The
tuning circuits has a large knob potentiometers tuning system
which use voltage controlled capacitances such as varactor diodes as
the frequency determining elements.
Therefore a stable AFC circuit is developed:
A
superheterodyne receiver having an automatic intermediate
frequency control circuit with means to prevent the faulty
regulation thereof. The receiver has means for receiving a radio
frequency signal and mixing the same with the output of a
superheterodyne oscillator. This produces an intermediate frequency
signal which is coupled to a frequency or phase discriminator to
produce an error signal for controlling the frequency of the
superheterodyne oscillator. A regulation circuit is provided having
an electronic switch to interrupt the feedback circuit when only
unwanted frequencies tend to produce faulty regulation of the
superheterodyne oscillator.
Power supply
is realized with mains transformer and Linear transistorized power
supply stabilizer, A DC power supply apparatus includes a rectifier
circuit which rectifies an input commercial AC voltage. The
rectifier output voltage is smoothed in a smoothing capacitor.
Voltage stabilization is provided in the stabilizing circuits by the
use of Zener diode circuits to provide biasing to control the
collector-emitter paths of respective transistors.A linear regulator
circuit according to an embodiment of the present invention has an
input node receiving an unregulated voltage and an output node
providing a regulated voltage. The linear regulator circuit includes a
voltage regulator, a bias circuit, and a current control device.
In
one embodiment, the current control device is implemented as an
NPN bipolar junction transistor (BJT) having a collector electrode
forming the input node of the linear regulator circuit, an emitter
electrode coupled to the input of the voltage regulator, and a base
electrode coupled to the second terminal of the bias circuit. A
first capacitor may be coupled between the input and reference
terminals of the voltage regulator and a second capacitor may be
coupled between the output and reference terminals of the voltage
regulator. The voltage regulator may be implemented as known to
those skilled in the art, such as an LDO or non-LDO 3-terminal
regulator or the like.
The bias circuit may include a bias
device and a current source. The bias device has a first terminal
coupled to the output terminal of the voltage regulator and a
second terminal coupled to the control electrode of the current
control device. The current source has an input coupled to the first
current electrode of the current control device and an output
coupled to the second terminal of the bias device. A capacitor may
be coupled between the first and second terminals of the bias
device.
In the bias device and current source embodiment, the
bias device may be implemented as a Zener diode, one or more
diodes coupled in series, at least one light emitting diode, or any
other bias device which develops sufficient voltage while
receiving current from the current source. The current source may
be implemented with a PNP BJT having its collector electrode
coupled to the second terminal of the bias device, at least one
first resistor having a first end coupled to the emitter electrode
of the PNP BJT and a second end, a Zener diode and a second resistor.
The Zener diode has an anode coupled to the base electrode of the
PNP BJT and a cathode coupled to the second end of the first
resistor. The second resistor has a first end coupled to the anode
of the Zener diode and a second end coupled to the reference
terminal of the voltage regulator. A second Zener diode may be
included having an anode coupled to the cathode of the first Zener
diode and a cathode coupled to the first current electrode of the
current control device.
A circuit is disclosed for improving
operation of a linear regulator, having an input terminal, an
output terminal, and a reference terminal. The circuit includes an
input node, a transistor, a bias circuit, and first and second
capacitors. The transistor has a first current electrode coupled to
the input node, a second current electrode for coupling to the
input terminal of the linear regulator, and a control electrode. The
bias circuit has a first terminal for coupling to the output
terminal of the linear regulator and a second terminal coupled to
the control electrode of the transistor. The first capacitor is for
coupling between the input and reference terminals of the linear
regulator, and the second capacitor is for coupling between the
output and reference terminals of the linear regulator. The bias
circuit develops a voltage sufficient to drive the control terminal
of the transistor and to operate the linear regulator. The bias
circuit may be a battery, a bias device and a current source, a
floating power supply, a charge pump, or any combination thereof.
The transistor may be implemented as a BJT or FET or any other
suitable current controlled device.
Power
Supply: The examples chosen are taken from manufacturers' circuit
diagrams and are usually simplified to emphasise the fundamental nature
of the circuit. For each example the particular transistor properties
that are exploited to achieve the desired performance are made clear. As
a rough and ready classification the circuits are arranged in order of
frequency: this part is devoted to circuits used at zero frequency,
field frequency and audio frequencies. Series Regulator Circuit Portable
television receivers are designed to operate from batteries (usually
12V car batteries) and from the a.c. mains. The receiver usually has an
11V supply line, and circuitry is required to ensure that the supply
line is at this voltage whether the power source is a battery or the
mains. The supply line also needs to have good regulation, i.e. a low
output resistance, to ensure that the voltage remains constant in spite
of variations in the mean current taken by some of the stages in the
receiver. Fig. 1 shows a typical circuit of the power -supply
arrangements. The mains transformer and bridge rectifier are designed to
deliver about 16V. The battery can be assumed to give just over 12V.
