Download LABORATORY 6 FREQUENCY RESPONSE OF A JFET AMPLIFIER

LABORATORY 6
FREQUENCY RESPONSE OF A JFET AMPLIFIER
OBJECTIVES
1. To determine quiescent point of a common source (CS) and common drain (CD)
amplifiers and calculate the values of the resistors Rd and Rs.
2. To measure upper and lower cutoff frequencies of a CS and CD amplifiers.
3. To simulate amplifier frequency response measurements using MicroCap software.
4. To study the frequency response of the common source (CS) and common drain
(CD) JFET transistor amplifiers in the frequency range of 50 Hz to 1 MHZ.
INFORMATION
In this lab, two JFET amplifier configurations will be investigated: the common source
(CS) and common drain (CD) amplifiers. Both amplifiers use a self-biasing scheme and
have a relatively linear output.
1 JFET Transistor Characteristics
The JFET transistor is a three-terminal device. The terminals are labeled drain (D), source
(S), and gate (G). For the N-channel JFET, which is what you will be using, the only
significant current that flows in the device is the drain-to-source current ID (Gate current is
zero). Basically you can think of a transistor as a valve (a switch), in other words it is
either turned on or turned off. The drain current flows into the drain terminal and exits
from the source terminal. The magnitude of ID is controlled by the voltage VGS applied
between the gate and source terminals. For N-channel JFETs, Vp and VGS are always
negative, i.e. the voltage at the gate is less than the voltage at the source terminal. For your
reference, Vp is also referred to as VGS-off (in some books).
2. Common Source amplifier
The Common Source Amplifier is one of the three basic FET transistor amplifier
configurations. In comparison to the BJT common-emitter amplifier, the FET amplifier has
much higher input impedance, but a lower voltage gain.
The Junction Field Effect Transistor (JFET) offers very high input impedance along with
very low noise figures. It is very suitable for extremely low-level audio applications as in
audio preamplifiers. The JFET is more expensive than conventional bipolar transistors but
offers superior overall performance. Unlike bipolar transistors, current can flow through
the drain and source in any direction equally. Often the drain and source can be reversed in
a circuit with almost no effect on circuit operation.
The bias levels in amplifiers based on BJTs are often stabilized using the emitter
degeneration technique; that is, a resistor is placed between the transistor’s emitter and
ground. The resistor creates negative feedback, which forces the quiescent collector
6-1
current to remain at its design value regardless of changes in the transistor’s parameters
(such as βF). A similar technique can be used to stabilize the biasing of FET amplifiers.
Shown in Figure 6.1 is a common-source JFET amplifier in which a resistor RS has been
added between the source and ground. In this circuit the gate has been connected to
ground through the resistor RG; thus, the gate is held at ground potential (0 V). If the drain
current ID begins to rise above its intended quiescent value, the voltage drop across RS will
increase. Since the gate-source voltage VGS is the difference between the gate potential
(fixed at 0 V) and the voltage across RS, a rise in the voltage across RS will cause VGS to
drop, lowering ID back to its original value. The opposite chain of events occurs if ID
begins to drop below its design value.
It is a common practice in the design of circuits based on JFETs to tie the gate to ground
potential via a large-valued resistor (typically around 1MΩ) as shown in Figure 6.1.
V DD
I
D
RD
C2 47uF
C1
J1
1uF
RL
RG
Vin
3.9k
1M
RS
Cs
47uF
.
Vout
.
Figure 6.1. The common source amplifier
3. Common Source amplifier biasing
Before starting the CS JFET amplifier biasing, you should obtain an input and output
characteristics of your JFET transistor, using the PC Characteristic Curve Tracer and
following the procedure, as described in details in the Pre-Lab section of this document.
Figure 6.2 shows typical Input characteristics and Figure 6.3 shows the Output
Characteristics of a JFET transistor. Using this characteristics diagrams you can set the
quiescent operating point of the JFET transistor as follows:
1. For ID = ½ IDSS, find the value of VGS:
•
In our example IDSS = 4.8 mA and the DC operating point conditions (VD, VG and
VGS) will be determined for ID = ½ IDSS = 2.4 mA.
•
From Figure 6.2, VGS = -0.8V and Vp = -2.4V;
•
Using the Output Characteristics diagram in Figure 6.3 determine the VDS value
for ID = 2.4 mA and VGS = -0.8V. In our example, VDS = 6V.
•
Draw the theoretical load line of this amplifier, as it is shown in Figure 6.3.
