Apparatus:
An npn transistor (BC547A), two resistors 1-2 and 20, 30k, power supply (SV), connecting wires, two-way key, LED and a bread board.
Theory:
NOT gate is a special gate having one input and output. It is also called inverter because it changes a high input to low input and vice versa. The symbol for NOT gate is. The truth table for NOT operation is
| Input | Output |
| A | F= |
| 0 | 1 |
| 1 | 0 |
The basic circuit used as NOT gat consist of a n-p-n transistor connected in a common emitter way as shown in figure. The output is taken between collector C and emitter E terminals. The circuit or device which is required to turn ON or OFF is connected across these terminals. The base-emitter terminals control the NOT gate. The basic equation for output circuit is
When terminal A is grounded i.e. (A=0)
Base current is zero. Transistor is at cut off condition. No current flow in
When maximum voltage is applied to terminal A(A=1)junction BE is forward biased. The transistor is in saturated condition i.e. IC=max
VO=5=4.8=0.2 volts
So that output at log 0.
Procedure:
1. Assemble the circuit diagram on bread board. Note that R must be nearly twice or r.
2. Ground the input terminal a so that (a=0), is =0, the transistor is in cut off region.
the voltage across collector emitter terminal in max i.e. Output F = 1. The led will give light. So the first row of truth table is verified.
3. Make the terminal a at high voltage so that a=1. A large potential injects a large current
into the base emitter circuit causing heavy current 1 to flow in CE circuit. As a result the resistance between c and e drops to such a small value that potential drop across CE terminal is nearly 0.1 volt. Hence output voltage f=0 the led will not give light. So second line of the truth table is verified.
Precautions:
1. The connection should be neat and clean.
2. The emitter collector junctions should be correctly identified.
3. The base resistance r, should be nearly ten time the collector resistance ro.
Viva voce
Q.1: Why is NOT gate known as an inverter?
Ans: Because it changes high input to low input and vice versa.
Q.2: What is the name of combination of “AND” gate and NOT” gate?
Ans: The name of combination of “AND” gate and NOT gate is NAND gate.
Q3: Why base resistor R has large value?
Ans: Because base need very small current of the order of micro amperes for proper operation. Base resistance controls the base current.
Q.4: How transistor is properly operated?
Ans: The transistor operates only when EB junction is forward biased and CB junction is reverse biased.