RADIO AMATEUR EXAM
GENERAL CLASS
By 4S7VJ
CHAPTER-4
4.1 VACUUM TUBE (VALVE)
Vacuum tube is a completely vacuum sealed glass tube
containing number of electrodes. The outstanding difference
between the vacuum tube and most other electrical devices is the
electric current flow through empty space or vacuum. Free
electrons in an evacuated space will be attracted to a positively
charged object within the same space, or will be repelled by a
negatively charged object. The movement of the electrons under
the attraction or repulsion of such charged objects constitutes
the current in the vacuum.
4.1.1 THERMIONIC EMISSION
If a metal plate or thin wire is heated to red-hot in a
vacuum, electrons near the surface are given enough energy of
motion to fly off into the surrounding space. The higher the
temperature, the greater the number of electrons emitted. This
emission of electrons called thermionic emission. The name for
the emitting metal is cathode. Usually the cathode is made out of
a thin metal tube and the filament is installed inside this tube
without electrical contact. For small valves there is no separate
cathode, filament itself acting as the cathode.
4.1.2 CLASSIFICATION OF VALVES
There are several types of valves according to the number
of electrodes. ( normally filament and cathode count as one
electrode)
1. diode (two electrodes)
2. triode (three electrodes)
3. tetrode (four electrodes)
4. pentode (five electrodes)
5. Double diode (two separate diodes in the same tube)
6. Diode triode
7. Triode pentode
4.1.2.1 DIODE VALVE
There are two electrodes in the diode valve. One is the
filament or cathode and the other one is the anode or plate,
placed surrounding the cathode. As indicated in the diagram
(Fig 4.1) the cathode is heating by the battery "A". "B" battery
is supply the positive voltage to the anode.
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Fig. 4.1
The anode current increases with increasing the anode voltage.
Anode current is zero with no anode voltage and the curve rises
until a saturation point is reached.
The anode voltage multiplied by the anode current is the
power input to the valve.
4.1.2.1.1 RECTIFICATION
Since current can flow through a valve in only one
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direction, a diode can be used to change a.c. into d.c.
Fig 4.2
4.1.2.2 TRIODE VALVE
The third electrode , called the control grid or, simply
grid is inserted between the cathode and plate as a coil or wire
mesh. It can be used to control the effect of the valve if the
grid is given a positive voltage with respect to the cathode the
flow of electrons will be accelerate and anode current will be
increase. If it is negative the electrons flow will be retard
and anode current will be decrease.
If any signal introduced to the grid, anode current will
be vary according to the signal or in other word the input signal
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will be amplify.
Fig 4.3
4.1.2.3 TETRODE VALVE
The fourth electrode, called the screen grid is inserted
between the control grid and plate as a coil or wire mesh. The
grid-plate capacitance can be reduced to a negligible value by
inserting the screen grid. The grid-plate capacitance is very
important for RF applications, because its reactance relatively
low at RF, offers a path over which energy can be fed back from
the plate to the grid and generates self-oscillations.
4.1.2.4 CATHODE RAY TUBE
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Fig 4.4
The heart of the Oscilloscope, Television and computer monitor
is the Cathode Ray Tube (CRT). As with other vacuum tubes the
filament heats the cathode (electron gun). The control grid
influences the amount of current flow , as in standard vacuum
tubes. Two cylindrical shaped anodes are employed, each having
a positive voltage. These anodes accelerate t he electron beam
and focus into a narrow beam.
The intensity of the beam is varied according to the
potential applied to the control grid. A high voltage is applied
to the second anode so the electron stream will attain high
velocity for increased intensity and visibility when it strikes
the tube face .
As shown in the diagram two sets of plates are present
in the tube beyond the second anode. These plates are for
deflecting the electron beam both horizontally and vertically.
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4.1.3 VACUUM TUBE AMPLIFIER
Fig 4.5
The diagram (Fig 4.5) shows a simple vacuum tube amplifier.
Input signal connected to the control grid. The voltage supplied
to the grid is called grid-bias (negative voltage). According to
the input signal the voltage at the control grid is varying
slightly. The same time anode current also varying considerable
amount according to the input signal. Therefor the input signal
amplified by the vacuum tube.
4.2 SOLID-STATE BASICS
The conductivity of a material is proportional to the number
of free electrons in the material. Pure germanium and pure silicon
crystals have relatively free electrons. If however, carefully
controlled amount of impurities are added , the number of free
electrons, and consequently the conductivity , is increased. When
certain other impurities are introduced an electron deficiency, or
hole, is produced.
Semiconductor material having more free electrons called
N-type material ; material having more electron deficiency is
called P-type material.