6-2
IDSS
ID
Vp `
VGS
Figure 6.2. Input characteristics of JFET
IDSS
ID
Figure 6.3. Output characteristics of JFET
4. Common Drain amplifier
The common drain FET amplifier is similar to the common collector configuration of the
bipolar transistor. A general common drain JFET amplifier, self-biased, is shown in Figure
6.4. This configuration, which is sometimes known as a source follower, is characterized
by a voltage gain of less than unity, and features a large current gain as a result of having a
very large input impedance and a small output impedance.
6-3
I
C1
V DD
D
J1
1uF
RG
1M
Vin
Cs
RS
47uF
.
R1
C2 47uF RL
3.9k
Vout
.
Figure 6.4.
a) The common drain amplifier
b) 2N5457 package
EQUIPMENT
1.
2.
3.
4.
5.
6.
7.
8.
Digital multimeter (Fluke 8010A, BK PRECISION 2831B).
Digital oscilloscope Tektronix TDS 210.
Function Generator Wavetek FG3B.
PROTO-BOARD PB-503 (breadboard).
JFET 2N5457.
Resistors 3.9 kΩ, 1ΜΩ.
Resistors R1, RS and RD according prelab calculations.
Capacitors 1 x 1µF, 2 x 47 µF.
PRE-LABORATORY PREPARATION
The lab preparation must be completed before coming to the lab. Show it to your TA for
checking and grading (out of 15) at the beginning of the lab and get his/her signature.
1. Common Source amplifier
1.1. Use the JFET curve tracer in lab 3108 and follow the procedure explained in details in
the Lab #9 of ECE240a course to obtain the input and output characteristics of the
JFET 2N5457 from your lab parts kit.
1.2. For the circuit in Figure 6.1 calculate the values of RS and RD using the curve tracer
plots. Assume that this circuit is biased for VDS=6V and ID=1/2 IDSS, with VDD=12V.
1.3. Calculate RS and RD resistors values.
The bias design procedure for the amplifier shown in Figure6.1 differs somewhat from that
for a general common-source amplifier. In the general case, the gate voltage VG can be
controlled by the designer, but with this circuit the gate is tied to ground potential, which
removes one degree of freedom in the design equations.
First calculate the value of a RS, using Equation (6.1)
VG = VGS + I D RS = 0
Equation (6.1)
Then the value of the drain resistor is easily determined using Equation (6.2)
6-4
V DS = VDD − I D (RD + RS )
Equation (6.2)
1.4. Determine the small signal ac model at mid frequencies (1 kHz) for the 2N5457. Use the
input and output characteristics and determine the ac small signal voltage gain using a
graphical method. All the calculations must be shown.
1.5. Simulate the above circuit in MicroCap using standard resistors values. Note that
1M resistance should be entered as 1 Meg in MicroCap. You must show the values of
the all the bias currents and voltages on your schematic printouts.*
1.6. Using the MicroCap AC analysis function obtain the gain and phase frequency
response for this amplifier from 10Hz to 1MHz and find the 3dB point. Print the
results and bring these plots with you to the lab session for comparing with the
experimental data.** Bring the print-out of the DC quiescent point values and the
Bode plots of the magnitude (in dB) and phase angle (in degrees) of the gain ratio to
the lab. You should use the Bode plot printouts as the graphs on which to plot your
experimental data.
Attention: You must plot the Bode plots, i.e. the ratio of the output voltage over the input
voltage, not the output voltage by setting the input voltage equal to 1.0∠0°!
MicroCap simulations tips:
• From MicroCap component library pick up a JFET 2N5545, which has a similar
characteristics to the JFET 2N5457 you have in your parts kit.
• * To provide a power supply to the circuit use the “Battery” source from the
MicroCap library and set it to a 12V value.
• *To obtain the values of the all the bias currents and voltages on your schematic
choose the Probe AC mode from Analysis menu and click on Node Voltages and
Currents icons on the toolbar.
• **For a sine wave signal source (used for simulating the Vin), use a 1MHz Sinusoidal
Source from the Micro–Cap library. Set the AC Amplitude to A= 0.1(V) in the model
description area of the signal source. Note that A=0.1V corresponds to a magnitude of
Vp-p=0.2V.
