So I was wondering, if, say, I'm getting a current from a transformer where the current is very low but the voltage is very high. And there's a diode to turn my circuit into a DC circuit. And I'm using an AND gate to turn on my transistor.

For the gate, we need the two inputs to be high, but do we mean by the high input a high voltage or a high current. And to turn on the transistor do I need a high voltage or a high current or both?

I wrote this question in the physics community but I was kindly told that it should be posted here and that there are so many kind of different transisors but I was wondering what kind of transistors should I think of in my level (highschool igsce OL physics)

This confusion got into me when I was solving this question enter image description here

and there were parallel and series connection—which contrary to normal questions where the high resistor would prevent a high voltage from reaching the other parts and it won't matter wether we think about voltage or current because they will always be low in the same time— here it was different. Even though it won't really matter which way I think about it here because in the end it's an AND gate but I still want to know.

PS: this isn't a hw question, I've the answers of the question already and understand how the circuit work, I'm just curious about how I should think about it.

  • 1
    \$\begingroup\$ This question is really too broad; the best answer is probably read "The Art of Electronics" by Horowitz & Hill. \$\endgroup\$
    – user16324
    Sep 11, 2020 at 11:14

1 Answer 1


A high level at a logic gate is a high voltage. Gates generally draw little or no current at their inputs.

If the logic gate in your diagram uses one of the most common technologies, that is CMOS or TTL, then it cannot be connected directly to that bipolar transistor without risk of damage. The transistor base will not rise more than about 0.7 V above the emitter, and will draw too much current from the logic gate output. There needs to be a series resistor between the logic output and the transistor base to provide a current drive to it. Typically you will aim to supply 3% to 10% of the collector current to the base terminal to make sure the transistor is on.

If the bipolar is replaced with a FET, then that will accept a high voltage directly, and will draw no current from the logic output.

There are many more options that could confound answers to general questions like these. Some logic gates have open collector outputs which cannot source current, and so need a pullup resistor, but then can be connected directly to a bipolar. FETs have different threshold voltages, some newer ones work well with 5 V logic levels, most FETs especially older ones really require much more than 5 V on the gate to work properly. There are other logic families than TTL and CMOS, and their logic levels would need to be designed with properly.


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