DESINING OF BASIC LOGIC GATES INTRODUCTION: Logic gates are electronic circuits with a number of inputs and one output. There are three basic logic gates normally. 1. NOT GATE 2. AND GATE 3. OR GATE • NOT GATE: NOT gate is a logical gate which only inverts the input digital signal. This is why, a NOT gate sometimes is referred as inverter. A NOT gate always have high or logical 1 output when its input is low or logical 0. On the other hand a logical NOT gate always have low or logical 0 output when input is high or logical 1. The logical symbol of a NOT gate is shown below. FIG 1(NOT GATE symbol)
If the input binary variable of a NOT gate is considered as A, then the output binary variable of the gate will be Ā. As the symbol of not operation is ( - ) bar. If the value of A is 1. then Ā = 0 and in opposite if the value of A is then Ā = 1. Truth table: DESINEING OF NOT GATE: COMPONENT REQUIRED: -A bunch of 5k and 10k resistors, and transistors -A LED -NPN type transistor (for example 2N3904, BC547, BC548, BC549 etc.) PICE LIST: SR.NO NAME OF PRODUCT 1 2 3 4 5 6 QUANTITY COST PER NO UNITS Rs BATTERY 1 num 20 SWITCH(toggle) 1 num 7 TRANSISTER 1 num 12 RESISTOR(200,10 2 num 3 PCB 1 num 30 ELECTRONIC .2 meter 6 TOTAL COST Rs 20 7 12 6 30 6
7 WIRES SOLDERING IRON 8 SOLDERING WIRE, FLUX 1 num 110 110 1 Pack 50 GRAND TOTAL 50 241 Fig 2(circuit of not gate using transister) WORKING: When initially our switch is in off state there is no power supply at the base of the transistor. At the base there is zero current and at the collector, we have small reverse saturation current (Ico) which makes the collector more positive as compared to the base. Thus base-collector junction is reverse biased. Now as we know the emitter is grounded or it is at zero volts but still there is a flow of non-zero reverse saturation current through the emitter to compensate the diffusion current. An equal and opposite current flows in the base-emitter junction (-Ico). This negative value of current (Ico) makes base-emitter junction reverse bias.
As a result, both base-collector junction and base-emitter junction are reversed biased. Now there is no flow of current from collector to emitter and they act as an open switch. In this state, the transistor is said to be in its Cutoff state. Therefore, all the current through the R2 resistance passes to the LED which turns it ON. Hence at the output, we get a high voltage corresponding to the low voltage of input. Now when we press the switch then our base gets a positive 5V supply which turns ON our transistor. Here the base voltage is pretty much high which drives the high base current. As the base current increases, it increases the collector current as well. The voltage drop across the resistor (R2) is big enough to lower the collector voltage below the base voltage since collector current is very high. Thus the collector to the base junction is in forward bias. The emitter to the base junction is already in forward bias state since emitter is connected with ground and base is with high voltage. This state of the transistor is termed as Saturation state. There is a smooth flow of current from collector to emitter and the transistor acts like a close switch. As we know current always passes through the low resistance path so here all the current gets a shorted path from collector to emitter. The current through R2 resistance then flows from collector to emitter and into ground. Since all the current passes through the transistor, there is not enough going through the LED, and therefore it goes OFF. Fig3 (analysis of circuit in simulator) • AND GATE: