F5-C5 Electronics

Question 1

Define thermionic emmision

Answer 1

Emission of electroms from a heated metal surface

Question 2

Define cathode rays

Answer 2

A beam of fast moving electrons from cathode to anode

Question 3

How to increase thermionic emmision?

Answer 3

. Imcrease temperature of metal
. Increase surface area of metal
. Coat metal with barium oxide/ strontium oxide

Question 4

Define doping

Answer 4

Addition of foreign objects into semi conductors to increase its conductivity

Question 5

Give an example of doping substance used in n type semiconductors

Answer 5

Anitmony
Phosphorus
Arsenic
(APA)

Question 6

Give an example of doping substance used in p type semiconductors

Answer 6

(BAGI)
Boron
Alluminium
Gallium
Indium

Question 7

Explain current flow mechanism in forward biased diode connection

Answer 7

• holes in p-type semiconductor moves towards n type semiconductor.
• electrons in n-type semiconductor moves to p type semiconductor
• depletion layer becomes smaller causing Junction voltage to decrease until EMF < junction voltage
• current is large enough to pass through diode to light up the bulb

Question 8

What is the function of capacitors? Explain its working principle

Answer 8

Capacitors are used to smooth out current.

In the first cycle, diode is forward biased. Current flows through the capacitor and it charges. Energy is stored in the capacitor. At the second cycle, diode is reversed biased. Current does notflow through capacitor. Capacitor discharge and current from capacitor flows through resistor

Question 9

What happens to collector current when base current is zero? Why?

Answer 9

Collector current is zero. Collector current is controlled by base current

Question 10

In a circuit where transistor acts as amplifiers, why is a capacitor needed

Answer 10

To prevent direct current from flowing to the speaker and the transistor

Question 11

Explain transistor as LDR in the morning

Answer 11

• resistance of LDR is low - > base voltage is low - > base voltage lower than minimum voltage required - > base current cannot flow - >transistor not on, collector current cannot flow - > bulb does not light up

Question 12

Explain transistor as thermistor when temperature is high

Answer 12

Resistance of transistor decrease -> base resistance at base circuit increase - > base voltage increase until larger than minimum voltage - > v case current can flow - >transistor turns on - >collector current flows - >alarm rings

Question 13

Explain transistor as a current amplifier.

Answer 13

1. Sound signal is converted into electrical signal.
2. Varying current flows through capacitor.
3. Small change in base current causes a big change in collector current.
4. Sound waves with larger amplitude but same frequency is produced.

Question 13

Explain transistor as a current amplifier.

Answer 13

1. Sound signal is converted into electrical signal.
2. Varying current flows through capacitor.
3. Small change in base current causes a big change in collector current.
4. Sound waves with larger amplitude but same frequency is produced.

Question 14

What is the formula to find base voltage?

Answer 14

Base voltage =Base resistance (R 2) / Total Resistance (R2+ R 1) X Input voltage.

Question 15

Explain full wave rectification

Answer 15

l. Consists of 4 diodes
2. During both first and 2nd half cycle , semiconductor diode is forward biased.. Current flows arcoss and complete the circuit.

Question 16

Why is npn transistor more frequently used than pnp transistors?

Answer 16

• npn transistor containes more electrons
• pnp transistor contains more holes
• mobility of electrons is better than that of holes.