Basic of AND Gate
AND gate gives high at the output only when all the inputs are high otherwise, it gives low.
SYMBOL OF AND GATE
|INPUT 1 A||INPUT 2 B||OUTPUT A.B|
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.
Two BC547 NPN transistor
Three 10k resistor
Two toggle switches
General purpose LED
5V power supply
Some jumper wires
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.
AND Gate Using Transistor
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.
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.
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.
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.