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68a Group Student Page

We are students from Faculty of Computer Systems, Technical University of Sofia. Our 68 group is divided into two sub-groups; we constitute the first 68a one. Here are our names:
Katerina Slavova, Eleonora Todorova, Radoslav Danchev, Hristina Malakova, Elena Hristova, Lubomir Nedyalkov, Dimitar Petkov, Ahmet Karaman, Alper Mutlu, Tamer Aydin and Kiril Keranov.

=== Lab 1: Investigating passive resistive circuits by Microlab system ===
In this laboratory exercise we were talking about how we can build a converters and how we may investigate them with MicroLab system. First, the instructor introduced to us what converter means.
Converter is a device that changes the continuous fluctuations in voltage from an analog device (such as a microphone) into digital information (?!?! Circuit-fantasist (talk) 06:29, 19 May 2008 (UTC)) that can be stored or processed in a sampler, digital signal processor, or digital recording device. Converters are generally two types:Voltage-to-current converers and Current-to-voltage converters.

Hristina Malakova68 (talk)

=== Lab 2: The genuine Ohm's experiment ===
In this laboratory exercise we went centuries ago. We felt like in Ohm primitive laboratory. Georg Simon Ohm lived in 1789 - 1854. His inventions are amazing. Even nowadays we know him as one of the most excentric and most creative people. During one of his experiments he took a piece of conductor. He stretched it in two ends. He started measuring voltage and current in the ends. But in these years there was not modern instruments nor such comforts like these we can use - he made the history of the electricity with primitive things. So he took a compass to measure current and an electroscope to measure voltage.
Following his ideas we tried to repeat his experiment with laboratory conditions. 
We decided to make our scientific experiment more interesting and more attractive.Instead of conductor we used graphitе. Graphite has impedance consequently it has voltage. 
We found out that, if we measure two ends of conductor - one of sites has 0V and the another has max V. Hristina Malakova68 (talk)
Thursday, March 20, 2008, 16.45 h

=== Lab 3: Building a transistor switch ===
Katerina Slavova, Tsarina Penkova 65gr, Eleonora Todorova, Radoslav Danchev, Hristina Malakova, Elena Hristova, Lubomir Nedyalkov, Pavlin Stoyanoff 64, Dimitar Petkov, George Gizdov 65, Ahmet Karaman 68, Alper Mutlu 68, Tamer Aydin 68, Maria Georgieva 65 and Kiril Keranov.
Thursday, April 03, 2008, 16.45 h

==== The history of the transistor ====
"The Transistor was probably the most
important invention of the 20th Century,
and the story behind the invention is one
of clashing egos and top secret research."

The problem started in the 1907 when the AT&T (American Telephone and Telegraph) faced the competition erupting from the expiration of Alexander Graham Bell's telephone patents. And the solution was transcontinental telephone service. In 1906, the eccentric American inventor Lee De Forest developed a triode in a vacuum tube. It was a device that could amplify signals, including, it was hoped, signals on telephone lines as they were transferred across the country from one switch box to another. AT&T bought De Forest's patent and vastly improved the tube. It allowed the signal to be amplified regularly along the line, meaning that a telephone conversation could go on across any distance as long as there were amplifiers along the way. But the vacuum tubes that made that amplification possible were extremely unreliable, used too much power and produced too much heat.

After the end of World War II, a team of scientists was collected to develop a solid-state semiconductor switch to replace the problematic vacuum tube.  This team included a mix of physicists, chemists and engineers. On November 17, 1947, Walter Brattain, an experimental physicist who could build or fix just about anything, made an experiment into a thermos of water. The silicon contraption he'd built was supposed to help him study how electrons acted on the surface of a semiconductor. The wet device created the largest amplification Brattain had seen so far. By turning on a positive voltage he increased the effect even more; turning it to negative could get rid of it completely. It seemed that whatever those electrons had been doing on the surface to block amplification had somehow been canceled out by the water - the greatest obstacle to building an amplifier had been overcome.
Unfortunately this giant jump in amplification only worked for certain types of current - ones with very low frequencies. That wouldn't work for a phone line, which has to handle all the complex frequencies of a person's voice. So the next step was to get it to work at all kinds of frequencies.

In fact the device worked as if there was no oxide layer at all. And as Brattain poked the gold contact in again and again, he realized that's because there wasn't an oxide layer. He had washed it off by accident. Brattain was furious with himself, but decided to fiddle with the point contact anyway. To his surprise, he actually got some voltage amplification - and more importantly he could get it at all frequencies! The gold contact was putting holes into the germanium and these holes canceled out the effect of the electrons at the surface, the same way the water had. But this was much better than the version that used water, because now, the device was increasing the current at all frequencies.

