By: Sajit Chandra Shakya
Preface How many types of logic Gates do you know? 100, 1000, or 10000. The more the gates you know, the more you are considered proficient in electronics.
Do you know the basics of the most elementary gates? It is important since until you know their operation, you would not know the intricate mechanisms of complex circuits of higher order (which would make logic gates appear trivial). This simple booklet is to enlighten you on the simplest gates. Expect a lot more from me.
Sajit Chandra Shakya (M.Phil.) Physics Educator Kathmandu, Nepal Contact: firstname.lastname@example.org
The “NOT” gate
The “NOT” gate is the circuit which gives a high voltage if it is input with low voltage and low voltage if a high voltage is provided. It is constructed with the help of a simple transistor in connection with a voltage source. The input is given to the base-emitter circuit and the output is taken across the collector-emitter. The circuit arrangement for constructing the “NOT” gate would be as in the figure and the truth table as well as overall action can be understood from the accompanying table. Vin High Low Or Vin 1 0 Vout 0 1 Vout Low High
When Vin is high, IB will be high, IC will be high ( Q IC = β IB), VC will be high ( Q VC = ICRC), Vout will be low ( Q Vout = V – VC).
When Vin is low, IB will be low, IC will be low ( Q IC = β IB), VC will be low ( Q VC = ICR C), Vout will be high ( Q Vout = V – VC).
The “OR” gate
D1 P D2 I Vin1 Vin2 T S U R Vout Q
The “OR” gate is the circuit consisting of two inputs and an output, in which a high output is obtained if any one of the input is high. The circuit otherwise would give a low output. It is constructed by using two diodes, through which inputs are given and a resistor, across which output is measured. The circuit arrangement for constructing the “OR” gate would be as in the figure and the truth table as well as overall action can be understood from the accompanying tables.
Vin1 Low High Low High
Vin2 Low Low High High
Vout Low High High High
Vin1 0 1 0 1
Vin2 0 0 1 1
Vout 0 1 1 1
When both Vin1 and Vin2 are low,
Both of them are not giving any current, so I is low and V out is low ( Q Vout = IR). Diode D1 will conduct because it is forward biased by Vi n1. Diode D2 will not because it is not given any potential difference ( Q Vin2 is low). So electrons from Vi n1 will pass through T, S (they can not go from S to diode D2 because D2 is already non-conducting), U, resistor R, Q and reach point P. They can not go through D2 because it is non-conducting. So they will pass through D 1 to complete a circuit, making the value of current I high, so V out (=IR) will also be high. Diode D2 will conduct because it is forward biased by Vi n2. Diode D1 will not because it is not given any potential difference ( Q Vin1 is low). So electrons from Vi n2 will come to S first, where they have two paths; to go towards T or U. They don’t go to T because Diode D1 is not conducting. So they go to U, resistor R, Q and reach point P. So they will pass through D2 to complete a circuit, making the value of current I high, so V out (=IR) will also be high. In this case, both the diodes D1 and D2 will conduct because both of them are forward biased by Vin1 and Vi n2. So electrons from both input sources will combine at S, then go through U, resistor R, Q and to P. Then they distribute themselves along the two paths towards the diodes and then to the cells, completing both the circuits, making the value of current I high, so V out (=IR) will also be high.
When Vin1 is high and Vin2 is low,
When Vin1 is low and Vin2 is high,
When both Vin1 and Vin2 are high,
The “NOR” gate
The “NOR” gate is the circuit consisting of two inputs and an output, in which a low output is obtained if any one of the input is high. The circuit otherwise...
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