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Understanding Electronic Components |
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7. Integrated circuits
Integrated circuits play very important part in many electronic devices of today. Their usage is widespread and common found in professional and amateur designs alike. For that matter, there is rarely a single device without them (except maybe your grandmother's TV from the 60's that just won't die with years). Integrated circuits are specially crafted electronic circuits, which could contain tens, hundreds, thousands or even tens of thousands of transistors, diodes and resistors. They are all interconnected by manufacturer in such a way that they form different purpose circuits like audio-amplifiers, voltage stabilizers, different logic circuits, certain parts of TV-receivers, a whole audio receiver or transceiver, etc. Several different integrated circuits are shown on 7.1.
Depending on the way they are manufactured, integrated circuits
could be divided into two groups: hybrid and monolithic. Hybrid
circuits have been around longer. When some transistor is opened,
one could find inside of it's casing a crystal plate whose volume
is hundred times smaller than the casing's. This means two things.
First, actual size of the transistor is very small and, second,
if many transistors were placed on a single non-conducting plate
connected in a way to form a certain purpose circuit, and then encase
that altogether in a single package you would get an instant to
use and experiment on circuit with lower price and form factor than
it would be if you made it from regular components yourself (not
to mention the time needed to solder all components together, or
to troubleshoot a faulty component if something cooks up). Solder
is replaced with conducting glues. Resistors are drawn with resistance
pastes, and the plate is then placed inside of the plastic or metallic
casing with leads coming out. Audio-power amplifiers with mark STK,
were manufactured like that.
As with transistors, integrated circuits which are housed in round
casing (one of them is the TO-99, as on picture 7.2a), angle of
view is from the bottom, which means that schematic depicts pins
as they are facing the viewer (lying on it's back). Small half-round
slit, or the "nose" as often referred to, is used as an
orientation help.
7.1 Analog integrated circuits While discussing analog circuits, we will look at the LM386 circuit as our example. It has all needed components in single package for a complete audio-amplifier. Picture 7.3a shows an example of an amplifier realized using this integrated circuit, which can be used as a complete low frequency amplifier for a walkman, interphone, cassette player or some other audio device. It could also be used as a test circuit for different experiment and troubleshoot situations. These techniques will be discussed more in forthcoming "Practical ELECTRONICS".
Schematic of the LM386 is on picture 7.3b. Although
it is a simplified representation of this circuit it clearly points
that if it was devised in discrete technique (from different single
components) it would take us ten transistors, two diodes and eight
resistors (one of them is placed instead of a supply source, represented
on the schematic as a rounded arrow), amplifier devised in this
way would be a lot more expensive, larger and with far worse characteristics
than it would be the case if integrated circuit was used. 7.2 Digital integrated circuits CD4011 will be our "show-and-tell" circuit
while learning the main characteristics of digital integrated circuits.
It is a 14 pin DIL packaged circuit. Pin placement is displayed
on picture 7.4a. The view is, as said previously, as with other
DIL packages, from above. Notice the small half-round slit on one
side of the circuit. It is already mentioned identifier, pin 1 is
on it's left side, and pin 14 on it's right side. No matter how
you use this circuit, between pins 7 and 14 are used to connect
a supply (battery or transformer). Negative battery pole is connected
to pin 7. This is a ground, to which all other voltages are measured.
All unused pins should be connected to ground as well. For example,
if a logic circuit with inputs on pins 1 and 2 was unused, those
pins would be connected to pin 7, or ground. Positive voltage is
connected to pin 14.
Logic circuits have many applications, but their
main field is in computer hardware. Picture 7.4c displays what we
would see if we attached an oscilloscope to pins of a logic NAND
gate inside of some computer. Regulated operational voltage of this
circuit is 5V. Input A (upmost diagram) is zero till moment t1 (there
is no voltage over it). Between moments t1 and t2 it is logic zero
(voltage is 5V), after it, it is zero till t3, and so on. Similar
voltage impulses (logic ones) occur in input B as well. Voltage
on output F is displayed on the bottom picture. It has a logic one
in those time intervals when at least one input is equal to zero.
When both inputs are one, output is, as said in the truth table,
zero.
Lets dig into the functionalities of this circuit.
