Talk:Circuit Idea/How to Make a Transistor Switch

Here is the building "scenario" that my students (group 68a) and I were using during Lab 3 to consider the popular circuit of transistor switch. Circuit-fantasist (talk) 07:31, 28 April 2008 (UTC)

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)

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. Normaly, 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 emmitter. 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 collegues 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
Finnaly we concluded that if we want our transistor switch to work properly, we have to observe some circumstances:


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

Hristina Malakova68 (talk)

