Brief NPN BJT OR Logic Gate Circuit 2N3904 Bipolar Junction Transistors

OR logic gates have the basic property of one or more high input resulting in a high output. Only if all of the inputs are low, will the output to be low.

Simple NPN BJT OR logic gate demonstration circuit using 2N3904 bipolar junction transistor schematic diagram by electronzap electronzapdotcom
Simple NPN BJT OR logic gate demonstration circuit using 2N3904 bipolar junction transistor schematic diagram by electronzap electronzapdotcom

Two transistors can be wired as emitter followers and connected in parallel, as shown in the diagram.

The emitter resistor that they share in the diagram, holds the output voltage at 0 volts while the transistors are off.

0V, which is often called ground, is considered to be a low input/output.

Vcc is the positive supply voltage. Notice that it is on the collector side of the NPN BJT. C = collector. Double c is commonly used to indicate the (NPN) collector side supply voltage.

The use of Vcc to indicate an unspecified positive supply voltage has been accepted in any circuit schematic.

Closing one switch allows a small amount of current flow through that NPN transistor’s Base to Emitter. That current also flows through the Emitter resistor and any low impedance/resistance load connected at the output.

Either transistor with base current flowing (high input) will result in the transistor connecting the positive supply voltage (Vcc) to the output.

However, the output will be approx. 0.6 less than the supply voltage in this circuit. That’s due to the base to emitter diode drop.

Outputting close to Vcc is considered as having a high output in this circuit. Make sure you limit current through the output. It’s practically a short circuit if you connect the output to ground when the output is high.

Video:

To support this site, check out the following links:

Home page

  • Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
  • Electronzap is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.