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This may be a very novice question, but I don't quite understand the point of transistors. I understand the functionality and potential uses but I don't quite understand the real purpose of using it in the first place. It will only activate when a (small) signal is applied to it to then activate another larger signal that is sent from it. But if the signal needs to be sent into the transistor and then another gets sent out, wheres the need for the transistor. Could one not just take that transistor activating signal and use it for whatever?

Thank you for your time :)

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    \$\begingroup\$ Keep studying. For now there is no point in answering your question with a detailed technical answer. You need to know a lot more about many things before you can benefit from an answer. Suffice it to say that everything electronic works using transistors. \$\endgroup\$
    – user57037
    Nov 13, 2016 at 5:47
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    \$\begingroup\$ I guess the jig is up. You've applied impeccable logic to the problem facing you and discovered the truth; transistors were an idea invented to obfuscate electronics and to distract nosy people into thinking there is something complicated when all that's really needed is to just simply use the signals "for whatever." \$\endgroup\$
    – jonk
    Nov 13, 2016 at 5:48
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    \$\begingroup\$ thanks for these rude comments people :) you really have a sense of community and helping others. think about how you understood difficult concepts in life. Not everyone has access to the same resources as others, as well as being particularly gifted in special areas. Most of us have asked "stupid questions" before, however, being able to answer them is a skill, not a chore. We all have to be able to understand these concepts somehow, and i figure that this forum would've been appropriate. I'm pretty sure that this level of inconsideration to others is intolerable in these communities! \$\endgroup\$
    – John Hon
    Nov 13, 2016 at 5:54
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    \$\begingroup\$ @John Hon since you are new to the community , you are not aware of jonk's wisdom with many decades of experience and your naivite and in this case his sense of humour. \$\endgroup\$ Nov 13, 2016 at 6:28
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    \$\begingroup\$ I know that you know that the output signal is larger than the input signal, because you said so. Isn't it obvious that without the transistor, you still have a small signal? \$\endgroup\$
    – user253751
    Nov 13, 2016 at 8:09

3 Answers 3

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When I was learning I remember thinking a similar thing. "If all a transistor does is relay my pushbutton switches or potentiometer changes, how does this get used in a computer?"

The reason is that, other transistors can control the transistor. This can be in the form of long term memory (for example a program stored on your magnetic drive) or short term memory (RAM which was just programmed by even more transistors), I/O input (such as keyboard + mouse), or combinational logic gates which decode the current state in memory into the proper value for whichever transistor you are talking about. As a whole this forms a system which can be used as a computer.

It is complicated to get it all at once. That is why so many pieces of the computer are broken into levels of abstraction. For example groups of transistors which produce boolean outputs are grouped into "logic gates". These "logic gates" can be combined to form larger functions such as adders or multipliers or the memory caches. These are put together to form the CPU, which is further joined with RAM and the rest of the peripheral hardware to form a complete computing system. But each part is built from transistors.

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For example, say you receive a radio signal from an antenna. There's really very little power in this signal, maybe a few microwatts. But you want to use that signal to drive 2 watt speakers so you can listen to the radio. A transistor circuit lets you use the tiny input signal to control the much more powerful signal driving the speakers.

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  • \$\begingroup\$ thank you :) but that lead me to wonder, why would people use it as a logic gate (like in computers) then? thx \$\endgroup\$
    – John Hon
    Nov 13, 2016 at 6:33
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    \$\begingroup\$ You can make logic gates with things like diodes and resistors, but then the gates do not have gain. That means the output amplitude of each gate is smaller than its input. If you chain several gates together, the output gets very small, eventually smaller than the noise in the circuit. By using transistors, we can make gates with gain, so that they can be cascaded essentially indefinitely without reducing the signal amplitude. \$\endgroup\$
    – The Photon
    Nov 13, 2016 at 6:42
  • \$\begingroup\$ Also, consider how would you make a useful flip-flop without a transistor (or other active element like a vacuum tube). \$\endgroup\$
    – The Photon
    Nov 13, 2016 at 6:54
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Transistors are called "Active components" due to their negative resistance and effective current gain characteristics.

In reality, they don't generate any power but the current gain draws power from the supply like a voltage controlled resistor or a small current control to control a larger current sink.

This current gain serves to ;

  • amplify input signal voltage
  • switch higher currents
  • lower the impedance of a signal (or buffer it) by current gain, so voltage does not drop as much due to load resistance.
  • Perform logic functions AND,OR,INV & NAND, NOR, XOR, XNOR etc

This is an example of 1960's Logic 2-input NAND enter image description here

  • mid 70's CMOS NAND characteristics enter image description here

There are many more applications but if these characteristics are not needed, then you do not need a bipolar transistor.

An Intel i7 uses 1.4 million MOSFET transistors http://download.intel.com/pressroom/kits/corei7/images/Nehalem_Die_Shot_3.jpg

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    \$\begingroup\$ "Transistors are called "Active components" due to their negative resistance and effective current gain characteristics." Negative resistance? Importance of current gain? I think, one could discuss a lot about these questionable statements. \$\endgroup\$
    – LvW
    Nov 13, 2016 at 10:20

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