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Fig 4.6
4.2.1 SEMICONDUCTOR DIODE
The vacuum tube diode has been replaced by the semiconductor
diode in modern equipment designs. Advantages of solid state diode
are as follows:-
1. More efficient because they do not consume filament power
2. They are very much smaller
3. They operate into the micro wave region, while most vacuum
tube diodes are inactive above 50 MHz.
4.2.1.1 TYPES OF DIODES
There are three main types of diode
1. Selenium diode
2. Germanium diode
3. Silicon diode
SELENIUM DIODE
Selenium diodes used as rectifiers in early stages
(before 1965). This is very low efficiency.
GERMANIUM DIODE
The Germanium diode is normally use for RF applications.
Characteristic curve shown in the diagram. (Fig 4.7) The forward
resistance is the order of 200 Ohms and reverse is around 100 k
to one Megohm. The junction barrier voltage is about 0.3 volts.
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Fig 4.7
SILICON DIODE
Most of power rectifier diodes are Silicon Junction Diodes.
Forward current will be several milli amperes to about 100 Amperes.
PIV (Peak Input Voltage) is 1000 V or greater. The junction barrier
voltage is about 0.7 volts. The device temperature is one of the
important parameters. Heat sinks are used with diodes that must
handle large amount of power, thereby holding the diode junction
temperature at a safe level.
4.2.1.2 VARIOUS APPLICATIONS OF DIODE
1. Diode as a rectifier
2. Diode as a gate
3. Zener Diode
4. Light Emitting Diode (LED)
5. Diode Detector (demodulator)
6. Switching diode
7. Vericap diode (variable capacitance)
8. Diode Frequency Multiplier
9. Gunn Diode
10. Solar-Electric Diode
11. Tunnel Diode
12. Current Regulator Diode
We will discuss only a few from the above list for the novice class.
4.2.1.3 Diode as a rectifier
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Half-wave rectifier
Fig 4.8
The diagram (Fig 4.8) shows a simple half wave rectifier
circuit. A diode will be conduct current in one direction but
not the other. During one half of the A.C. cycle the diode will
conduct and current will flow through the diode to the load.
During the other half cycle the diode is reverse biased and no
current will flow.
Full-wave rectifier
Center tap type
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Fig 4.9
A commonly used rectifier circuit is shown in the diagram.
(Fig 4.9) A transformer with a center tapped secondary is required
for this circuit.
Bridge rectifier
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Fig 4.10
Another commonly used rectifier circuit is illustrated in
the diagram (Fig 4.10) IN this arrangement two diodes operate in
series on each half cycle.
4.2.1.4 Diode as a gate
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Fig 4.11
Diode can be placed in series with D.C. leads to function as a
gate. It is active as a protective device. Should the operator
mistakenly connect the supply leads in reverse the current will
not flow through D1 (Fig 4.11-A).
A power type diode D2 can be used in shunt with the supply
line (Fig 4.11-B) to the solid state device for protector purposes.
If the supply polarity reversed accidentally, the fuse will be
blown off due to the high current flow through D2.
4.2.1.5 ZENER DIODE
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Fig 4.12
Some electronic devices needs a stable voltage, otherwise
it will not function properly. For example VFO stage of transceiver
Zener diode can be used as a voltage regulator.
4.2.1.6 Light-Emitting Diode (LED)
The primary component in optoelectronics is the LED. This
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diode contains a P-N junction of crystal material that produces light around the junction when forward bias is applied. LED
Fig 4.13
junctions are made from gallium arsenide (GaAs) , gallium phosphide (GaP)or a combination of both materials. The commonly available LED colors are red, green and yellow. Recently developed White and blue LEDs, and also multi coloured.
There are valuable advantages to the use of LEDs. Notable among them are the low current drain, long life and small size. They are useful as visual indicators. One of their most common applications is in digital display units , where arrays of tiny LEDs are arranged to provide illuminated segments in numeric display assemblies. The forward bias current for a typical LED ranges between 10 and 20 mA.
4.2.2 TRANSISTOR
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The transistor was invented by Shockley, Bardeen and Brattain at Bell laboratories in 1947. It has become a standard amplifying device
in electronic equipment.
Fig 4.14
There are three terminals in the transistor called Colector,
Emitter and Base.
The diagram shows a sandwich made from two layers of P-type
material with a thin layer of N-type between. There are in effect
two PN junction diodes back-to-back. If a positive bias is applied
to the P-type material at the left, current will flow through the
left hand junction to the ight. That means holes moving from left
to right and electrons moving from right to left. Some of the holes
moving into the N-type material will combine with the electrons
there and be neutralized, but some of them also will travel to the
region of the right hand junction.
If the PN combination at the right is biased negatively ,as
shown there would normally be no current flow in this circuit.
However, there are now additional holes available at the junction
to travel to point C (collector) and electrons can travel toward
point E (emitter) so current can flow even though this section of
sandwich is bias to prevent condition. Most of the current is
between E and C and does not flow out through the common connection
to the N-type material in the sandwich.
