There are kits you can by to introduce you to electronics. In the UK Maplin's sell them. Choose one that inspires you. http://www.maplin.co.uk/c/gadgets-toys-and-hobbies/projects-kits-and-modules?page=3&sort=&productsPerPage=

The application determines whether you use transistors, gates or op.amps.

The kits will give you an idea of what can be done with the devices. I stated learning about electronics when I was 11 years old by looking at magazines. here is a reference: http://en.wikipedia.org/wiki/Practical_Electronics

I think you can now buy everyday electronics magazine that does a similar function.

Once you get confident in the terminology and what applications are available, you can then progress to designing your own circuits and use the professional data sheets.

Many semiconductor manufacturers have tools to help design. One is Tina-Ti

http://www.ti.com/tool/tina-ti it is free but is optimised for Texas parts.

Hope this helps

Regards

Dr. Paul

Thank you for the answer, Paul. But I already know about the Semiconductors Theory, or Relays, for example. What I want to know is if there are other ways to build amplifiers, or gates. Other way but really old are the Electronic Vacuum Valves. But more than that I really don't know.

Yes, you can build gates from transistors, or Valves. Why would you want to?

If you are just curious then a good beginners book will explain. In my youth, I built gates, from discrete devices, a 32 bit divider and amplifiers . Tell me what you want to build and I can look up some references.

Regards

Paul

Thank you Paul.