Both feed the regulator circuit Trl, Tr2, Tr3, which gives an 11V output
and can be regarded as a three -stage direct -coupled amplifier. The
first stage Tr 1 is required to give an output current proportional to
the difference between two voltages, one being a constant voltage
derived from the voltage reference diode D I (which is biased via R3
from the stabilised supply). The second voltage is obtained from a
preset potential divider connected across the output of the unit, and is
therefore a sample of the output voltage. In effect therefore Tr 1
compares the output voltage of the unit with a fixed voltage and gives
an output current proportional to the difference between them. Clearly a
field-effect transistor could do this, but the low input resistance of a
bipolar transistor is no disadvantage and it can give a current output
many times that of a field-effect transistor and is generally preferred
therefore. The output current of the first stage is amplified by the two
subsequent stages and then becomes the output current of the unit.
Clearly therefore Tr2 and Tr3 should be current amplifiers and they
normally take the form of emitter followers or common emitter stages
(which have the same current gain). By adjusting the preset control we
can alter the fraction of the output voltage' applied to the first stage
and can thus set the output voltage of the unit at any desired value
within a certain range. By making assumptions about the current gain of
the transistors we can calculate the degree of regulation obtainable.
For example, suppose the gain of Tr2 and Tr3 in cascade is 1,000, and
that the current output demanded from the unit changes by 0.1A (for
example due to the disconnection of part of the load). The corresponding
change in Tr l's collector current is 0.1mA and, if the standing
collector current of Tr 1 is 1mA, then its mutual conductance is
approximately 4OmA/V and the base voltage must change by 2.5mV to bring
about the required change in collector current. If the preset potential
divider feeds one half of the output voltage to Tr l's base, then the
change in output voltage must be 5mV. Thus an 0.1A change in output
current brings about only 5mV change in output voltage: this represents
an output resistance of only 0.0552.
GENERAL BASIC TRANSISTOR LINE OUTPUT STAGE OPERATION:
The
basic essentials of a transistor line output stage are shown in Fig.
1(a). They comprise: a line output transformer which provides the d.c.
feed to the line output transistor and serves mainly to generate the
high -voltage pulse from which the e.h.t. is derived, and also in
practice other supplies for various sections of the receiver; the line
output transistor and its parallel efficiency diode which form a
bidirectional switch; a tuning capacitor which resonates with the line
output transformer primary winding and the scan coils to determine the
flyback time; and the scan coils, with a series capacitor which provides
a d.c. block and also serves to provide slight integration of the
deflection current to compensate for the scan distortion that would
otherwise be present due to the use of flat screen, wide deflection
angle c.r.t.s. This basic circuit is widely used in small -screen
portable receivers with little elaboration - some use a pnp output
transistor however, with its collector connected to chassis.
Circuit Variations:
Variations
to the basic circuit commonly found include: transposition of the scan
coils and the correction capacitor; connection of the line output
transformer primary winding and its e.h.t. overwinding
in series; connection of the deflection components to a tap on the
transformer to obtain correct matching of the components and conditions
in the stage; use of a boost diode which operates in identical manner to
the arrangement used in valve line output stages, thereby increasing
the effective supply to the stage; omission of the efficiency diode
where the stage is operated from an h.t. line, the collector -base
junction of the line output transistor then providing the efficiency
diode action without, in doing so, producing scan distortion; addition
of inductors to provide linearity and width adjustment; use of a pair of
series -connected line output transistors in some large -screen colour
chassis; and in colour sets the addition of line convergence circuitry
which is normally connected in series between the line scan coils and
chassis. These variations on the basic circuit do not alter the basic
mode of operation however.
Resonance
The
most important fact to appreciate about the circuit is that when the
transistor and diode are cut off during the flyback period - when the
beam is being rapidly returned from the right-hand side of the screen to
the left-hand side the tuning capacitor together with the scan coils
and the primary winding of the line output transformer form a parallel
resonant circuit: the equivalent circuit is shown in Fig. 1(b). The line
output transformer primary winding and the tuning capacitor as drawn in
Fig. 1(a) may look like a series tuned circuit, but from the signal
point of view the end of the transformer primary winding connected to
the power supply is earthy, giving the equivalent arrangement shown in
Fig. 1(b).
