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FLORIDA ATLANTIC UNIVERSITY Department of Electrical Engineering EEL 4119L Electronics Laboratory II Experiment 2 Negative Feedback Amplifiers using BJT Abstract
Careful design is a must in the construction of BJT amplifier circuits. For feedback BJT amplifiers, emphasis must be placed on transistor quality and component specifications as well as on feedback design and bias point stability in order to achieve stable and well balanced, gain efficient and frequency response efficient (wideband)
amplifiers. Theory alone will suffice to plan the design, but hands-on practice is a necessary complement in order to guarantee the desired result: a stable (non-oscillating) high gain, wide band (high range of frequency response) BJT amplifier.
Objectives: 1.
To study the influence of the negative feedback in BJT amplifier circuits. 2.
To examine via experimentation the properties of the Current-Series, Current-Shunt, Voltage-Series and Voltage-Shunt feedback BJT amplifiers. 3.
To determine the input impedance, output impedance, gain, bandwidth of BJT amplifiers with and without feedback. Pre Lab Work: Read about feedback Amplifiers from Microelectronic Circuits by Sedra, Smith & Chandorkar. I. Introduction Fig. a1: Frequency response of a feedback amplifier with and without feedback Feedback plays a very important role in electronic circuits and the basic parameters, such as input impedance, output
impedance, current and voltage gain and bandwidth, may be altered considerably by the use of feedback for a given
amplifier. In feedback, a portion of the output signal is taken from the output of the amplifier and is combined with the
normal input signal and thereby the feedback is accomplished. In this experiment, the effects of ac feedback on the bias point stability, voltage gain and frequency response of a several
BJT amplifiers are examined. The circuit is first analyzed using PSPICE and then the circuit is constructed so the actual
performance of the amplifier can be compared with the predicted performance. The experiment demonstrates that unless a
high-gain circuit is carefully constructed; paying close attention to possible means of feeding back a signal 360
o
out of
phase with the input, an amplifier will inadvertently become an oscillator. 2-2
There are two types of feedback. They are i) Positive Feedback and ii) Negative Feedback. Negative feedback helps to increase the bandwidth, decrease gain, distortion, and noise, modify input and output
resistances as required. An amplifier circuit incorporating some amount of negative feedback is not only more stable, but
it distorts the input waveform less and is generally capable of amplifying a wider range of frequencies. The tradeoff for
these advantages is decreased gain. If a portion of an amplifier's output signal is "fed-back" to the input to oppose any
changes in the output, it will require a greater input signal to drive the amplifier's output to the same amplitude as before. This constitutes a decreased gain. However, the advantages of stability, lower distortion, and greater bandwidth are usually worth the tradeoff in reduce gain for many applications. Positive feedback can produce undesired oscillations in an otherwise stable feedback amplifier. Spring 2018 Revision 2-3
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A. MEASURING INPUT AND OUTPUT IMPEDANCE FOR ANY AMPLIFIER CIRCUIT (Read before starting Experiment) Measuring Amplifier Input Impedance Fig. (a): Measuring Input Impedance.
1.
Connect your Feedback Amplifier circuit as per the diagram shown in Fig. (a). Use a RDB decade box for the Variable Resistance. 2.
Turn DC Power Supply ON. Turn scope ON. Signal generator is set to provide a sine wave output at 5 kHz. The amplitude of
the input signal should be adjusted so that the display on the oscilloscope is noise free (large enough) and distortion free (not too large) say 100mV peak-to-peak. Because of the attenuator the net input to the amplifier will be 0.1 times signal value i.e., 10mV. The display on the oscilloscope should be as large as is practical and set so that its amplitude and half its amplitude can be easily estimated. If distortion is observed due to too large an input signal level, reduce the input signal level to 50mV peak-to-peak. 3.
