Chapter 4: Electronics

1. Form 5 Chapter 4: Electronics Physics Next > The study of matter 1

2. < Back Next > Physics: Chapter 4 Objectives: (what you will learn)1) uses of Cathode Ray Oscilloscope2) understanding semiconductor diodes3) understanding transistors4) analysing logic gates 2

3. < Back Next > Maltese-cross Tube Thermionic emission = emission of electrons from hot metal surface in vacuum Cathode rays = electrons moving at high speeds after acceleration through high potential difference A Maltese-cross tube is used to show the first two properties of cathode rays. Properties:1. electrons moving at high speeds in straight lines2. cause fluorescent material to emit light 3. deflected by magnetic field4. deflected by electric field 3

4. < Back Next > Maltese-cross Tube “Maltese Cross” Crookes Tube Invented in the 1880s by William Crookes during his investigations into the nature of cathode rays. It demonstrates that radiant matter is blocked by metal objects. The direction of deflection of cathode rays by magnetic field is found with Fleming’s left-hand rule. 4

5. < Back Next > Cathode Ray Tube The oscilloscope is capable of following changes that occur within billionths of a second. It is widely used throughout industry and in laboratories to test and adjust electronic equipment, and to follow rapid oscillations in electric voltages. Special converters attached to oscilloscope can convert mechanical vibrations, sound waves, and other forms of oscillatory motion into electrical impulses that can be observed on the face of CRT. 5 The Cathode Ray Tube (CRT)

6. < Back Next > Cathode Ray Tube 6 The Cathode Ray Tube (CRT)

7. < Back Next > Cathode Ray Oscilloscope The Cathode Ray Oscilloscope (C.R.O.) is divided into 3 parts: • Electron gun • Deflection system • Fluorescent screen 7

8. < Back Next > Velocity, v = 2eV m Cathode Ray Oscilloscope • Electron gun: • The cathode emits electrons when heated • The grid controls the number of electrons reaching anodes – control with brightness knob • The anode focus electrons into fine beam – control with focus knob • The potential difference between anode and cathode accelerates electrons to high velocity • Deflection system: • Y-plates: electric field deflects electrons vertically • X-plates: electric field deflects electrons horizontally • Fluorescent screen: • When fast electrons hit fluorescent screen, their kinetic energy is converted into light – a spot of light is seen on the screen • The walls of C.R.O. after anode is coated with graphite and grounded to keep out external electric field doctronics Kinetic energy of electrons emerging from anode = eV ½ mv2 = eV 8 where e = charge of electron, m = mass of electron

9. V0 1 ln √2 √2 2 < Back r.m.s. voltage, Vrms = = volts Next > x l Given: Y-sensitivity = n V per division Cathode Ray Oscilloscope • Uses of C.R.O. • 1. Measure potential difference • Switch off time-base • Connect voltage to be measured to Y-input • d.c. voltage: if x = deflection of light spot, voltage = xn volts • a.c. voltage: 2 x (peak voltage, V0) = ln 9

10. Distance travelled 2d Time taken 3T < Back Speed of sound, v = = Next > wall A B d microphone d 3 divisions Cathode Ray Oscilloscope • 2. Measure short time interval • Switch on time-base; one horizontal division = time interval, T • Pulse A represents sound detected by microphone • Pulse B represents the echo • Say, time interval between A and B is 3 divisions = 3T • If d = distance of wall from microphone • 3. Display waveform • Connect input voltage to Y-input • Switch on time-base • Adjust frequency to a steady trace formed on screen • The trace or waveform is the graph of voltage V against time t 10

11. < Back Next > band p-n junction + – + – p n – + symbol structure actual diode Semiconductor diodes Semiconductors have resistance between that of metals and insulators; e.g. carbons, germanium, silicon Pure semiconductor: negative charge carriers = positive charge carriers or free electrons = holes Doped semiconductor (with added impurity): n-type: free electrons > holes (impurity of valency 5; arsenic or phosphorus) p-type: holes > free electrons (impurity of valency 3, indium or gallium) Semiconductor diode 11

12. + no current < Back + Next > + current + Forward bias Reverse bias Semiconductor diodes • Ideal diode • Allows current through when connected in forward bias • Stops current when connected in reverse bias (infinite resistance) 12

13. VD a.c.V VR R < Back Next > Half-wave rectification Semiconductor diodes A diode is used as a rectifier to convert a.c. to d.c. Current only flows through the diode during the positive half cycle (as shown by +V). 13 The voltage across the load, VR is direct voltage and the current is d.c.

14. VD a.c.V VR R < Back C Next > smoothing capacitor Semiconductor diodes A capacitor, C is connected across load, R to smoothen voltage, VR. 14

15. < Back Next > Semiconductor diodes 2 diodes are used in a simple full-wave rectification. 4 diodes are used in a bridge full-wave rectification. 15

16. C C B B E E < Back n-p-n transistor p-n-p transistor Next > Some samples of the actual transistors Structure of an n-p-n transistor Transistors Transistor is an electronic device containing at least 3 layers of semiconductor and electrical contacts, used in a circuit as amplifier, detector, or switch. B: base C: collector E: emitter 16

17. Ic C mA B µA E Ib < Back Next > Transistors Transistor as a current amplifier The base current Ib controls the collector current Ic Ic is many times larger than Ib. When Ib = 0, Ic = 0 When Ib changed, it is amplified by the transistor, producing larger change in Ic. 17

18. < Back Next > Transistors Transistor as a switch The transistor can be used as a switch to switch on a lamp, L. The light-dependent resistor (LDR) has resistance of 2 kΩ in bright light and 20 kΩ in the dark. During the day, resistance R1 is much less than resistance R2. So the potential difference across LDR is much smaller than across R2. The base current Ib is small, the collector current Ic is small, and the relay is not activated. The lamp L is off. The reverse happens when in the dark. R1 increases to maximum, potential difference across LDR increases, and Ib increases. The transistor amplifies the increase resulting in large Ic, thus activating relay and lamp L is switched on. Other devices may be used in place of LDR for other functions. 18

19. < Back Next > A X NOT logic gate It is also called the inverting buffer. Input Output Truth table Boolean equation X = A Logic Gates Logic gates= switching circuits used in computers and electronic devices A logic gate has one or more inputs but only one output. Its action is summarized by an equation in Boolean algebra, or with a truth table. 19

20. A X = A • B B A Making a NAND gate out of transistors and resistors X = A • B < Back B Next > Input Output A B AND NAND 0 0 0 • 0 = 0 0 = 1 0 0 0 • 1 = 0 0 = 1 1 0 1 • 0 = 0 0 = 1 1 1 1 • 1 = 1 1 = 0 Logic Gates AND and NAND logic gates AND NAND 20

21. < Back Next > Input Output A B OR NOR 0 0 0 • 0 = 0 0 = 1 0 1 0 • 1 = 1 1 = 0 1 0 1 • 0 = 1 1 = 0 1 1 1 • 1 = 1 1 = 0 Logic Gates OR and NOR logic gates A OR X = A + B B A NOR X = A + B B 21

22. Summary What you have learned: • Uses of Cathode Ray Oscilloscope < Back 2. Semiconductor diodes 3. Transistors 4. Logic gates 22 Thank You