The Flyback Period:
Since the operation of the
circuit depends mainly upon what happens during the line flyback period,
the simplest point at which to break into the scanning cycle is at the
end of the forward scan, i.e. with the
beam deflected to the right-hand side of the screen, see Fig. 2. At
this point the line output transistor is suddenly switched off by the
squarewave drive applied to its base. Prior to this action a linearly
increasing current has been flowing in the line output transformer
primary winding and the scan coils, and as a result magnetic fields have
been built up around these components. When the transistor is switched
off these fields collapse, maintaining a flow of current which rapidly
decays to zero and returns the beam to the centre of the screen. This
flow of current charges the tuning capacitor, and the voltage at A rises
to a high positive value - of the order of 1- 2k V in large -screen
sets, 200V in the case of mains/battery portable sets. The e
nergy
in the circuit is now stored in the tuning capacitor which next
discharges, reversing the flow of current in the circuit with the result
that the beam is rapidly deflected to the left-hand side of the screen -
see Fig. 3. When the tuning capacitor has discharged, the voltage at A
has fallen to zero and the circuit energy is once more stored in the
form of magnetic fields around the inductive components. One half -cycle
of oscillation has occurred, and the flyback is complete.
Energy Recovery:
First
Part of Forward Scan The circuit then tries to continue the cycle of
oscillation, i.e. the magnetic fields again collapse, maintaining a
current flow which this time would charge the tuning capacitor
negatively (upper plate). When the voltage at A reaches about -0.6V
however the efficiency diode becomes forward biased and switches on.
This damps the circuit, preventing further oscillation, but the magnetic
fields continue to collapse and in doing so produce a linearly decaying
current flow which provides the first part of the forward s
can,
the beam returning towards the centre of the screen - see Fig. 4. The
diode shorts out the tuning capacitor but the scan correction capacitor
charges during this period, its right-hand plate becoming positive with
respect to its left-hand plate, i.e. point A. Completion of Forward Scan
When the current falls to zero, the diode will switch off. Shortly
before this state of affairs is reached however the transistor is
switched on. In practice this is usually about a third of the way
through the scan. The squarewave applied to its base drives it rapidly
to saturation, clamping the vol
tage
at point A at a small positive value - the collector emitter saturation
voltage of the transistor. Current now flows via the transistor and the
primary winding of the line output transformer, the scan correction
capacitor discharges, and the resultant flow of current in the line scan
coils drives the beam to the right-hand side of the screen see Fig. 5.
Efficiency:
The
transistor is then cut off again, to give the flyback, and the cycle of
events recurs. The efficiency of the circuit is high since there is
negligible resistance present. Energy is fed into the circuit in the
form of the magnetic fields that build up when the output transistor is
switched on. This action connects the line output transformer primary
winding across the supply, and as a result a linearly increasing current
flows through it. Since the width is
dependent on the supply voltage, this must be stabilised.
Harmonic Tuning:
There
is another oscillatory action in the circuit during the flyback period.
The considerable leakage inductance between the primary and the e.h.t.
windings of the line output transformer, and the appreciable self
-capacitance present, form a tuned circuit which is shocked into
oscillation by the flyback pulse. Unless this oscillation is controlled,
it will continue into and modulate the scan. The technique used to
overcome this effect is to tune the leakage inductance and the
associated capacitance to an odd harmonic of the line flyback
oscillation frequency. By doing this the oscillatory actions present at
the beginning of the scan cancel. Either third or fifth harmonic tuning
is used. Third harmonic tuning also has the effect of increasing the
amplitude of the e.h.t. pulse, and is generally used where a half -wave
e.h.t. rectifier is employed. Fifth harmonic tuning results in a
flat-topped e.h.t. pulse, giving improved e.h.t. regulation, and is
generally used where an e.h.t. tripler is employed to produce the e.h.t.
The tuning is mainly built into the line output transformer, though an
external variable inductance is commonly found in colour chassis so that
the tuning can be adjusted. With a following post I will go into the
subject of modern TV line timebases in greater detail with other models
and technology shown here at Obsolete Technology Tellye !
- The EHT Output is realized with a selenium rectifier.
The EHT selenium rectifier which is a Specially designed selenium rectifiers were once widely used as EHT
rectifiers in television sets and photocopiers. A layer of selenium
was applied to a sheet of soft iron foil, and thousands of tiny discs
(typically 2mm diameter) were punched out of this and assembled as
"stacks" inside ceramic tubes. Rectifiers capable of supplying tens of
thousands of volts could be made this way. Their internal resistance was
extremely high, but most EHT applications only required a few hundred
microamps at most, so this was not normally an issue. With the
development of inexpensive high voltage silicon rectifiers, this
technology has fallen into disuse.A selenium rectifier is a type
of metal rectifier, invented in 1933. They were used to replace vacuum
tube rectifiers in power supplies for electronic equipment, and in high
current battery charger applications.
The photoelectric
and rectifying properties of selenium were observed by C. E. Fitts
around 1886 but practical rectifier devices were not manufactured
routinely until the 1930s. Compared with the earlier copper oxide
rectifier, the selenium cell could withstand higher voltage but at a
lower current capacity per unit area.
