Experiment 2
Title: Junction Field Effect Transistor (JFET)
Objectives
1. To investigate the JFET common-source circuit biasing characteristics.
2. To obtain the dc quiescent voltages and currents of the JFET
3. To obtain the frequency response of the JFET amplifier circuit.
4. To determine the lower cutoff frequency and the upper cutoff frequency of the JFET
amplifier circuit.
5. To determine the midband gain, Avo of the JFET amplifier circuit.
List of Apparatus
Components
1. Transistor: 2N5461 (or any equivalent P-channel JFET)
2. Resistors: 8.2 MΩ, 1.2 MΩ, 2.4 KΩ, 2.7 K Ω
3. Capacitors : 0.22 µF , 4.70 µF, 1 nF , 10 nF
Equipments
1. Power Supply
2. Oscilloscope
3. Function Generator
4. Digital Multimeter
5. Protoboard
Theory
A junction field-effect transistor is a unipolar device because only one type of charge
carrier (electrons or holes) is involved in its operation. There are two types of JFET, the
N-channel and P-channel. The N-channel and the P-channel of the FET have their own
operating activities. The P-channel JFET transistor conducts from drain-to-source when
the voltage from gate-to-source is positive. When the voltage from gate-to-source is 0V,
the gate-to-drain junction is reversed-biased and no current can flow through the resulting
depletion region. The output current ID is controlled by the value of input voltage VGS.
JFET characteristics
Figure 1 shows the relationship between output current ID and input voltage VGS for the
JFET circuit. If VGS = 0 V, ID is a maximum and is known as IDSS, or the drain saturation
current. If VGS is increased (made more positive), a point is reached where the current ID
is 0 A. The value of VGS where this occurs is called the pinch-off voltage, Vp.
Once all the required values obtained, the transconductance gm, which is the ratio of ac
output current to input voltage, can be evaluated by performing the following equation:
gm =
id
2 IDSS
=
Vgs | Vp |
 | VGS |  2 IDSS
1 −
=
| Vp |  | Vp |

1
ID
IDSS
ID
IDSS
ID RS = VG –VGS
Where VG = 0 V
Q
ID = IDSS( 1 – VGS/Vp)2
VG
Vp
VGS
Figure 1. The relationship between output current, ID and input voltage VGS for p-channel
JFET.
JFET Frequency response
A typical JFET common-source amplifier circuit is shown in figure 4. The JFET is biased
using the combination of a voltage divider across the gate and a self-biasing Rs in the
source circuit.
The frequency response of the JFET amplifier plotted in the form of voltage gain versus
frequency is shown in figure 2. The gain is null at zero frequency, then rises as frequency
increases, level off for further increases in frequency, and then begins to drop again at
high frequencies. The frequency response of an amplifier can be divided into three
frequency regions.
Voltage
Gain
Midband
region
Avo
0.707Avo
0
fL =lower
cutoff frequency
fH = upper
cutoff frequency
frequency
Figure 2. Frequency response of an amplifier.
The frequency response begins with the lower frequency region designated between 0 Hz
and lower cutoff frequency. At lower cutoff frequency, fL ,the gain is equal to 0.707 Avo.
Avo is a constant midband gain obtained from the midband frequency region. The third,
the upper frequency region covers frequency between upper cutoff frequency and above.
Similarly, at upper cutoff frequency, fH, the gain is equal to 0.707 Avo. After the upper
cutoff frequency, the gain decreases with frequency increases and dies off eventually.
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The Lower Frequency Response.
Approximately, the following equations can be used to determine the lower cutoff
frequency of the amplifier, where the voltage gain drops 3 dB from its midband value
(=0.707 times the midband Avo):
(1)
f1 = 1/ ( 2πrinCc1 )
where,
f1 = lower cutoff frequency due to Cc1
Cc1 = input coupling capacitance
rin = input resistance of the amplifier
(2)
f2 = 1/ ( 2πro Cc2 )
where,
f2 = lower cutoff frequency due to Cc2
Cc2 = output coupling capacitance
ro = output resistance of the amplifier
Provided that f1 and f2, are not close in value the actual lower cutoff frequency is
approximately equal to the larger of the two.
The Upper Frequency Response
The upper cutoff frequency fH due to the effect of the shunt capacitances in the transistor.
