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 !
PHILIPS TDA2549 I.F. amplifier and demodulator for multistandard TV receivers:
GENERAL DESCRIPTION
The TDA2549 is a complete i.f. circuit with a.f.c., a.g.c., demodulation and video preamplification facilities for
multistandard television receivers. It is capable of handling positively and negatively modulated video signals in both
colour and black/white receivers.
Features
• Gain-controlled wide-band amplifier providing complete i.f. gain
• Synchronous demodulator for positive and negative modulation
• Video preamplifier with noise protection for negative modulation
• Auxiliary video input and output (75 Ω)
• Video switch to select between auxiliary video input signal and demodulated video signal
• A.F.C. circuit with on/off switch and inverter switch
• A.G.C. circuit for positive modulation (mean level) and negative modulation (noise gate)
• A.G.C. output for controlling MOSFET tuners.
THOMSON TDA3190 COMPLETE TV SOUND CHANNEL:
The TDA3190 is a monolithicintegratedcircuit in a
16-lead dual in-line plastic package.It performsall
the functionsneededfor the TV soundchannel :
.IF LIMITER AMPLIFIER
.ACTIVE LOW-PASSFILTER
.FM DETECTOR
.DC VOLUMECONTROL
.AF PREAMPLIFIER
.AF OUTPUT STAGE
DESCRIPTION
The TDA3190 can give an output power of 4.2 W
(d = 10 %) into a 16 Ω load at VS = 24 V, or 1.5 W
(d = 10 %) into an 8 Ω load at VS = 12 V. This
performance,togetherwiththe FM-IF sectionchar-
acteristicsof high sensitivity, highAM rejection and
low distortion, enables the device to be used in
almost every type of televisionreceivers.
The device has no irradiation problems, hence no
externalscreening is needed.
The TDA3190 is a pin to pin replacement of
TDA1190Z.
The electrical characteristics of the TDA3190 re-
mainalmost constantover the frequencyrange4.5
to 6 MHz, therefore it can be used in all television
standards (FM mod.). The TDA3190 has a high
inputimpedance,soitcanwork withaceramicfilter
or with a tuned circuit that provide the necessary
input selectivity.
The value of the resistors connected to pin 9,
determinethe AC gain of the audio frequencyam-
plifier. This enablesthe desiredgainto be selected
in relation to the frequency deviation at which the
output stage of the AF amplifier, must enter into
clipping.
CapacitorC8, connectedbetween pins10 and11,
determinesthe uppercutofffrequencyof the audio
bandwidth.Toincreasethebandwidththe valuesof
C8 and C7 must be reduced, keeping the ratio
C7/C8 as shown in the table of fig. 16.
The capacitor connected between pin 16 and
ground, togetherwith the internal resistor of 10 KΩ
forms the de-emphasis network. The Boucherot
cell eliminates the high frequency oscillations
causedbytheinductiveloadandthewiresconnect-
ing theloudspeaker.
ULTRAVOX (SEMAR) P12 CHASSIS 2T/1 Preset tuner:
A preset tuner adapted for selecting a desired one out of a plurality of
preset channels, comprising: a memory for storing digital data
concerning a plurality of channels to be preset, push-buttons/rotatable knob for
addressing the memory for reading the digital data of a desired channel,
a digital/analog converter for converting the read digital data into an
analog signal, a manually operable variable voltage generator to obtain search mode AFC assisted, a
write-in/channel select mode selector, a switch circuit responsive to
the mode selector switchable between the digital/analog converter and
the variable voltage generator, a voltage controlled oscillator
responsive to the output of the switch circuit, a tuner employing the
voltage controlled oscillator as a local oscillator, a comparator for
comparing the outputs of the digital/analog converter and the variable
voltage generator, a counter to be reset responsive to the push-button search mode AFC
and to be enabled responsive to the write-in mode output of the mode
selector to make a counting operation as a function of the output of the
comparator, the output of the counter being loaded as the digital data
concerning a channel in the memory, as addressed, a reference
oscillator, a frequency comparator for comparing the output of the
reference oscillator and the intermediate frequency output of the tuner,
a filter for filtering the frequency comparator output to provide a
correction control signal to the digital/analog converter, the
digital/analog converter being adapted to be corrected for deviation of
the intermediate frequency of the tuner as a function of the correction
control signal.
A typical conventional preset tuner such as employed in tv
receivers and the like comprises a mechanical preset scheme. For
example, such a preset tuner employing variable inductance devices as a
tuning element is adapted to preset a plurality of channels by varying
the inductance value of the variable inductance devices in association
with the manual operation of a tuning knob. Another example of such a
tuner using a variable capacitance device such as a variable capacitance
diode as a tuning element employs variable resistors adjustable in
association with the manual operation of a tuning knob for the purpose
of a presetting operation.
Such preset tuners as described above as employing a variable inductance
device, a variable capacitance device and the like require provision of
the same number of variable inductance devices, variable resistors and
the like as that of presetting channel selection switches, which makes
the tuner large in size, with the result that such tuner is
disadvantageous particularly in case where such tuner is employed in an
indash type portable tv receiver, where the tuner is provided in a limited
space.