• **To obtain the gain and phase frequency response plots for this circuit you must
run “AC ANALYSIS”. To get best results for your plots set the AC Analysis
Limits as follows:
Frequency Range: 1E6,10 which corresponds to a frequency range from 10 Hz to 1MHz
Plot parameters:
P
X-Expression
Y-Expression
X-Range
Y-Range**
Voltage Gain
Phase
1
2
F
F
dB(V(3)/V(1))* 1e6,10,1k
ph(V(3)/V(1))* 1e6,10,1k
20,10,1
-160,-190,5
Note: * V(1) and V(3) are the AC voltages at corresponding nodes(1) at the input
and (3) at the output of the simulation circuit set up. In your particular case they could
have different numeration.
2. Common Drain amplifier
1.1. For the circuit in Figure 6.4 calculate the values of R1 and RS. Bias this circuit for
VDD=12V, VDS=6V and ID=1/2 IDSS using the curve tracer plots.
6-5
The procedure for calculation of R1 and RS values is very similar to that used for commonsource amplifier. The only unknown component values in this circuit are resistors R1 and
RS. The following Equations (6.3) to (6.6) could be used for calculations:
VS = VDD − VDS
Equation (6.3)
VG = VGS + VS
Equation (6.4)
VS −VG
ID
Equation (6.5)
VS = I D (RS + R1 )
Equation (6.6)
RS =
2.2. Determine the small signal ac model at mid frequencies (1 kHz) for the 2N5457. Use the
input and output characteristics and determine the ac small signal voltage gain using a graphical
method. All the calculations must be shown.
2.3. Simulate the above circuit in MicroCap using standard resistors values and attach
the bias point results. For this you must show the values of the all the bias currents and
voltages on your schematic.*
2.4. Using the MicroCap AC analysis function, obtain the gain and phase frequency
response for this amplifier from 10Hz to 1MHz and find the 3dB point. Print the results
and bring these plots with you to the lab session for comparing with the experimental
results. Bring the print-out of the DC quiescent point values and the Bode plots of the
magnitude (in dB) and phase angle (in degrees) of the gain ratio to the lab. You should use
the Bode plot printouts as the graphs on which to plot your experimental data.
Attention: You must plot the Bode plots, i.e. the ratio of the output voltage over the input
voltage!
MicroCap simulations tips:
• For a sine wave signal source use a 1MHz Sinusoidal Source. Set the AC Amplitude
to A= 1(V) in the model description area of the signal source. Note that A=1V
corresponds to a magnitude of Vp-p=2V.
• To get better results for your plots set the AC Analysis Limits as follows:
Frequency Range: 1E6,10 which corresponds to a frequency range from 10 Hz to 1MHz
Plot parameters:
P
X-Expression
Y-Expression
X-Range
Y-Range**
Voltage Gain
Phase
1
2
F
F
dB(V(3)/V(1))* 1e6,10,1k
ph(V(3)/V(1))* 1e6,10,1k
-1.5,-2.5,0.2
10,-2,2
Note: *V(1) and V(2) are the AC voltages at corresponding nodes (1)- input and (2)output of the simulation circuit set up. In your particular case they could have different
numeration.
Note: For each simulation, print the Micro-Cap circuit set-up with node numbers and the
supplementary text file with components description. This will help your TA to correct any
mistakes in your simulations. Bring all required plots to your lab session and submit them to
your TA. You will use these plots to draw practical results of your experiments during the lab
session.
6-6
PROCEDURE
1. Common Source amplifier frequency response.
1.1. Build the circuit in Figure 6.5 using the components given in the pre-lab. Use standard
parts when building the amplifier. Do not use multiple resistors to match the specified
values. Also measure the actual value of the resistors using the multi-meter.
CH1
CH2
PHASE METER
RD
V DD
C1
RED
CH1 1uF
CH1
CH2
CH2
C2 47uF
J1
RL
FG
BLACK
Vin
GND
GND
.
RG
1M
3.9k
Rs
Cs
Vout
GND
47uF
.
GND
OSCILLOSCOPE
CH1
CH2
Figure 6.5. Common source amplifier circuit measurements
1.2. Initially apply only DC power to the circuit and measure the amplifier's Q point using
the Digital multimeter (DC quiescent conditions). Make sure your circuit is biased
correctly, your measurements should deviate no more than 15% from the calculated
values for ID and VDS. If your values deviate more than the allowable, adjust the
resistance values, and provide an explanation in your report.
1.3. Connect the Function Generator (FG) to supply the input ac signal to the CS amplifier
circuit.
