Transistor Logic AND Gate

This is a Transistor-Transistor Logic (TTL) AND Gate circuit using bipolar junction transistors. A basic circuit using any general-purpose bipolar transistor such as the BC549, BC548, or BC547, can be used to construct the AND gate.

In this configuration there are two transistors connected in series. This is similar to two switches connected in series. The current through the diode will flow only when both transistors are conducting. This can only happen when both transistors receive an input voltage of 5 V (TTL) representing logic 1.

If either of the transistors receives an input voltage of 0 V (logic 0) then the transistor does not conduct breaking the flow of current.

AND Circuit Built on Breadboard

Here is a simple breadboard construction of the AND gate. Make sure you connect the transistors the right way round, and follow the links in the parts list to see what the terminals are and how they are connected.

The diode has a flat side, which is the cathode. To learn how to use the diode, please refer to the LED Resistor Calculator article.

The power to the circuit is applied through the red wire, which is positive, and the blue wire, which is negative. I used a PP3 9 V dry cell for this circuit. You can connect the power supply in the usual manner to the breadboard power supply rails.

The green wires are for the signal input, and the black wire is for the ground. The input signal is with respect to ground. For practical purposes, you could use a 1.5 V AA size battery for the signal inputs. A low voltage such as this is enough to make the transistor conduct.

In an AND gate if the input terminals (green wires) are joined together, the circuit then behaves as a buffer.

Parts List

Semiconductors
Tr1, Tr2: BC549, BC548, or BC547
LED: 2.1 V forward voltage drop

Resistors
2 × 100 kΩ
3 × 1 kΩ

Misc
Solderless Breadboard
PP3 9 V Dry Cell
1 × AA Dry Cell for signal input

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