Brattain had managed to get a large amplification at some frequencies and he'd gotten a small amplification for all frequencies - now he just had to combine the two. He knew that the key components were a slab of germanium and two gold point contacts just fractions of a millimeter apart. Walter Brattain put a ribbon of gold foil around a plastic triangle, and sliced it through at one of the points. By putting the point of the triangle gently down on the germanium, he saw a fantastic effect - signal came in through one gold contact and increased as as it raced out the other. The first point-contact transistor had been made.

He showed this little plastic triangle at a group meeting on December 23. It was official - this tiny bit of germanium, plastic and gold was the first working solid state amplifier.

(The text is written by Radoslav Danchev)

==== Building the circuit on the whiteboard ====

===== Transistor as an amplifier =====

The first part of this laboratory exercise was to build a transistor switch on the whiteboard. And we started with a conversation about the behavior of the transistor. The lecturer asked us what are the real functions of a transistor and what a transistor consists of. Those who knew it were just few. What we knew from the previous year was that the transistor amplifies electrical current. But in the exercise we took a deep look in its structure and understood that the transistor is nothing more than electrically driven resistor. Actually there is no real amplification. The real function of a transistor is to regulate not to amplify. (The text is written by Radoslav Danchev)
In laboratory classes we imagined transistors the way they are normally explained in electronics - like a variable resistors. They are electrically-managed resistors, known as active transistors. Normally, the voltage across the transistor would increase its value, but actually it fades. The transistor decreases the supply voltage. It seems that the transistor can regulates it - this is one of the main funcions of transistors.

===== Making transistor act as a switch =====
After discussion of what is transistor, we started talking about the meaning of the transistor switch. We concluded that this is an element with two states - OFF and ON. When the switch is off, no current flows through it, and therefore the transistor is off. The depletion region across the emitter-base junction is large. There is no potential differences between the base and emitter. The emitter-collector terminals act as open. The second state is ON - then the transistor turns on as hard as it can. A voltage is present across the base-emitter junction. The depletion region across the emitter-base junction shrinks as much as possible. 
Here is a presentation about transistor switch : http://www.wisc-online.com/objects/index_tj.asp?objID=SSE3703 
It is very interesting! Take a look!
Eventually, we understood what exactly transistor and transistor switch are, along with their general purpose. So we started building one. The building scheme is in the photo. Hristina Malakova68 (talk)

===== Assembling a transistor switch by simpler converters =====
Here are some questions about the circuit of the transistor switch. What does the base resistor RB actually do in this circuit? Can we apply 1000 V or even more to the transistor switch input? What does the collector resistor RC actually do in this circuit? A tip: present the transistor switch as a circuit composed by three converters (a voltage-to-current, a current-to-current and a current-to-voltage converter). See the pictures below. Circuit-fantasist (talk) 14:32, 20 April 2008 (UTC)

==== Mounting the circuit on a prototyping PCB ====

The picture above shows how we have built a transistor switch. It was very interesting task for us. Some of my colleagues took part in this. I watched them with extreme attention. First, the lecturer found a prototyping printed circuit board (PCB), which he gave to us. After that we started building up our "invention". We soldered the supply voltage to the PCB. Then we put n-p-n transistor on the board. It was very difficult to solder its pins... but we made it!

==== Investigating the circuit ====
Then we put a lamp to use it like a collector load. The lamp had current charge of 100mA. We attached a button to control switching on and off the lamp. We soldered the button between the plus of the supply and the base of the transistor. When the lamp is off then the button is off, too. The transistor in this case is cut off. We attached also a contact point where we could measure the output (collector) voltage of our transistor switch. This was the final touch we made on our "invention". Our transistor switch worked very well and we tested it a couple of times. It was quite amazing. It was worth doing it.

==== Drawing conclusions ====
Finally we concluded that if we want our transistor switch to work properly, we have to observe some circumstances:

The transistor must be saturated very well. Otherwise it would start warming up.
If Rb isn't small enough, the transistor would be blocked and it would start warming up. Its behavior would be like an amplifier.
The transistor must be in its two final states - saturated (fully ON state) or cut off (OFF state).Hristina Malakova68 (talk)

== Lab 4: Op-amp trigger circuits with positive feedback ==
Making the comparator with hysteresis memorize ("inventing" an elementary flip-flop)

=== A simple inverting comparator ===

A problem: false switching.
In this laboratory exercise we had the chance to talk about one of the most interesting electrical elements- amplifier and especially operating amplifier.
The operating amplifier can be with or without back connection. Those ampplifiers which do not have back connection are super sensitive. The lecture told us the simplest amplifier without back connection is comparator. This device function is to render an account of the current when it passes through the amplifier. Comparator is  a logical signal which means: “+” corresponds to “1” and “-“corresponds to “0”.
Hristina Malakova68 (talk)

