Logic gates using transistors and diode | Proteus design suite simulation

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Logic gates are semiconductor devices. It is building block of any digital device. It has basically two input terminals and one output terminal. On the input terminals, we provide digital signal and on the output terminal it provides a digital output. To know operations of various types of logic gates we first need to know what digital signal is?
Now, let’s get to the main topic which is logic gates. There are three basic logic gates which are AND, OR and NOT. Many logic gates can be formed through these three universal logic gates. Important extracted gates are NAND, NOR, Exclusive-OR (XOR), Exclusive-NOR (XNOR) etc. XOR and XNOR are called universal logic gate because using these gates we can implement any boolean expression and build that ciruit.
AND gate
It has two or more input terminal and one output terminal. It performs AND operation which means input 1 and input 2 both should be true or high for high output. Let it understand through figures and truth table.
Symbol and truth table of AND gate is shown in the video
Boolean expression for this gate is “Y = A.B”.
You can clearly observe in the video that output is high only when both inputs are high. Now, let’s see how we can implement AND gate using transistors.
Implementing AND Gate using transistors
In the circuit diagram, you can see when input A and input B is zero LED is not glowing. But when the both input is high LED is turned on. See the circuit in the video.
You can see two BC547 transistors are connected in series whose base terminals are inputs. When both input terminals are low, not even one transistor is in ON state. In this condition transistors are blocking the current flow through the LED, so, LED is not glowing, that means output is low. When input A or B is high and other is low, any one transistor is ON and other is OFF. Only when both transistors are on then current will flow and the led will glow.
The AND gate is also implemented using diode. By watching that you will be able to create AND gate using diode.
OR Gate
This logic gate performs OR operation. It means for high output, any one or both input terminals should be high.
Symbol and truth table of the OR gate is shown in the video.
Boolean expression for this gate is “Y = A + B”.
You can observe that output is high only if any one or both input is high. In the circuit in video, OR gate is implemented through transistors.
There are two transistors are connected in parallel in this time and base of both transistors are connected to input terminals. If any one or both transistors allow the current flow then current will flow through the LED and LED will glow. When both input terminals are low both transistors are blocking current flow through the LED.
The AND gate is also implemented using diode. By watching that you will be able to create AND gate using diode.
NOT Gate
This logic gate inverts the input signal. When input is high, output will be low and when output is low then output will be high. Therefore, it is also known as inverter.
Symbol and truth table is shown in video
Boolean expression for this gate is “Y = Ā”.
Circuit of NOT gate using transistor is as shown in video:
In not gate input terminal is low but output is high, because the transistor is in OFF state and current is not flowing through it rather it is flowing through the LED. So, LED is glowing.
And input is high but output is low, because the transistor is in ON condition and current will flow through the transistor but not through the LED due to its high resistance.
NAND Gate
This gate is combination of AND gate and NOT gate. The output of AND gate is connected to the input of NOT gate. Its output is reverse of AND gate.
NAND Gate truth table and symbol is shown in the video.
Now, let’s see how we can implement this gate using transistors.
NOR gate
This gate is made by combining OR gate and NOT gate. The output terminal of OR gate is connected to input terminal of NOT gate and same here, the output is become opposite to the OR gate.
Truth table and symbol of NOR gate is in the video.
In the circuit, transistors are connected same as the OR gate but output is same as NOT gate. When any input terminal is high then current will flow through any transistor not through LED. When both inputs are low then only current will pass through LED.
XOR gate
It is also pronounced as EX-OR or Exclusive OR. It gives high output when one of its inputs is high and other is low. Let’s see how we can implement this gate using the gates we know.
Truth table and the symbol of the XOR gate are given in the video.
Methods of implement the XOR gate usig basic gates is shown in the video.
XNOR gate
This gate is made by connecting XOR gate and NOT gate. Output of XOR gate is connected to the output of the NOT gate. Output of XNOR gate is reverse of XOR gate because of NOT gate.
Truth table and the truth table are shown in the video.

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