EXPERIMENT #2
TITLE: TUNED AMPLIFIERS AND FREQUENCY MULTIPLICATION
WORK GROUP:
1. Neel
Patel
353-72-3740
2. Ihab Abusaba 330-90-3687
3. Manuel Damian 322-78-5733
4. Chee Sen Loong 353-94-0863
OBJECTIVE:
1. To investigate the behavior of negative clamping.
2. To study class C bias and amplification.
3. To understand the theory of frequency
multiplication.
EQUIPMENT AND COMPONENTS:
1. dual-trace
oscilloscope
1-2N2222 transistor
2. sinusoidal function
generator
1-1N914/1N4148 diode
3. low-voltage power
supply
Resistors: 1Kohms, 120Kohms
4. prototype
board
Capacitors: 3.3nF, 1uF(2)
5. frequency
counter
Inductors: 10mH
THEORY:
Negative clamping is the
process that occurs when the waveforms of Vin and Vo are identical and as the
amplitude of Vin is increase, Vin and Vo are no longer identical to their DC
offset. Negative clamping is used to bias a transistor for class C
amplification. Class C bias puts the Q-point beyond cutoff into the active
region.
The main
limitation of a class C amplifier is its narrow bandwidth of frequencies which
can be amplified. Class C amplifiers are often used for power
amplification of FM signals and they are also used for frequency
multiplication. Since the current pulses are rich in the harmonics of the
input waveform, the class C amplifier can be used as a frequency multiplier by
tuning the output circuit to the desired harmonic.
PROCEDURE:
STEP 1:
Vin =4Vp-p
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Vin-max=2v
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Vin-min=-2v
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Vo =2Vp-p
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Vo-max=.531v
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Vo-min=-1.469v
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Determine at what critical voltage level of Vin the waveforms of Vin and Vo no longer posses the same DC offset.
Critical level = 0.7mV
STEP 2:
Vin=1Vp-p
--------
Vb=1.038Vp-p
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Vo was too distorted to get a good waveform
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Vo=125mVp-p
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Vin=2Vp-p
--------
Vb=2.047Vp-p
200us/div
Vo=531mVp-p
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Vin=4Vp-p
--------
Vb=4.094Vp-p
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Vo=656mVp-p
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STEP 3:
Fr=1/2pi(LC)^(1/2)
Fr=27.705Khz
Vin =1.781Vp-p
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- *--l--*l---l*--l---*--t Fin=30.5KHz
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Vo =25.94Vp-p
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- *---l-*--l---l*---l---*l--t Fo=30.5KHz
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STEP 4:
Calculate the duty cycle for each of the waveforms of STEP 2 using the equation,
%D = duty cycle = (change in t / T) *100%
Vin = 1Vp-p
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%D =
0, couldn't get a good waveform for Vo.
Vin = 2Vp-p
---------
change in t =
215us
%D = 215/970 = 22.2%
T= 970us
Vin = 4Vp-p
---------
change in t =
160us
%D = 160/960 = 16.7%
T = 960us
STEP 5:
Determine the bandwidth of the amplifier.
-3dB = 20 log(Vo/Vo-max) Vo-max = 25.94V
10^(-3/20) =
Vo/Vo-max
Vo = 18.36V, setting to find F+, F_.
F+ =
34.27KHz
BW = (F+) - (F_)
F_ =
29.14KHz
BW = 5.13KHz
STEP 6:
Q-calculation
Q = Fr / BW = 5.95
STEP 7:
FREQUENCY MULTIPLICATION
frequency -doubler
--------------
Vin =1.391Vp-p
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- *--l---l---l*--l---l---l-*-l--t Fin =
15.25KHz
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Vo=8.125Vp-p
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--*-l----l--*-l----*----l--t Fo = 31.6KHz
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STEP 8:
X3, X4, X5 frequency multiplication
| TYPE | Vin(V) | Fin(KHz) | Vo(V) | Fo(KHz) |
| X3 | 1.375 | 10.17 | 8.125 | 31.5 |
| X4 | 1.391 | 7.625 | 7.812 | 31.7 |
| X5 | 1.375 | 6.1 | 7.187 | 31.4 |
CONCLUSION/ QUESTIONS:
1) The waveforms of Vin and Vo are identical and as Vin is increased the waveforms are no longer identical with respect to their DC offset. Negative clamping is used to bias a transistor for class C amplification by putting the Q-point at a point "beyond" cutoff into the active region.
2) The advantages of a class C amplifier in audio and RF signal applications are that class C amplifiers provide high efficiency and that they have a narrow bandwidth so that it can filter out undesired frequencies. The disadvantage of class C amplifiers is the distortion in the waveforms. The distortion can be minimized by the use of higher Q components within the tank circuit.
3) The process of frequency
tripling in a X3 multiplier stage is as follows: The tank circuit is
recharge on every third cycle of the sine wave by setting the frequency of Vin
to exactly 1/3 of the resonant frequency. A single-stage X7 multiplier
stage does not work as well as a X3 multiplier stage due to excessive
distortion. To get better results, more that one stage of multiplication
can be used.
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