AND Gate using Transistor

Basic of AND Gate

AND gate gives high at the output only when all the inputs are high otherwise, it gives low.

Fig. 1: Symbol of AND Gate

INPUT 1 A	INPUT 2 B	OUTPUT A.B
0	0	0
0	1	0
1	0	0
1	1	1

The TRUTH TABLE OF AND GATE

The use of transistor for AND gate operation depends on the transistor switching speed. For AND gate operation we use transistor as a switch.

Components Required

 Two BC547 NPN transistor

 Three 10k resistor

 1k resistor

 4.7k resistor

 Two toggle switches

 General purpose LED

 5V power supply

 Some jumper wires

 Breadboard

 Multimeter

Two Input AND Gate Using Transistor

The BC547 is used here in common emitter configuration. This transistor utilizes low power and also has low-frequency. In the common emitter configuration, transistor gives a phase shift of 180 degrees. Due to change in 180 degree in phase shift, it is able to give high at the output when our input is low and vice-versa. The biasing of the transistor is done in a way so that the operating point of the transistor comes closer to the origin in the transfer-characteristic curve of the transistor.

This causes an immediate switching of the transistor from its cutoff to the saturation state. Hence when we apply enough voltage at the base of the transistor it immediately reaches its saturation state and the transistor starts conducting. In this project, we have used BC547general purpose NPN bipolar junction transistor. Other range of BC transistor (BC548, BC549) also works fine here. By using RTL (resistor transistor logic) we have designed the AND gate by two transistors and some resistor.

Fig. 2: Circuit Diagram of Transistor based AND Gate

How it Works

The transistors are connected in series and their bases are used as input. The base of both the transistors act like inputs and one of the emitter of either of the transistors is used to derive the output. Initially, both the switches are in OFF state so none of the transistor bases get a power supply. The base to emitter junction and base to collector junction of both the transistors have a voltage lower than 0.65V, which is the practical threshold voltage of the diode.

Both junctions are in reverse bias hence both the transistors turn off and go into their cutoff state. Therefore, the transistors act like an open switch. Since all the current coming from the collector through resistor R3 blocks by the transistor. Hence at the output, we get a low voltage which turns off the LED.

In next case when we press switch 1 then the base of the first transistor gets a positive value of voltage but its emitter is connected to another transistor collector. As the second transistor is still in its cutoff state, the emitter of the first transistor is disconnected. The base to emitter junction and base to collector junction of both the transistors have a voltage lower than the threshold voltage and again they reach their cutoff state. All the current is again blocked by the transistor and we get low voltage at the output, which turns OFF the LED.

Fig. 3: Circuit Diagram showing working of Transistor based AND Gate

Likewise, when switch 1 is opened and switch 2 is closed, the current is blocked by the first transistor and we get low at the output.

Fig. 4: Circuit Diagram showing working of Transistor based AND Gate

When we press our second switch, the junction of both the transistors has a voltage greater than threshold voltage so both junctions are forward bias. Therefore, both transistors are in saturation state and act like a short circuit. The current now gets a short circuit path and flows from the collector of the first transistor to the emitter of the second transistor which derives high at the output. At the output, our LED lights up.

Fig. 5: Circuit Diagram showing working of Transistor based AND Gate

We can say for AND logic, transistors are connected in series and both the transistors must be in their conducting state to derive high at the output. If one of the inputs or both the inputs have low value then we get low at the output, otherwise high.