However, capacitors Cp is used to subdue these shunt capacitance effects. So, fH can be
evaluated approximately by
f’H = 1/2πroCp
PART I PRELIMINARY WORK
(1) Get the equivalent circuit for the JFET circuit given in figure 4.
(2) What condition must the JFET circuit acquire to determine the pinch-off voltage, Vp
and the drain saturation current, IDSS, respectively?
(3) By referring to a relevant JFET datasheet, approximately determine the gm for ID = 1
mA. State your assumptions if required.
(4) You are required to produce a simulated frequency response, voltage gain versus
frequency, for the amplifier circuit given in figure 4 by using MULTISIM or of any
equivalent software. Print the response. Observe the simulated response and
determine the following values; the midband gain, Avo, the lower cut-off frequency,
fL and the upper cut-off frequency, fH. Record all the values in your preliminary
report.
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PART II. EXPERIMENTAL PROCEDURE
(a) Determining the drain saturation current, IDSS
Connect the circuit as shown in figure 3a. Make sure apply short circuit between the gate
and the source of the JFET. Supply voltage of -12 V (negative supply) to the circuit.
Measure the drain voltage, VD. Calculate the IDSS.
-12V
-12V
VDD
VDD
RD
2.4k
R1
8.2M
D
G
2N5461
short
circuit
R2
1.2M
RD
2.4k
R1
8.2M
0-30V
Vin
S
+
D
G
2N5461
S
+
RS
2.7k
R2
1.2M
Figure 3a.
RS
2.7k
Figure 3b.
(b) Determining the pinch-off voltage, Vp
Connect the circuit as shown in figure 3b. Supply voltage of –12 V to the VDD of the
circuit. Vary the input voltage, Vin, and observe the drain voltage, VD, until the drain
voltage just reaches -12V. Measure the gate voltage, VG. This measured voltage is the
pinch-off voltage, Vp.
(c) Determining the transconductance, gm.
Connect the circuit as shown in figure 4. Supply voltage of –12 V to the VDD of the
circuit. Measure the drain voltage, VD, the source voltage, VS and the gate voltage, VG.
Calculate the ID and VGS. Then calculate the gm.
(d) Obtaining frequency response for the JFET
Connect the circuit as shown in figure 4 and setup your apparatus accordingly to perform
the work in determining the frequency response of the JFET amplifier. Apply supply
voltage to the circuit. Measure and record the drain, the gate and the source voltage of the
transistor. Next, apply a sinusoidal input signal, Vi, with vo1tage of 1volt peak-to-peak
value at frequency of 5 Hz to the test circuit. Observe, measure, record the output
voltage, VO and calculate the voltage gain, Av = Vo/Vi. Also measure the phase
difference between input and output signals and record in a table. With input signal
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always constant, increase signal frequency, measure and record the output voltage, VO.
Again calculate the voltage gain and obtain the signals phase difference. You are required
to create a frequency response table to tabulate your readings with frequency, input
voltage, output, voltage gain, phase difference and phase angle as the table entry list.
Produce all necessary readings to cover frequency band between 5 Hz and 100 KHz.
Make sure the input voltage is always constant at all frequency settings.
Plot the frequency response of (i) voltage gain in dB versus frequency, and (ii) phase
response verses frequency on a semi-log paper. Explain and discuss your observation on
the measured results.
-12V
VDD
RD
2.4k
R1
8.2M
Cc1
10nF
0.22uF
D
G
2N5461
S
rs
50
Vi
Vs
R2
1.2M
RS
- Cs
2.7k
+
Cp
1nF
Vo
4.7uF
Figure 4. The JFET circuit for determining gm and the frequency
response experiment.
QUESTIONS
1.
2.
3.
4.
5.
Obtain the equivalent circuit for the circuit conducted in the experiment and state the
value of the components constituted in this equivalent circuit.
What are the measured values of fL, Avo and fH for the common-source amplifier in
figure 4?
Which capacitor affects on the lower cutoff frequency and the upper cutoff
frequency of the amplifier circuit? What you can conclude about the capacitors’
impact on the cut-off frequencies when they are increased and decreased?
Obtain the empirical function to describe the measured frequency response of the
amplifier circuit?
Give your comment on the measured frequency response.
Prepared by
Ismail Ariffin,
Abd Rahim Rahman,
Mohd Azhar Razak
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