There are various types of transistors. Some of them are
1. Point-contact transistor
2. Junction transistor
3. Bipolar transistor
4. Field effect transistor (FET)
5. Metal oxide FET (MOSFET)
Normally transistor has three terminals (emitter, collector, base) FET also has three terminals named as gate, drain and source. MOSFET has more than one gate. These gates are very sensitive to static charges which can quickly puncture the insulation and cause a short circuit between the junctions . Similar care must be exercised when soldering a MOSFET. Power transistors need to be mounted on a heat sink. Small signal transistors (low power) do not require heat sinks.
TRANSISTOR CHARACTORISTICS
4.2.2.1 POWER AMPLIFICATION
According to the diagram (Fig 4.14) collector is reverse biased. So collector-base resistance is high. On the other hand the emitter and collector currents are substantially equal. So the power in the collector circuit is larger than the power in the emitter circuit. The powers are proportional to the respective resistances because the currents (Ie and Ic) are almost same and
Power, P = I²R
In practical transistors emitter resistance is on the order of a few Ohms while the collector resistance is hundred or thousand times higher, so power gain of 20 or 40 dB or even more are possible.
4.2.2.2 CURRENT AMPLIFICATION FACTOR
(Beeta-Factor ß or HFE)
An important characteristic of a transistor is its current
amplification factor or 'BEETA'. It is defined as the ratio of the collector current to the base current.
ß = Ic/Ib
Thus if base current of 5 mA causes the collector current
to rise to 200mA, the á = 40 Typical BEETAs for transistors range from as low as 10 to as high as several hundreds.
4.2.2.3 MAXIMUM RATINGS
We need to be ever mindful of maximum safe ratings for semiconductors. These are listed on the manufacturer's data sheets and in various transistor manuals. Let's use the 2N3904 npn silicon transistor as an example. The maximum Vceo (collector to emitter voltage with the base open) is +40. The maximum Ic (steady collector current) is 200 mA . Maximum power is 1.5 watts. Now, let's learn what these ratings mean to us.
Transistors are likely to self-destruct if the forgoing ratings are exceeded, even for a moment. We should never operate any semiconductor at its maximum ratings. Better to operate your transistors at about half of the maximum ratings. Keep in mind that the transistor Vce rises to twice the supply voltage when ac or RF energy is being amplified.
Transistor should not be allowed to operate when it is hot. Heat is the most dangerous enemy of semiconductors. Reduce the operating parameters or use a heat sink when a transistor is more than comfortably warm to the touch.
4.2.2.4 TRANSISTOR AMPLIFIER
Amplifier circuits used with transistors fall into one three types, known as the common emitter (grounded emitter) , common base(grounded base) and common collector (grounded collector) circuits.
COMMON EMITTER CIRCUITS
The grounded emitter circuit shown in the diagram (Fig 4.15-a) corresponds to the ordinary grounded cathode vacuum tube triode amplifier. Input signal apply to the base and amplified output is going out from the collector. The base current is small and the input impedance is therefor fairly high, several kilo Ohms in the average case. The phase of the output (collector) current is opposite to that of the input (base) current so such feedback as occurs through the small emitter resistance is negative feedback and the amplifier is stable.
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Fig. 4.15
COMMON BASE CIRCUIT
As shown in the diagram (Fig. 4.15-b)input signal is apply to the emitter for the common base amplifier and it is with low impedance. Output signal is going out from the collector, and the output impedance is few kilo Ohms. Input and output both are in phase.
COMMON COLLECTOR CIRCUIT
As shown in the diagram (Fig. 4.15-c) input signal is apply to the base and output is taking from the emitter. This amplifier has high input impedance and low output impedance.
4.2.2.5 CLASSES OF AMPLIFIERS
4.2.2.5.1 CLASS-A AMPLIFIER
A class-A amplifier is one operated so that the wave shape of the output is the same as the input. The operating region is in the linear portion of the curve.(Fig 4.16)
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4.2.2.5.2 CLASS-B AMPLIFIER
The diagram (Fig 4.17) shows two npn transistors connected in push-pull circuit as a class-B amplifier. If the bias set at the point where the collector current just cut off (when no signal applied) then a signal can cause collector current to flow in either transistor only when the signal voltage applied to the particular transistor is positive with respective to the emitter. Since in the balanced base circuit the signal voltage in the base of the two transistors always have opposite polarization collector current flows only in one transistor at a time.
Upper half of the input signal is amplifying with one transistor and the other half is amplifying by the other transistor. The graph shows the operation of such an amplifier. At the operating point Ib and Ic both equals to zero. Only one half of the signal is amplifying and the other half is getting cut off completely.
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EXERCISES