I am making a NOT gate using a BC337/BC547A transistor (tried both) and I have made this circuit (image shown below) using a simulator:

working schematic

This is working properly. However when I remove the resistor R1, as shown below, it is not working.

this is not working

Can you please explain what the importance of the resistor is here?

  • \$\begingroup\$ When the switch is on, without the resistor, what do you expect to be the voltage across LED1? \$\endgroup\$
    – user253751
    Sep 8, 2020 at 13:31
  • 1
    \$\begingroup\$ It saves replacing the transistor every time you close the switch. \$\endgroup\$
    – user16324
    Sep 8, 2020 at 16:16
  • \$\begingroup\$ In the 2nd case LED1 is connected directly to Vcc so nothing else occurring in the circuit can influence it. Also in this case since Vcc will exceed the forward voltage for LED1 it will burn out (I wonder how they will simulate the smoke :-) ) \$\endgroup\$
    – mhaselup
    Sep 17, 2020 at 4:22

1 Answer 1


Without the resistor, it is impossible for the output to be at any voltage other than that of the VCC supply rail, to which it is then directly wired. All the transistor can do in such a case is short out the power supply.

What you drew in the first picture is called "resistor transistor logic" as it depends on a pulling resistor to counterbalance the transistor and provided the limited drive in the other direction. But it wastes power (in the resistor) in one state, and it is slow in the direction where the resistor sources current.

More typically, "transistor transistor logic" was used - there other transistors would function to drive the output high when needed, or to act like the resistor does as something of a current source.` But that's largely faded from use.

Today most logic is CMOS - Complementary Metal Oxide Semiconductor. It is made of field-effect transistors with oxide-insulated gates. And it is "complementary" because there's an approximate mirror image to the schematic, with an upper P-channel FET and a lower N-channel FET which work alternately to drive the signal high or low.


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