Note the output signal amplitude on the oscilloscope. The resistance of the decade box at the amplifier input should then be increased until the output waveform is exactly half its previously set value at 5 kHz. At this setting, the signal is equally shared between the decade boxes and the input impedance of the amplifier, meaning the resistance and impedance are equal. After switching off the power and removing the decade box, measure the decade box resistance with an Ohm meter to obtain a value equivalent to the input impedance of the amplifier. Record the input impedance in the appropriate table. Measuring Amplifier Output Impedance 2-4
Fig. (b): Measuring Output Impedance.
1.
Connect your Feedback Amplifier circuit as per the diagram shown in Fig. (b). Use a RDB decade box for the Variable Resistance Load. 2.
The measurement of output impedance uses the same method as for the input impedance but with different connections. In this case the amplifier load is replaced with a resistor decade box. Temporally remove any load resistor attached to the amplifier. 3.
Signal generator is set to provide a sine wave output at 5 kHz. The amplitude of the input signal should be adjusted so that the
display on the oscilloscope is noise free (large enough) and distortion free (not too large) say 100mV peak-to-peak. Because of the attenuator the net input to the amplifier will be 0.1 times signal value i.e., 10mV. The display on the oscilloscope should be as large as is practical and set so that its amplitude and half its amplitude can be easily estimated. If distortion is observed due to too large an input signal level, reduce the input signal level to 50mV peak-to-peak. 4.
Note the output signal amplitude on the oscilloscope. The resistance of the decade box at the amplifier output should then be adjusted until the output waveform is exactly half its previously set value. At this setting, the signal is equally shared between
the decade boxes and the output impedance of the amplifier, meaning the resistance and impedance are equal. After switching
off the power and removing the decade box, measure the decade box resistance with an Ohm meter to obtain a value equivalent to the output impedance of the amplifier. Record the output impedance in the appropriate table. B: Voltage-series Feedback Amplifier
2-5 Scope CH1 Scope CH2
Fig. 1: Voltage-Series Feedback Amplifier circuit Fig. 1 shows a Voltage-Series Amplifier circuit. The combination of R
f
and C
s
provide ac feedback when these elements are present in the circuit. Simulations - 1 1.
Start by simulating the circuit in Fig. 1 with PSPICE. Plot the circuit with the dc analysis results on the circuit. 2.
Simulate the AC frequency response of the following Rf and Cf combinations between 10 Hz and 30 MHz; Set markers at the mid band gain and at the -3dB upper and lower frequency. •
R
f
= 1M
and C
s
= 200pF •
R
f
= 10k
and C
s
= 200pF •
R
f
= 4.7k
and C
f
= 200pF Case (a): Without Feedback Using the breadboard, construct the circuit shown in Fig. 1.0 using R
f
= 1M
and C
s
= 200pF
. Laboratory Measurements -1
3.
Connections are made as per circuit diagram Fig. (1). 4.
Measure input and output impedance of the amplifier as describe in section A. 5.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across R12 and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 6.
Turn function generator ON. Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine = × V
OUT (This is the gain at mid-band which corresponds to maximum gain.) the voltage gain. |
Gain | 20 Log
( )
V
IN
2-6
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7.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
8.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
9.
10.
Calculate the bandwidth of the amplifier using the expression Bandwidth: Bandwidth = f
H
-
f
L
Calculate
the gain-bandwidth product of the amplifier using the expression: •
Gain-Bandwidth Product = (Magnitude of mid-band gain) X (Bandwidth) Case (b): With Feedback Using the breadboard, construct the circuit shown in Fig. 1.0 using R
f
= 10k
and C
s
= 200pF
.
Laboratory Measurements -2 11.
Connections are made as per circuit diagram Fig. (1). 12.
Measure input and output impedance of the amplifier as describe in section A. 13.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across R12 and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 14.
Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
15.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
16.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
17.
Calculate the bandwidth of the amplifier using the expression Bandwidth: BW = f
H
-
f
L
18.
Calculate the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (Magnitude of Mid-Band Gain) X (Bandwidth) 19.