Both inputs of NAND1 are connected to each other, so when input
P occurs, output is zero. This logic zero is passed on to NAND2,
so no matter what is on the input 6, output 4 is logic one. This
means that, between the ground and pin, voltage is equal to 12V.
Current flows through capacitor C and resistor R, so capacitor begins
to fill. Every empty capacitor behaves like a short circuit. Because
of that, when 12V appears on the pin, it is also present on the
resistor R. It is a voltage between it's upper end and ground, and
that also means with the connected pins 8 and 9. Pin 10 shows logic
zero because of this which is connected to pin 6. From now on, logic
zero on pin 5 is no longer needed because only one input needs to
be zero for output to be logic one. So input P is no longer needed
as well. Gates NAND2 and NAND3 are self sufficiently maintaining
logic zero on pin 4. How long will this last? It depends on the
capacitance of the capacitor and on the resistance of the resistor.
As capacitor is filled, voltage on resistor rises so the current
and voltage drop. When this voltage drops to 1/2 of the supply voltage
(6V in our case), NAND3 detects zero on it's inputs, and so logic
one appears on pin 10. Since logic one is now on input 5 (no logic
one present on P), and on input 6, output 4 is zero, capacitor dumps
it's charge, and the circuit starts operating again. As we saw,
for a certain period of time, which is equal to T=0.7*RC output
of pin 10 was logic zero. During that time output E (pin 11) is
logic one. For example, if R = 2M? and C=47µF, for time T
= 2*10^6*47*10^-6 = 94 s from the moment impulse on input P subsided,
voltage on output E is 12V. 7.3 Practical examples Interesting and useful applications, which would demonstrate
properly all possiblities and broad usage of these electronic components
are too much of a scope than could be addressed by this book. So,
we will demonstrate another three circuits, and some others will
be covered in some future issues.
Another example is a audio amplifier using a LM386 circuit, with an additional simple preamp using the transistor BC107. Serially connected capacitor and resistor between pins 1 and 5 are facilitating low frequency amplification (around 100Hz) improving characteristics of the circuit which is important if it is to be used with small speakers. Capacitor between pin 7 and ground is added on occasions when amplifier doesn't work properly. Another troubleshooting measure would be replacing the resistor marked with an asterisk on the schematic. This amplifier could be used with any low frequency source (gramophone, microphone, some transformer, etc.), it could also be connected instead of a BC107 on schematic 4.8, and thus develop a radio-receiver with speaker reproduction.
Third example is a simple alarm device, whose schematic
is on picture 7.8. Base circuit for this device is again the CD4011.
It's gates NAND3 and NAND4 form a 600Hz audio oscillator. This signal
is amplified using BC286 transistor and reproduced using a speaker.
To achieve a 600Hz sound, cut the connection between pins 4 and
8, and connect pin 8 to pin 9. This makes sound a constant tone.
Gates NAND1 and NAND2 are forming a 4Hz oscillator, whose output
is connected to pin 8. This makes alarm produce repetitive 600Hz
sounds with very short breaks. If you want to use this alarm in
your home, on doors for example, you could make switch S using two
brass plates, connected to pins 1 and 7, which will be adjusted
to both a door and a door frame making them conduct current when
doors are closed, so, when door is opened, circuit is signaled to
start the alarm. You could combine this circuit with already mentioned
time interval counter, only instead of a light bulb, you can connect
it to your alarm. This would prevent shutting the alarm if the door
was immediately closed upon entering the room since alarm will sound
until time period T is over.
Some other sensor could be used instead of the S switch.
What is important is that it's resistance is very small in ordinary
conditions, and when an alarming situation arises (burglary, fire,
flood, global thermo nuclear war, anything that this alarm is supposed
to detect and announce) it has a very large resistance. For example,
if we connected a photo resistor to the circuit, alarm would be
silent while there is light, but it would sound when the light is
off.
Tuning to a low frequency station is done automatically
by pressing the RUN switch. That turns on part of the integrated
circuit which is designated for scanning over the given range. When
it finds a station it stays locked on it until some other pressing
of the RUN button. When it reaches 108MHz it waits for the RESET
signal which brings it back to the beginning of the predefined interval
(88MHz). |
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