=== Introducing a hysteresis into the circuit ===
A system with hysteresis can be summarised as a system that may be in any number of states, independent of the inputs to the system. Many physical systems naturally exhibit hysteresis. Human-designed systems will sometimes intentionally exhibit hysteresis. For example, consider a thermostat that controls a furnace. The furnace is either off or on, with nothing in between. The thermostat is a system; the input is the temperature, and the output is the furnace state. If we wish to maintain a temperature of 23 degrees, then we might set the thermostat to turn the furnace on when the temperature drops below 21 degrees, and turn it off when the temperature exceeds 25 degrees. This thermostat has hysteresis. Let us say that the temperature is 24 degrees. Given this information, we cannot predict whether the furnace will be on or off; it's not possible to predict the instantaneous output of the thermostat, knowing only its instantaneous input.(The text is written by Eleonora Todorova)

How we said earlier when the operating amplifier has no back connection it is super sensitive  and its voltage-ampere characteristic varies. To ignore this variety it is used the phenomenon called histeresis. Histeresis actually is a positive back connection. It works like a pen-push down or push up. There is no average state-pen is either push-up or push-down and this is the mechanism of the histerezis. It enhances performance of the amplifier on the same direction and that is how the varies disappear. To use the histerezis   in the comparator scheme we drew it is necessary to change the kind of source. It must not to be a hard source but a source that the schema (operating amplifier) can use to control by itself in positive direction. We put e voltage divider to our schema. This allows the current to pass through the resistors R1 and R2 which values are equal. 
Hristina Malakova68 (talk)

=== Visualizing the circuit on the whiteboard ===

=== Investigating the circuit on the whiteboard ===

This is a Schmidtt trigger, which is made by an operational amplifier. As you can see on the pictures, first we draw its transfer characteristic on the whiteboard. Then, one of my colleagues and the teacher made a demo at the whiteboard showing to us how exactly voltages were changing. It was like an animation. Hristina Malakova68 (talk)

=== Illustrating the circuit operation by a hysteresis curve ===

Hysteresis represent the history dependence of physical systems. If you push on something, it will yield: when you release, does it spring back completely? If it doesn't, it is exhibiting hysteresis, in some broad sense. The term is most commonly applied, as Webster implies, to magnetic materials: as the external field with the signal from the microphone is turned off, the little magnetic domains in the tape don't return to their original configuration (by design, otherwise your record of the music would disappear!) Hysteresis happens in lots of other systems: if you place a large force on your fork while cutting a tough piece of meat, it doesn't always return to its original shape: the shape of the fork depends on its history.
Hysteresis can be used to filter signals so that the output reacts slowly by taking recent history into account. (The text is written by Eleonora Todorova)

=== What is Schmitt trigger? ===

The Schmitt trigger is a comparator application, which switches the output negative when the input passes upward through a positive reference voltage. It then uses negative feedback to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point. 
In the top diagram, the input voltage increases from zero, along the bottom horizontal line. The output voltage remains at zero on the vertical line. However, when the input voltage reaches 1.7 volts, the output shoots up from zero to 5 volts. Reducing the input voltage, as shown in the top horizontal line does not cause the output to drop to zero immediately. This only happens when the input voltage is reduced to 0.9 volts. The input level at which the output increases to maximum, and the level at which it drops to zero are different (the text is written by Radoslav Danchev).
Radoslav, I do not yet understand why the hysteresis curve is inclined to right. Please, elucidate me. Circuit-fantasist (talk) 13:44, 28 April 2008 (UTC)Well, now I agree... Circuit-fantasist (talk) 06:31, 19 May 2008 (UTC)

In the lower diagram Schmitt Trigger action is demonstrated in another manner. The black graph represents a noisy logic signal. This is the input to the Schmitt Trigger. The green graph is the output signal. The output remains at zero until the input exceeds 1.7 volts. The output then shoots up to 5 volts and remains at 5 volts until the input drops to 0.9 volts. The output then drops to zero. An almost perfect output is recovered from a very noisy input (the text is written by Radoslav Danchev).

=== A problem: How do we make it memorize? ===
At the end of the exercise we were talking about a very interesting thing - how exactly we can make a device to memorize. This was very exciting for us because like a future computer engineers we had to get an idea about the problem how exactly a computer RAM is built in?
The recipe of this creation is as follows:
The goal is to make a RS trigger (latch, flip-flop) that can memorize the signals applied to it. To get a trigger we have to get a 100% positive back connection (a feedback). Thus we can get an amplifier with back positive connection that means we have to create a hysteresis. To get a positive feedback we have to connect consecutively two invertors.
The secret of one trigger is to supply 10V  for logical 1 and to supply -10V for logical 0 in the input and at definite moment trigger switch over one to another state.
We have also to supply biasing voltage and that means that the incoming signal has to get a value which is in the middle of the hysteresis cycle. It said in another way this value has to be between two thresholds. Hristina Malakova68 (talk)

==== Considering the idea on the whiteboard ====

What happens, if we apply an initial input quantity whose magnitude is inside the hysteresis cycle?

==== "Inventing" a flip-flop ====
A tip: can we use here the powerful idea of biasing? Circuit-fantasist (talk) 13:49, 28 April 2008 (UTC)

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