Take a photo of your breadboard and include in your lab report. Table 2: Voltage-Series Feedback Amplifier Without Feedback With Feedback Input Impedance Output Impedance Gain (Mid-Band) in dB 2-7
7.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
8.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
9.
10.
Lower cut-off frequency (f
lo
) Higher cut-off frequency (f
ho
) Bandwidth (f
ho
– f
lo
) Gain-Bandwidth Product Fig. 2 shows a Current-Shunt Feedback Amplifier circuit. The combination of R
f
and C
f
provide ac feedback when these elements are present in the circuit. 2-8 Lower cut-off frequency (f
lo
) Higher cut-off frequency (f
ho
) Bandwidth (f
ho
– f
lo
) Gain-Bandwidth Product
Simulations - 1 1.
Start by simulating the circuit in Fig. (2) with PSPICE. Plot the circuit with the dc analysis results on the circuit. 2.
Simulate the frequency response of the following Rf and Cf combinations between 10 Hz and 30 MHz; Set markers at the mid band gain and at the -3dB upper and lower frequency. •
R
f
= 1M
and C
f
=10uF •
R
f
= 10k
and C
f
= 10uF Case (a): Without Feedback Using the breadboard, construct the circuit shown in Fig. (2) using R
f
= 1M
and C
f
= 10uF
. Laboratory Measurements -1
3.
Connections are made as per circuit diagram Fig. (2). 4.
Measure input and output impedance of the amplifier as describe in section A. 5.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across R12 and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 6.
Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
Calculate the bandwidth of the amplifier using the expression Bandwidth: BANDWIDTH = f
H
-
f
L
Calculate
the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (Magnitude of Mid-Band Gain) X (Bandwidth) Case (b): With Feedback Using the breadboard, construct the circuit shown in Fig. (2) using R
f
= 10k
and C
f
= 10uF
.
Laboratory Measurements -2 11.
Connections are made as per circuit diagram Fig. (2). 12.
Measure input and output impedance of the amplifier as describe in section A. 13.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across R12 and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 14.
Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. 2-9
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7.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
8.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
9.
10.
= ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
15.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
16.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
17.
Calculate the bandwidth of the amplifier using the expression Bandwidth: BW = f
H
-
f
L
18.
Calculate the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (Magnitude of Mid-Band Gain) X (Bandwidth) 19.
Take a photo of your breadboard and include in your lab report. Table 2: Current-Shunt Feedback Amplifier Without Feedback With Feedback Input Impedance Output Impedance Gain (Mid-Band) in dB 2-10 Lower cut-off frequency (f
lo
) Higher cut-off frequency (f
ho
) Bandwidth (f
ho
– f
lo
) Gain-Bandwidth Product
Fig. (3) Shows a Voltage-Shunt Feedback Amplifier circuit. The combination of R
f
and C
f
provide ac feedback when these elements are present in the circuit. Simulations - 1 1.
Start by simulating the circuit in Fig. 3 with PSPICE. Plot the circuit with the dc analysis results on the circuit. 2.
Simulate the frequency response of the following Rf and Cf combinations between 10 Hz and 100 MHz; Set markers at the mid band gain and at the -3dB upper and lower frequency. •
R
f
= 1M
and C
f
=10uF •
R
f
= 10k
and C
f
= 10uF Case (a): Without Feedback Using the breadboard, construct the circuit shown in Fig. (3) using R
f
= 1M
and C
f
= 10uF
. Laboratory Measurements -1
3.
Connections are made as per circuit diagram Fig. (3). 4.
Measure input and output impedance of the amplifier as describe in section A. 5.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across R4 and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 2-11
7.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
8.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
9.
10.
6.
Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
Calculate the bandwidth of the amplifier using the expression Bandwidth: BANDWIDTH = f
H
-
f
L
Calculate
the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (Magnitude of Mid-Band Gain) X (Bandwidth) Case (b): With Feedback Using the breadboard, construct the circuit shown in Fig. (3) using R
f
= 10k
and C
f
= 10uF
.
Laboratory Measurements -2 11.
