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                   292                         Fundamentals of Computers                           NPP




                                                      A         B         C


                                       Battery
                                                                                     Y

                                                                                 Bulb


                  AND Gate using Diodes                       S>m`moS> H$s ghm`Vm go AND JoQ> ~ZmZm
                      Consider the following circuit containing   à{VamoY d S>m`moS> go ~Zo hþE n[anW H$mo XoImo…
                  diodes and a load resistor:

                                          A

                                                       D
                                                        1


                                          B                                       Y

                                                       D 2               R
                                                                          L



                                                                       + 5V
                      A, B are the input signals which may have   A, B BZnwQ> {g½Zëg h¢ {OZ_| Xmo dmoëQ>oO d¡ë`yO
                  two voltage values 0 volts and +5 volts (0, 1).  0 dmoëQ²>g VWm +5 dmoëQ²>g (0, 1) hmo gH$Vo h¢Ÿ& AmCQ>nwQ>
                  The output Y may also have two values 0 and 1.  Y _| ^r Xmo d¡ë`yO 0 VWm 1 hmo gH$Vr h¢Ÿ& O~ A `m
                  When A or B or both are Zero, at least one of the  B `m XmoZm| eyÝ` hmoVo h¢ Vmo H$_ go H$_ EH$ S>m`moS²>g
                  diodes will be forward biased. In forward bias  \$madS>© ~m`ñS> hmo Om`oJmŸ& \$madS>© ~m`g H$ÊS>reZ _|
                  condition a diode  works as a  short  circuit.
                  Therefore the point for Y will be shorted to 0  EH$ S>m`moS> EH$ em°Q>© g{H©$Q> Ho$ ê$n _| H$m`© H$aoJmŸ& AV:
                  volts. Thus, Y = 0. When both the inputs are 1(at  Y Ho$  {bE nm°BÝQ>  H$mo  0 dmoëQ²>g Ho$ {bE em°Q>© {H$`m
                  High Voltage), both the diodes will be reversed  Om`oJmŸ& Bg àH$ma Y = 0 hmoJmŸ& O~ XmoZm| BZnwQ> 1 (hmB©
                  biased and work as open circuit. Therefore the  dmoëQ>oO) hmoVo h¢ Vmo XmoZm| S>m`moS²>g [adg© ~m`ñS> hm|Jo VWm
                  voltage at Y is +5V. Thus Y = 1. Therefore we  AmonZ g{H©$Q> Ho$ ê$n _| H$m`© H$aVo h¢Ÿ& AV: Y na dmoëQ>oO
                  can say that Y = 1 only when A and B both are 1.  +5V h¡Ÿ& Bg àH$ma Y = 1 h¡Ÿ& AV: h_ H$h gH$Vo h¢
                  Therefore  the above  circuit  follows the  truth  {H$ Y = 1 Ho$db V^r hmoVm h¡ O~ A VWm B XmoZm| 1
                  table of AND Gate. By increasing  number of  h¢Ÿ& AV: CnamoŠV g{H©$Q> AND JoQ> H$s Q¯>W Q>o~b H$m
                  diodes we can draw the circuit for AND Gates  AZwgaU H$aVm h¡Ÿ& S>m`moS²>g H$s g§»`m ~‹T>mH$a h_ Xmo
                  having more than two inputs. The circuit for 3-  go A{YH$ BZnwQ²>g dmbo AND JoQ> ~Zm gH$Vo h¢Ÿ& 3-
                  input AND Gate is:                          BZnwQ> AND JoQ> Ho$ {bE g{H©$Q> h¡:
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