material covered in Analog Electronics; specifically in terms of gain and amplifier performance. This lab introduces the RFC, which was used to filter noise in projects worked on while Co-oping.
Table 1: Measurements on CS amp
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