Connections are made as per circuit diagram Fig. (3). 12.
Measure input and output impedance of the amplifier as describe in section A. 13.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across R4 and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 14.
Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
15.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
16.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until
the output voltage gain
falls to 70.7 percent 2-12 Lower cut-off frequency (f
lo
) Higher cut-off frequency (f
ho
) Bandwidth (f
ho
– f
lo
) Gain-Bandwidth Product
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of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
17.
Calculate the bandwidth of the amplifier using the expression Bandwidth: BW = f
H
-
f
L
18.
Calculate the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (Magnitude of Mid-Band Gain) X (Bandwidth) 19.
Take a photo of your breadboard and include in your lab report. Table 3: Voltage-Shunt Amplifier Feedback Amplifier Without Feedback With Feedback Input Impedance Output Impedance Gain (Mid-Band) in dB Fig. (4) shows a Wideband Feedback Amplifier circuit. The combination of R
f
and C
s
provide ac feedback when these elements are present in the circuit. Simulations - 1 1.
Start by simulating the circuit in Fig. (4) With PSPICE. Plot the circuit with the dc analysis results on the circuit. 2.
Simulate the frequency response of the following Rf and Cs combinations between 10 Hz and 30 MHz; Set markers at the mid band gain and at the -3dB upper and lower frequency. •
R
f
= and C
s
= 0 •
R
f
= 10k
and C
s
= 200pF 2-13
7.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
8.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
9.
10.
•
R
f
= 4.7k
and C
s
= 1nF Case (a): Without Feedback Using the breadboard, construct the circuit shown in Fig. (4) using R
f
= and C
s
= 0
. Laboratory Measurements -1
3.
Connections are made as per circuit diagram Fig. (4). 4.
Measure input and output impedance of the amplifier as describe in section A. 5.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across Rload
and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 6.
Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
2-14 Lower cut-off frequency (f
lo
) Higher cut-off frequency (f
ho
) Bandwidth (f
ho
– f
lo
) Gain-Bandwidth Product
7.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator 8.
9.
10.
until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
Calculate the bandwidth of the amplifier using the expression Bandwidth: BANDWIDTH = f
H
-
f
L
Calculate
the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (Magnitude of Mid-Band Gain) X (Bandwidth) Case (b): With Feedback Using the breadboard, construct the circuit shown in Fig. (4) using R
f
= 10k
and C
s
= 200pF
.
Laboratory Measurements -2 11.
Connections are made as per circuit diagram Fig. (4). 12.
Measure input and output impedance of the amplifier as describe in section A. 13.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across Rload and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 14.
Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
15.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
16.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower
cut-off frequency f
L
. Take a screen shot of
scope image.
2-15
Lower cut-off frequency (f
lo
) Higher cut-off frequency (f
ho
) Bandwidth (f
ho
– f
lo
) Gain-Bandwidth Product
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7.
17.
Calculate the bandwidth of the amplifier using the expression Bandwidth: BW = f
H
-
f
L
18.
Calculate the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (3dB mid-band gain) X (Bandwidth) 19.
Take a photo of your breadboard and include in your lab report. Table 4: Wideband Feedback Amplifier Without Feedback With Feedback Input Impedance Output Impedance Gain (Mid-Band) in dB F. Construction and Testing of Soldered Wideband Amplifier Circuit 1.
Select the proper components and construct the Wideband Amplifier in Fig. (4) on a soldered Vero-board as presented in Fig. (4a). Keep all wires as short as possible, avoiding the use of jumper wires, and be sure that components connected to the collector of the output BJT are kept as far away as possible from the components connected to the base of the input BJT. Construct the circuit shown in Fig. (4) using R
f
= and C
s
= 0
. Fig. 4a: Strip Board Soldering for Wideband Amplifier Circuit 2.
