Radio,Television And Electronic Paper 2, WASSCE (SC), 2019

Question 4

1. State the three types of multivibrator.
2. Give one property each of the multivibrators stated in 4(a)(i).
Fig. 3 is the diagram of a Colpitt’s oscillator.
State the function of:
1. resistors R1 and R2;
2. capacitor Cin;
3. capacitor Cout.
State three types of oscillators other than the Colpitt’s oscillator.

Observation

The Chief Examiner reported that a large percentage of the few candidates attempted question 4 did not respond well to 4(b) as majority of them stated the conventional functions of the components as against the functions of the components in the circuit.

The expected answers were:

1. Types of multivibrators
  - Astablemultivibrator
  - Monostablemultivibrator
  - Bistablemultivibrator
2. Properties of multivibrators
  - Astablemultivibrator
    - Generates square waves of its own without any external trigger pulse
    - It is a free running multivibrator
    - Has no stable state
    - Has two quasi-stable states
  - Monostablemultivibrator
    - Does not generate square wave of its own
    - Has one permanent state
    - Has one temporary/quasi stable state
    - Generates one shot/pulse
    - Requires a trigger signal
  - Bistablemultivibrator
    - Requires trigger pulse to switch states
    - Has two stable states
    - It is flip-flop/trigger circuit/binary in nature
1. Function of R1 and R2
  - Form the voltage divider for the circuit
  - Provide bias to the transistor
  - Used to establish transistor Q-point
  - Used to regulate current flow in amplifier
2. Function of capacitor C_in
  - Feeds back RF voltage from output to input
  - Acts as coupling capacitor
  - Acts asd.c. blocking capacitor
3. Function of capacitor C_out
  - Acts as blocking capacitor/blocks d.c. from the tank circuit
  - Acts as coupling capacitor to
  - Couples output to the load
Types of oscillators
- Wien-bridge/RC oscillator
- Phase-shift/RC oscillator
- Hartley /LCTuned oscillator
- Tuned base/gate/LC oscillator
- Tuned collector/drain/LC oscillator
- Crystal oscillator