BME 323
MEDICAL ELECTRONICS
BJT-JFET CASCADE AMPLIFIER
EXPERIMENT 05
BJT-JFET CASCADE AMPLIFIER
Objective:
1. To investigate the common-drain amplifier
2. To investigate multiple gain of cascade amplifiers
Theory:
A typical JFET common-drain amplifier configuration, commonly called a source follower, is
shown in Figure 1. The common-drain amplifier characterized by a voltage gain of slightly
less than one, a high input resistance, and a low output resistance. The input and output
resistances can be measured by the same method used for previous experiments.By analyzing
the small-signal equivalent circuit of a common-drain amplifier, you can show the voltage
gain from source-to-load.
Preliminary Work:
Please consider the circuit –source follower- in Figure 1. Find voltage gain formula (in terms
of R1 ,R2 ,RS ,RL , rS ,rD , gm). Leave gm as gm and do not forget there is not any resistance
connected on drain pin of JFET!
(rS : inner resistance of signal generator)
!!! ANYBODY WHO DID NOT PREPARE PRELIMINARY WORK CAN NOT JOIN THE EXPERIMENT !!!
Procedure:
1. Construct the circuit in Figure 1 and find voltage gain by oscilloscope display in
XY mode, draw the Vout-Vin graph as shown in XY mode osc. display. Let the
input signal as vS=250.sin(2.π.1000.t) mV –millivolts-
NJFETblock
Figure 1
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BME 323
MEDICAL ELECTRONICS
BJT-JFET CASCADE AMPLIFIER
2. With vS =0 V, measure and record the DC voltages VGS and VRS. Use them and
your knowledge on JFET characteristics, calculate ID and gm. (Hint: for this
circuit as you know; gm ≈1/rout where rout : output resistance )
3. Now consider the four pinned dash lined block of the circuit (we call it as
NJFETblock from now on) in Figure 1.The pins are called as IN,OUT,VDD,GND.
IN pin have to be used for input signal (like function generator sinusoidal signal)
which comes into the NJFETblock.
OUT pin have to be used for output signal which goes out from the NJFETblock.
VDD pin have to be used for power supply +voltage of the NJFETblock.
GND pin have to be used for power supply ground of the NJFETblock.
Construct the circuit in Figure 2, which is described as cascade amplifier and includes
a common-emitter amplifier and a source follower amplifier. Don’t forget
NJFETblock described above and don’t mess your board up!
Distribute your circuit on board and connect elements with wires
Figure 2
4. Find the voltage gain of cascade amplifier in terms of Vo, Vi shown in Figure 2.
Set oscilloscope display in XY mode, draw the Vout-Vin graph as shown in XY
mode osc. display.
5. Find the total current drawn from power supply for the circuit in Figure 2. Be
careful, current is drawn by both transistors -BJT, JFET-
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BME 323
MEDICAL ELECTRONICS
BJT-JFET CASCADE AMPLIFIER
Question:
What is your comment on the NJFETblock ? Does it increase or decrease the input signal at
the output? Does it amplify signal. If your answer is NO, please explain the reason of usage
such a circuit! If your answer is YES, please explain how can it be!
Fill the report by necessary measured values and graphs
Equipment List:
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2N5459 N-channel JFET or equivalent
2N2222 NPN BJTor equivalent
1*220 KΩ, 1*100 KΩ, 2*10 KΩ, 1*22KΩ, 1*12 KΩ, 1*1 KΩ, 1*1.5 KΩ resistors
2*10 µF capacitors
Dual channel oscilloscope
Power supply (+15V)
Signal generator
References:
Microelectronic Circuits, Fourth Edition, Sedra&Smith, FET Small Signal Analysis
Electronic Devices, Third Edition, Floyd, JFET Amplifers
Electronic Devices and Circuits, Fifth Edition: Section 5-1, AC Amplifier
Fundamentals; Section 5-3, Amplifier Analysis Using Small Signal Models
Experiments in Electronic Devices and Circuits, Fifth Edition, Exp. 5, Exp. 9
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BME 323
MEDICAL ELECTRONICS
BJT-JFET CASCADE AMPLIFIER
Student ID # :
Name
:
REPORT:
1.
Vout
AV =Vout/Vin= ...........
Vin
2. VGS= .......................
VRS= .......................
gm= 1/rout = .................
ID =
..............
4.
Vo
AV =Vo/Vi= ...........
Vi
5.
Itotal =
Answer of the Question:..................................................................................................
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