Apply power to the circuit. Compare all dc voltages with the simulated values. If measured values are not within a few percent of the simulated values, there is a good chance that something is wrong with the circuit. Check to see that no wires are touching that should not be touching. Enter your measured dc voltages in Table 4a. Variable DC
Voltage V Simulated Operating
Point Value (V) Measured Operating Point Value (V) V(Base1) V(Base2) V(Collector1)
V(Collector2)
V(Emitter1) 2-16
V(Emitter2) V(In) V(Out) Table 4a: Vero-Board DC Voltage Levels at Transistor Pins and Input and Output Signals (Fig. (4)) 3.
Case (a): Without Feedback Using the breadboard, construct the circuit shown in Fig. (4) using R
f
= and C
s
= 0
. Laboratory Measurements -1
4.
Connections are made as per circuit diagram Fig. (4). 5.
Measure input and output impedance of the amplifier as describe in section A. 6.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across Rload
and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. Adjust the circuit and signal generator to produce clean 5 kHz sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
8.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
9.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
10.
Calculate the bandwidth of the amplifier using the expression Bandwidth: BANDWIDTH = f
H
-
f
L
11.
Calculate the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (Magnitude of Mid-Band Gain) X (Bandwidth) Case (b): With Feedback Using the breadboard, construct the circuit shown in Fig. (4) using R
f
= 10k
and C
s
= 200pF
.
Laboratory Measurements -2 12.
Connections are made as per circuit diagram Fig. (4). 13.
Measure input and output impedance of the amplifier as describe in section A. 2-17
7.
14.
Turn oscilloscope ON. To view the output and the input signal simultaneously, connect one probe of oscilloscope (CH1) to Vin and another probe (CH2) across Rload and use AC coupling to probe the signal amplitude. Connect ground of oscilloscope, DC power supply and Function Generator very close together to a single point on breadboard and to amplifier ground. 15.
Adjust the circuit and signal generator to produce 5 kHz clean sine-waves on the input and output. Measure and record the amplitude of both input and output, and then take the ratio to determine the voltage gain. = ×
V
OUT
(This is the gain at mid-band which corresponds to maximum gain.) |
Gain |
20
Log
(
)
V
IN
16.
Keeping the input voltage at constant 10mV peak-to-peak, slowly increase the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to higher cut-off frequency f
H
. Take a screen shot of scope image.
17.
Keeping the input voltage at constant 10mV peak-to-peak, slowly decrease the frequency of the signal generator until the output voltage gain falls to 70.7 percent of its value at 5 kHz. Stop and record this frequency which corresponds to lower cut-off frequency f
L
. Take a screen shot of scope image.
18.
Calculate the bandwidth of the amplifier using the expression Bandwidth: BW = f
H
-
f
L
19.
Calculate the gain-bandwidth product of the amplifier using the expression: Gain-Bandwidth Product = (3dB mid-band gain) X (Bandwidth) 20.
Comment on the difference in the frequency response of the Vero-board wideband amplifier and the proto board wideband amplifier. 21.
Measure the frequency response of the Wideband Feedback Amplifier using the example table in Appendix A. 22.
Take a photo of your soldered Vero-board Wide-Band Amplifier and include in your Technical Report. Table 4b: Soldered Wideband Feedback Amplifier Without Feedback With Feedback Input Impedance Output Impedance Gain (Mid-Band) in dB Lower cut-off frequency (f
lo
) Higher cut-off frequency (f
ho
) Bandwidth (f
ho
– f
lo
) Gain-Bandwidth Product 2-18
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Lab Report 1. Complete the tables of gain-bandwidth products that you started with your simulation results. Compare the simulation results with the measured results and offer explanations for any discrepancies. •
Submit your completed set of tables and graphs along with calculations, answers, conclusions and comments to the questions posted. 2-19
Appendix A: Examples of simulated frequency response m1
freq=2.808k
Hz Gain=62.207
Eqn Gain=dB(Vout/Vin)
Eqn Rin=mag(Vin/I_Probe1.i)
m2
freq=1.235k
Hz Rin=9.764E3
2-20
Example table of measured frequency response
2-21
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Related Documents
Related Questions
(1)
Describe in detail the relative advantages of Class A and Class B
amplifiers. In what types of circuits would Class B be advantageous
over Class A
(1I) With the aid of signal diagrams, describe two forms of distortion you
would expect to observe on an output signal of a Class B amplifier.
