electronics blanks

8/2/2019 Electronics Blanks

1/7

9 ELECTRONICS9.1 Cathode Ray OscilloscopeTHERMIONIC EMISSION

1 Metals contain .. electrons.2 The free electrons are in . motion.3 When a metal is heated, the free electrons gain energy.4 The free electrons move . and faster.5 Some free electrons can overcome the attraction of the positive ions and . from the surface of the

metal.6 Definition of Thermionic EmissionThermionic emission is the emission of free electrons from the surface of a hot metal .7 The Cathode Ray Tube

8 Cathode rays are a beam of . moving and high. electrons.

9 Properties of cathode rays(a) Move in a . line(b) Deflected by an .. field

(c) Deflected by a magnetic field(d) Has . and momentum.

The kinetic energy of the electrons will beconverted to .. energy when the beamof electrons strike a fluorescent screen.

CATHODE RAY OSCILLOSCOPE

Maltese Cross+

3 kV

Flourescent screen

Filament


2/7

1 The Electron Gun(a) Hot filament/cathode emits electrons by emission.(b) Electrons are . and . by the anode.(c) The electrons emerge from the anode as a beam of cathode rays.

2 The Deflection System(a) Y plates - to deflect the electron beam

- connected to the external Y-input

(i) The electron strikes the (ii) The electron beam is deflected (iii) The electron beam oscillatescentre of the screen upwards up and down

(b) X plates - to deflect the electron beam

- connected to the time base circuitWhen the time base is switched on, the electronbeam isdeflected from the left to the right at aconstant speed.This motion is repeated over and over again.

3 The Display which consists of a .. screen(a) When the electron beam strikes the screen, kinetic energy of the electrons is converted to light energy(b) A .. spot is seen on the screen

No Y-input DC voltage at Y-input Larger DC voltage at Y-input

Time base OFF

Time base ON

Time base ONat higherfrequency


3/7

No Y-input AC voltage at Y-input Larger AC voltage at Y-input

Time base OFF

Time base ON

Time base ON

at higherfrequency

4 Uses of the Cathode Ray OscilloscopeMEASURING POTENTIAL DIFFERENCE or VOLTAGE

(a) Voltage gain is 0.5 V cm1 (b) Voltage gain is 2.0 V / div. (c) Voltage gain is 5.0 V cm1

Type of voltage : .. Type of voltage : .. Type of voltage : ..Peak voltage = Peak voltage = Peak voltage =

MEASURING SHORT TIME INTERVALS

(a) Time base is set to 2.0 ms cm1 (b) Time base is set to 5 ms cm1

Time interval between the two signals = Period of the wave, T=

Frequency,

T

f1

= =

DISPLAYING WAVEFORMS(a) The signal from the waveform to be displayed is connected to the Y-input.

The time base is adjusted until a suitable waveform is displayed on the screen.


4/7

9.2 Semiconductor DiodesSEMICONDUCTORS1 In terms of conduction of electricity, materials can be divided into 3 categories.

Conductors Semiconductors Insulators

GoldSilverCopper

AirRubberPolythene

Conducts electricity Does not conduct electricity

2 Common properties of semiconductors(a) Elements in Group of the Periodic Table (b) Each atom has .. valence electrons(c) Covalent bonds between atoms (d) Contains free electrons and ...(e) Conducts small currents when a small amount of impurity is added. Does not obey Ohm's law.

3 DOPING is the process of replacing some the atoms in a semiconductor with atoms of other elements.(a) Doping with pentavalent atoms (5 valence electrons)

i. Examples of pentavalent atoms are : phosphorus (P), arsenic (As), antimony (Sb)

ii. Each pentavalent atom donates a electroniii. Free electrons become the . charge carrier in the semiconductoriv. The semiconductor becomes a n-type semiconductor

(b) Doping with trivalent atoms (3 valence electrons)i. Examples of trivalent atoms are : aluminium (Al), gallium (Ga), indium (In)ii. Each trivalent atom contributes a .. (which is a positive charge carrier)iii. Holes become the majority charge carrier in the semiconductoriv. The semiconductor becomes a p-type semiconductor

4 There are 2 types of semiconductors

n-type semiconductor p-type semiconductor

Doping with pentavalent atoms Doping with trivalent atomsMajority charge carrier : electrons Majority charge carrier : holes

Minority charge carrier : holes Minority charge carrier : electrons

P-N JUNCTION DIODE = SEMICONDUCTOR DIODE

Symbol for a diode

1 When a diode is forward bias: 2 When a diode is reversed bias:(a) The depletion layer becomes . (a) The depletion layer becomes .(b) The resistance of the diode (b) The resistance of the diode (c) The diode conducts a current (c) No current flows through the diode

Current flows. Bulb lights up. No current. Bulb is not lighted.

p n


5/7

2 The diode conducts a current when it is bias. The current is NOT directly proportional to the potentialdifference. Therefore, a diode does NOT obey .. law.

RECTIFICATION1 A diode can act as a RECTIFIER to convert an alternating current to a . current.2 Half-wave rectification

During the first half-cycle:the diode is forward bias and conducts a current.

For the next half-cycle:the diode is reverse bias and no current flowsin the circuit

3 The output direct current voltage can be smoothed by connecting a . parallel to the resistor.a. When the diode is forward bias, the capacitor is ...b. When the diode is reverse bias, the capacitor discharges to maintain an almost constant voltage across the

resistor.

Current

Voltage


6/7

1

2 3

4

R

4 Full-wave rectification4 diodes connected in a bridge circuit is used.

During the first half-cycle:Diodes 1 and 3 are forward bias while diodes2 and 4 are reverse bias. The current flowsthrough the resistor Rfrom Q to P.

For the next half cycle:Diodes 2 and 4 are forward bias while diodes1 and 3 are reverse bias. The current flowsthrough the resistor Rfrom Q to P also.Therefore the current in resistor Ris a direct current.

9.3 Transistors1 There are 2 types of transistors : n-p-n transistor and p-n-p transistor.

NPN TRANSISTOR PNP TRANSISTOR

2 Two Important Currents in a Transistor

BC

E

BIBC

E

IC

IE

BIBC

E

IC

IE

BC

E

IB

IC

IB

+I

C

a. If IB = 0, IC = 0

b. IB switches on ICTherefore the transistor can act as a ..

c. IB > ICTherefore the transistor can act as an

Basecurrent

Collectorcurrent


7/7

3 The transistor as an amplifier

4 The transistor as a switchLight operated switch (b) Heat operated switch

The base current IB depends on VBE. The resistance of the thermistor

As VBE increases, IB also increases. decreases when its temperature

increases.

(a) VCBis the voltage across the resistor R(b) VBEis the voltage across the LDR(c) The 6 V from the power supply is shared by Rand the LDR.(d) A higher voltage is applied across the higher resistance.

(e) When conditions are bright:The resistance of the LDR is low.

The voltage VBE across the LDR is low.

The base current is very small, too small to switch on the collector current. The bulb is not lighted.

(f) When the surroundings suddenly become dark:The resistance of the LDR increases rapidly.

The voltage VBE across the LDR increases.

The base current increases, and switches on the collector current. The bulb lights up.

Microphone

IC

I E

I B

Earphone

I B

I C

C

B

E

LDR

R

I BIC

C

B

E

R

T

I BIC

electronics blanks

Documents