Experiment title:
JFET CHARACTERISTICS:
OBJECTIVES:
i) To investigate the drain characteristics of JFET.
ii) To investigate the transfer characteristics of JFET.
THEORY:
INTRODUCTION TO JFET(JUNCTION FIELD-EFFECT TRANSISTOR):
The JFET ( junction field-effect transistor) is a type of FET that operates with a
reverse-biased pn junction to control current in a channel. Depending on their
structure, JFETs fall into either of two categories, n channel or p channel.
Basic Structure
Figure (a) shows the basic structure of an n-channel JFET (junction field-effect transistor).
Wire leads are connected to each end of the n-channel; the drain is at the upper end,
and the source is at the lower end. Two p-type regions are diffused in the n-type material
to form a channel, and both p-type regions are connected to the gate lead. For simplicity,
the gate lead is shown connected to only one of the p regions. A p-channel JFET is shown
in Fig B given below.
Basic Operation
To illustrate the operation of a JFET, Figure alongside shows dc
bias voltages applied to an n-channel device. VDD provides a drain-tosource voltage and supplies current from drain to source. VGG sets the
reverse-bias voltage between the gate and the source, as shown.
The JFET is always operated with the gate-source pn junction reversebiased. Reverse biasing of the gate-source junction with a negative gate
voltage produces a depletion region along the pn junction, which extends
into the n channel and thus increases its resistance by restricting the
channel width.
The channel width and thus the channel resistance can be controlled by varying the gate
voltage, thereby controlling the amount of drain current, ID. The white areas represent the
depletion region created by the reverse bias. It is wider toward the drain end of the channel
because the reverse-bias voltage between the gate and the drain is greater than that between the
gate and the source.
Drain-Source Characteristic Curve
If Vgs is increased (it's more negative to n-channel), depletion will be immediately
generated in the channel so that the current required to pinch off the channel will be decreased.
The curve corresponding to Vgs = -1V is shown in above fig. From this result we can find out
that the gate voltage functions as a controller capable of decreasing the drain current (at a
specific voltage Vds). If Vgs is more positive for p-channel JFET, the drain current will be
decreased from Idss (as shown in Fig 7.4 (b)). If Vgs is continuously increased, the drain current
will be decreased correspondingly. When Vgs reaches a certain value, the drain current will be
decreased to zero and will be independent of the value of Vds. The gate-source voltage at this
time is called pinch-off voltage which is usually denoted as Vp or Vgs (cutoff). From above we
can find out that Vp is a negative voltage for n-channel FET and a positive voltage for p-channel
FET.
Transfer Characteristic Curve
Another characteristic curve for JFET is transfer characteristic curve. This is a variation
curve of drain current Id corresponding to gate-source voltage Vgs while the drain-source
voltage Vds is constant. Two points, Idss and Vp are the most important points in this transfer
characteristic curve. When these two points are fixed in the coordinate axes, the remaining points
can be looked up from this transfer characteristic curve or can be solved from the formula Id =
Idss(1- Vgs/Vp)2. From this formula,we can calculate Vgs = 0, Id = Idss, I d = 0, Vgs = Vp.
The design of JFET is typically designed in the middle between Vp and Idss of the transfer
curve.
EQUIPMENTS AND COMPONENTS:
(1) JFET BFW11-1 pc.
(2) Solid state VOM- 1pc
(3) DC milliammeter- 0-10mA – 1pc.
(4) DC power supply 0-20 volts(VDD) – 1pc.
(5) DC bias supply, 0-5 volts(VGG) – 1 pcs.
PIN CONFIGURATION OF JFET BFW 10/11:
CIRCUIT DIAGRAM FOR DRAIN AND TRANSFER
CHARACTERISTICS OF JFET:
OBSERVATIONS:
Observation table for drain characteristics of JFET:
VGS(V)
DRAIN CURRENT ID (mA)
0.25
0.5
1
0
1.25
VDS(V)
0.1
0.3
0.7
1
2
3
5
6
Observation Table for transfer characteristics:
For transfer characteristics VDS=6V
ID(mA)
VGS(V)
0
0.25
0.5
0.75
1
1.25
1.5
Experiment title:
IC AUDIO POWER AMPLIFIER:
OBJECTIVES:
i) To investigate the gain efficiency of an IC audio power amplifier.
ii) To investigate the efficiency of an IC audio power amplifier.
THEORY:
INTRODUCTION TO AUDIO POWER AMPLIFIER:
An audio amplifier is an electronic device that increases the strength (amplitude) of audio signals
that pass through it. An audio amplifier amplifies low-power audio signals to a level which is
suitable for driving loudspeakers. The input signal of an audio amplifier may only measure a few
hundred microwatts, but its output may be tens or even thousands of watts. Design parameters for
audio amplifiers include gain, frequency response, distortion and noise.
Usage:
The basic purpose of using an audio power amplifier is based on the fact that while the input
signal to an audio power amplifier may measure only a few hundred microwatts, its output may
be tens or hundreds of watts for a home stereo system or thousands or tens of thousands of watts
for a concert sound reinforcement system.
Application:
Important applications include public address systems, theatrical and concert sound
reinforcement systems, and domestic systems such as a stereo or home-theatre system.
Instrument amplifiers including guitar amplifiers and electric keyboard amplifiers also use audio
power amplifiers.
Voltage amplification or gain of the audio power amplifier:
The Voltage Amplification (Av) or Gain of a voltage amplifier is given by:
With both voltages measured in the same way (i.e. both RMS, both Peak, or both Peak to Peak),
Av is a ratio of how much bigger is the output than the input, and so has no units. It is a basic
measure of the Gain or effectiveness of the amplifier.
Efficiency of a power amplifier:
The efficiency of a power amplifier basically refers to the ratio of output power to the input
power of the power amplifier.
For an amplifier, the efficiency is simply the ratio of DC power in rather than the AC drive
signal. It varies within a number between 0 to 1,or a percentage from 0 to 100. it is important
with audio amplifiers intended to drive a speaker or radio frequency amplifiers intended to
provide a signal to an antenna to ensure the efficient data transmission.
Efficiency or power amplification =output power(Pout)
.
input power(Pin)
Bandwidth of an amplifier:
The bandwidth represents the amount or "width" of frequencies, or the "band of frequencies,"
that the amplifier is most effective in amplifying. However, the bandwidth is not the same as the
band of frequencies that is amplified. The bandwidth (BW) of an amplifier is the difference
between the frequency limits of the amplifier. For example, the band of frequencies for an
amplifier may be from 10 kilohertz (10 kHz) to 30 kilohertz (30 kHz). In this case, the
bandwidth would be 20 kilohertz (20 kHz).
Bandwidth, BW=f2-f1
EQUIPMENTS AND COMPONENTS REQUIRED:
1. IC Audio amplifier -1 pc.
2. Signal generator -1 pc.
3. Oscilloscope – 1pc.
PRACTICAL CIRCUIT DIAGRAM FOR AN AUDIO POWER AMPLIFIER:
OBSERVATION TABLE FOR AUDIO AMPLIFIER:
VO(P-P)
VIN(P-P)
AV
POUT