(III) Describe in circuit terms the advantages of a Class AB amplifier.
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3E
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Activity 2:
The amplifier circuit below has a single ac input and two ac outputs. Assuming transistor
parameters of B= 130 and VBE=0.7 V:
15 V
15 V
W
350
ΚΩ
300
ΚΩ
H11
13 ΚΩ
10 ΚΩ
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Draw the input current-voltage characteristic, the output current-voltage characteristics
and the graph giving dependence of the output current as function of the input current.
Define the amplification gain of this circuit.
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This problem is AC analysis problem. DC analysis is not needed to answer the question.
A) Convert this bias circuit into a bypassed common source amplifier that has an output across
a load resistor (RL). To do this you should draw three capacitors on the figure below, an
input voltage source, and any resistors you think that should be added.
B) In the space below the figure, Draw the hybrid n model for this amplifier circuit including all
voltages and resistors. Label Vi, Vgs, and vo on the model. Assume the capacitors you
add act as short circuits at AC. Be sure to include resistors R1, R2, R3, R4, and RL in the
hybrid pi model.
> When you "verify" a mode of operation you will need to calculate all three voltages (Vc,
V8, VE for BJTS and VG, Vs, Vo for MOSFETS) and show the correct two conditions are
satisfied.
> Assume Capacitors acts like open circuits at DC and short circuits for AC.
> Assume the following:
o Beta = 100
O VBE = 0.7
12V
o V (Thermal) = 26 mV
o V (Threshold) = 2V
O…
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Q3 a)
Show that negative feedback can reduce the sensitivity of an amplifier circuit,
by deducing the expression of dAd/dAo]-
b) Figure Q3.b shows a discrete transistor feedback configuration.
Vcc
RC
RF
O vo
Vi
Rs
Figure Q3.b
1) Identify the feedback topology of this configuration.
2) What type of amplifier is this configuration? How the feedback can affect
its input and output resistances?
3) Draw the small-signal equivalent circuit and derive the closed loop
transfer function.
c) Copy the following table into your answer book and complete it with either
'infinite' or 'zero'. Please note that table is for ideal amplifiers.
Input Resistance
Output Resistance
Voltage amplifier
Current amplifier
Transresistance amplifier
Tranconductance amplifier
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Provide the detailed explanation -
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kn = kn°(W/L) = 2 mA/V² and A=0.
(a) Calculate the parameter values at the opeating point, Ip, VGs, and Vps
(b) Draw the small-signal equivalent circuit
(c) Determine Av=V/Vi
(d) Determine the input resistance Rin and the output resistance Rout.
+5V
RD=7kQ
R;=165k2
Vo
Cc
Answer:
(ab In =,
Rout
Vi
Vcs =
R2=35k2
R3=0.5k2
Vps =
Rin
(c) Av = V/VI =
(d) Rin =
-5V
Rout =
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Lecturer Karrar Al bayat
=
Consider the circuit shown in Figure below with transistor parameters ß
120 and VA =00. (a) Determine the small-signal parameters gm, I, and to for both
transistors. (b) Plot the dc and ac load lines for both transistors. (c) Determine the overall
small-signal voltage gain Av = vo/vs. (d) Determine the input resistance R₁, and the output
resistance R.. (e) Determine the maximum
Vcc=+12 V
undistorted swing in the output voltage.
< R₁ =
< 67.3 ΚΩ
R₂ =
Σ 15 ΚΩ
Ro
R₂ =
R₁=
12.7 K
345 ΚΩ
Ris Co
RC1 =
ΤΟ ΚΩ
"98
21
REL=
<2k2=CE
22
CC3
RE2=
RL=
1.6 kΩ < 250 Ω
-OU
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