1.4. Connect CH1 of the Oscilloscope and CH1 of the Phase meter in parallel with the
input of the CS amplifier to measure the parameters of the input signal Vin. Connect
CH2 of the Oscilloscope and CH2 of the Phase meter in parallel with the load resistor
RL to measure the parameters of the output signal Vout.
1.5. Set the input voltage level to Vin =100mV (RMS), as measured by the CH1 of the
oscilloscope. Starting from 50 Hz, sweep the input frequency up to 1 MHz.
1.6. For each of the selected frequencies read the RMS voltage of the Vin (CH1) and Vout
(CH2) from the oscilloscope display and record the data in Table 6.1. For the same
frequencies record the Phase Meter readings (display will show the phase angle
between these two signals in [deg]).
1.7. For each of the measurements calculate the voltage gain in (dB), using the Equation
(6.7).
AV (dB) = 20 log
6-7
Vout
Vin
Equation (6.7).
Vin [V]
f [Hz]
50
75
100
200
500
800
1k
2k
5k
10k
20
50k
100k
200k
500k
1M
Θ[deg]
Vout [V]
Av[dB]
Table 6.1. CS and CD amplifier frequency response
1.8. Plot the obtained data on top of your MicroCap simulations. Do they agree? If there is
a difference, explain what could be the reason for it.
1.9. Increase the input signal level until output voltage clipping occurs. Record the
maximum input and output levels of undistorted sine wave signal.
1.10. Note the phase shift between output and input. Is your amplifier inverting or noninverting?
2. Common Drain amplifier frequency response.
2.1. Build the circuit in Figure 6.6 using the components given in the pre-lab. Use standard
parts when building the amplifier. Do not use multiple resistors to match the specified
values. Also measure the actual value of the resistors using the multi-meter.
CH1
CH2
PHASE METER
RED CH1
CH1
C1
1uF
FG
BLACK
V DD
RG
1M
Vin
GND
GND
J1
Cs
Rs
47uF
CH2 CH2
.
R1
C2
RL
Vout
47uF 3.9k
OSCILLOSCOPE
.
GND
GND
Figure 6.6. Common Drain amplifier circuit measurements
6-8
CH1
CH2
2.2. Initially apply only DC power to the circuit and measure the amplifier's Q point using
the Digital multimeter (DC quiescent conditions). Make sure your circuit is biased correctly,
your measurements should deviate no more than 15% from the calculated values for ID and
VDS. If your values deviate more than the allowable, adjust the resistance values, and
provide an explanation in your report.
2.3. Connect the Function Generator (FG) to supply the input ac signal to the CS amplifier
circuit.
2.3. Connect CH1 of the Oscilloscope and CH1 of the Phase meter in parallel with the
input of the CS amplifier to measure the parameters of the input signal Vin. Connect CH2
of the Oscilloscope and CH2 of the Phase meter in parallel with the load resistor RL to
measure the parameters of the output signal Vout.
2.4. Set the input voltage level to Vin=1V (RMS), as measured by the CH1 of the
oscilloscope. Starting from 50 Hz , sweep the input frequency up to 1 MHz.
For each of the selected frequencies read the RMS voltage of the Vin (CH1) and Vout
(CH2) from the oscilloscope display and record the data in Table 6.1. For the same
frequencies record the Phase Meter readings (display will show the phase angle between
these two signals in [deg]).
For each of the measurements calculate the voltage gain in {dB), using the Equation (6.7).
2.5. Plot the obtained data on top of your MicroCap simulations. Do they agree? If there is
a difference, explain what could be the reason for it.
2.6. Increase the input signal level until output voltage clipping occurs. Record the
maximum input and output levels of undistorted sine wave signal.
2.7. Note the phase shift between output and input. Is your amplifier inverting or non-inverting?
2.8. Answer the following questions:
• Do your experimental results agree with the MicroCap simulation? Comment on
both the DC and the AC values. Explain any discrepancies.
• Is your CD amplifier suitable for an audio amplifier?
• What is a suitable application for the source follower?
REPORT
Your Lab report is due one week later. Please submit it to your TA in the beginning of
your lab session.
Note: You must copy/print the Signature and Marking Sheet from your manual
before coming to the lab session.
6-9
SIGNATURE AND MARKING SHEET – LAB 6
To be completed by TA during your lab session
Student Name:____________________
TA Name:___________________
Student # : _____________________
Check
boxes
Task
Max.
Marks
Pre-lab completed
15
CS Amplifier completed
10
CD Amplifier completed
10
Overall Report Preparation
65
TOTAL MARKS
100
6-10
Granted
TA
Marks Signature