Accordingly, a principal object of the present invention is to
provide an improved preset tuner, wherein the data concerning the local
oscillation frequencies corresponding to the respective preset channels
is preloaded in a memory in a digital representation format.
Another
object of the present invention is to provide an improved preset tuner,
which is adapted for implementation by large scale integration
integrated circuits.
A further object of the present invention is
to provide an improved preset tuner, which is adapted for
implementation in an electronic structure rather than mechanical
structure.
How AFC Circuit Works in B/W Analog Television Receiver:
Push-Button
tuning on u.h.f. while being very convenient often leaves a margin of
mistuning, especially after some wear and tear has occurred on the
mechanism. Even dial tuning can lead to errors due to the difficulty
many people experience in judging the correct point. Oscillator drift
due to temperature changes can also cause mistuning. Automatic frequency
control (a.f.c.) will correct all these faults. The vision carrier when
the set is correctly tuned on u.h.f. is at 39.5MHz as it passes down
the i.f. strip. Thus if at the end of the i.f. strip a discriminator
tuned circuit is incorporated centred on 39.5MHz the discriminator
output will be zero at 39.5MHz and will move positively' one side of
39.5MHz and negatively the other as the tuning drifts. This response is
shown in Fig. 1.
If the tuning is not correct then the
discriminator output is not zero and if this output is applied to change
the reverse bias on a tuning diode mounted in the oscillator section of
the u.h.f. tuner it will correct most of the error. Tuning, varicap or
varactor diodes-to give them a few of their names-are junction diodes
normally operated with reverse bias but not sufficient to bias them into
the breakdown region in which zener diodes operate. The greater the
reverse bias the lower their capacitance: a typical curve, for the
PHILIPS BB105 or STC BA141 tuning diode, is shown in Fig. 2. All diodes e
xhibit
this basic type of characteristic but special diodes have to be used
for u.h.f. because they must not introduce any excessive loss into the
tuned circuits they control. In other words, just as a coil has to have a
good Q so does a varicap diode. Normally, we don't worry about the Q of
a capacitor as it is usually very good. However, a tuning diode is not a
true capacitor. It has, for example, leakage current so the Q of the
diode is a factor which has to be considered. The diode manufacturer
however will have considered these points and if you buy a diode
specified for u.h.f. use you will have no trouble. These points have
been mentioned to clear up any misunderstandings and to show why any old
diode won't do.
Basic AFC SystemTo return to our TV
set, if the oscillator frequency is too high then the vision carrier
frequency will also be too high and in the simple arrangement shown in
Fig. 3 the discriminator will give a negative signal to decrease the
bias on the tuning diode thus increasing its
capacitance
and in turn reducing the oscillator frequency and correcting the error.
Note that in this diagram the reverse bias on the diode is applied to
its cathode. It is therefore positive with respect to ground so that a
negative signal from the discriminator will reduce the positive voltage
on the diode thus reducing its bias and increasing its capacitance. In
this arrangement the diode is biased somewhere near the mid point of its
characteristic by the positive d.c. bias fed into one side of the
discriminator. The discriminator thus adds to or subtracts from this
d.c. bias.
AFC Loop Gain:The amount by which the error
is reduced depends on the gain of the circuit. An estimate of the gain
required must first be made by guessing how much error is likely to be
given by your push -buttons or hand tuning: 1MHz would be an outside
figure as a tuning error of that magnitude would produce a very bad
picture of low definition in one direction and badly broken up in the
other. This error should be reduced to
about
100kHz to be really unnoticeable, indicating a required gain of ten. In
fitting a.f.c. to an existing set some measure- ments should be done as
an experiment before finally deciding on the circuit gain. The first
thing to do is to add the suggested discriminator to the i.f. strip. As
the circuit (Fig. 4) shows a Foster -Seeley type discriminator is used
and with the coils specified and the driver circuit shown it should give
±4V for 0.5MHz input variation.
EXAMPLE of Circuit Description:The
driver stage Tr1 takes a small sample signal from the i.f. strip but
this should be large enough to drive Tr1 into saturation. That is to say
Tr1 is a limiter stage so that the signal amplitude applied to the
discriminator coil L2 stays constant over the normal range of signal
levels. Trl is biased at approximately 7mA which, according to the
original report ("Simple a.f.c. system for 625 -line TV receivers" by P.
Bissmire, PHILIPS Technical Communications, March, 1970), gives the
best limiting performance. C1, R
14
and R3 damp the stage to prevent oscillation. C2 decouples the power
feed and should be close to the circuit. The coil former and can are the
normal ones used for TV sets and so should be easily obtainable: the
former diameter is 5mm. and length 40mm. and winding details are given
in Fig. 5.
The invention being
thus described, it will be obvious that the same may be varied in many
ways. Such variations are not to be regarded as a departure from the
spirit and scope of the invention, and all such modifications as would
be obvious to one skilled in the art are intended to be included within
the scope of the developed apparates both tubes or transistors.
