Archive for July, 2011
Reading is the most simplest way for human to derive and constructing meaning in order to gain a particular knowledge. An electronic book or e-book or digital book is a book-length publication in digital form which consists of text, images, or both, and produced and published through computers or other electronic devices.
Introduction to Sequential Circuits:
The Basic R-S FLIP-FLOP
Consider the above schematic circuit of MC724 quad 2-input NOR gate of R-S Flip-Flop. When power is switched on and no inputs are applied(R=0 and S=0), due to imbalance of the circuit let say Q3 goes into conduction and Q1 and Q2 are in cut-off. Since Q3 is conducting, output at its collector will be zero i.e.Q=0 and Q2 is in cut-oof, it has maximum output voltage at its collector terminal 3 i.e. Q bar=1. The flip-flop is said to be in the RESET position. This is one stable state as shown below.
When the inputs S=1 and R=0, Q1 goes into conduction, making Q bar=0. This makes the collector of Q3 and Q4 high giving Q=1. The flip-flop is now in another stable state i.e SET state and will remain in this state if no input is applied further as shown below.
So,from this the truth of R-S Flip-Flop as follow.
Watch the following video for animation of R-S Flip-Flop
(1) Click on following image and go through the tutorial of Flip-Flop and upload the answer of quiz in it.
The exclusive-OR gate has a high output only when an odd number of inputs is high.
As shown in above circuit of Exclusive-OR gate, the upper AND gate gives output A’B and lower AND gate gives AB’ and both these output are given as input to OR gate whose output is given by Y=A’B+AB’
As shown in above circuit when both the inputs are low, both AND gates have low outputs, therefore final output Y is low.
If input A is low and B is high, the upper AND gate has high output, so the OR gate has high output as shown below.
Similarly, when A is high and B is low, we get final output high as shown below
If both A and B inputs are high, both AND gates have low outputs and the final output is low as shown below.
From the above discussion, the truth table of two input exclusive-OR gate can be given as follow.
From this truth table, we can say output is high when either A or B is high. When both the inputs are low or high, the output is low. This why circuit is known as exclusive-OR gate.
Four Inputs Exclusive-OR Gate:
As shown in above circuit, a pair of exclusive-OR gate driving an exclusive-OR gate. If all inputs are low (A to D), input gates have low output, so the final gate has a low output.(verify from two input exclusive-OR gate truth table). If A to C are low and D is high, the upper gate has a low output, the lower gate has a high output, and the output gate has a high output.
Here is an important property of the above circuit’s truth table. Each ABCD input with an ODD number of 1s produces an output 1 as shown below truth table.
In second row of above truth table, when ABCD=0001, it has ODD number of 1s which gives output 1. When ABCD=0010, it produces output 1.
For EVEN number of input ABCD=0011, it produces output low.
Any Number of Inputs Exclusive-OR Gates:
Encoder is a digital circuit that converts an active signal into a coded output signal.
General idea of encoder is illustrated in following block diagram
There are n input lines, only one of which is active. Encoder converts this active input to the binary coded output with m bits.
When button 3 is pressed, the C and D OR gates have high inputs, therefore output is ABCD=0011 i.e. decimal 3 is converted into its equivalent binary number 0011.
(Verify this conversion using online calculator by clicking on http://www.easycalculation.com/binary-converter.php)
Similarly, if you press button 5, the output will be ABCD=0101
and for switch 9, we get ABCD=1001
The IC-74147 of Encoder:
The above figure shows the pinout diagram for IC-74147, a decimal-to-BCD encoder.Pins 1 to 5 and 10 to 13 are for X1 to X9 decimal inputs and pins 14,6,7 and 9 are for BCD output. Pin 16 is for supply voltage and pin 8 is grounded. Notice bubble at input and output, which indicated active-low signal.
When all X inputs are high, all outputs are high. When X9 is low, the ABCD output is LHHL or 0110 which is binary equivalent of 9. When X8 is only low input, ABCD is LHHH or 0111 which is binary equivalent of 8.
IC-74147 is called priority encoder because it gives priority to highest order input. That means if all inputs from X1 to X9 are low, it gives priority to highest order 9 which is encoded into out as LHHL. i.e. X9 has priority over all others. When X9 is high it gives priority to X8 and gets encoded if it is low.
As shown in above circuit, PNP transistor is connected with emitter-base PN junction as forward bias by VEE whereas collector-base diode is reverse-biased by Vcc. AC signal is given by the source Vs through resistance Rs and output voltage is obtained across collector and base as voltage Vo’.
Two-Diode Analogy of CB Amplifier:
PNP Two-Diode Analogy of CB Amplifier
As shown above, PNP transistor is connected as two-diode analog. Both the diodes are connected in front-to-front position, since arrow on the emitter points inward.
Operation of Circuit:
With no input AC signal, the emitter current is DC bias current IE. Most of this current is swept into collector current of the transistor as base current IB is small. i.e. IE=IB+IC…………(1) In presence of input AC signal Vs, there will be DC as well as current in the circuit and therefor above equation becomes iE=iB+iC
The above graph of IE —–>VEB gives input characteristics of CB amplifier. Since diode is in forward-biased, this characteristic is similar to that of diode in forward bias. But input or controlling current is much larger than in the CE amplifier.
The above graph of Ic——>VCB for different values of IE gives output characteristics of a CB amplifier. Due to high output resistance, the output characteristics curve is almost horizontal.