# help with revision for exams

1.  use the superposition method to calculate the current through a 12 ohm resistor in figure 1:

2. refer to figure 2 and calculate the following:

2.1. IA

2.2. ZA

3.1. the following information is available for an LC  π – filter circuit:

RL=15kΩ

L=0,5H

Vdc=100V

Vr(rms)=10V

C1=10µF

C2=15µF

Rs=300Ω connected across the supply of 250V, 50Hz

Type = full-wave

Calculate the following:

3.1.1. the output ripple voltage

3.1.2. the output direct current voltage

3.1.3. the percentage ripple factor across the first capacitor

3.1.4. the percentage ripple factor across the second capacitor

3.2.1. TRUE OR FALSE? When a zener diode is used as a voltage regulator, it must be connected in forward bias.

3.2.2. TRUE OR FALSE? The tunnel diode can be used in high speed applications.

4.1. state five points that can be affected by negative feedback to improve the performance of a circuit.

4.2. draw a neat push-pull amplifier circuit which uses two NPN transistors. Clearly show the input and output waveforms.

4.3. state three applications of a uni-junction transistor.

4.4. an amplifier has a voltage gain of 50. If the input signal is 2mV, the value of the output voltage will be 0,01V. True Or False?

4.5. name three regions in which a transistor can operate.

4.6. Field effect transistor are (current/voltage/both) operated. Choose the correct answer.

5.1. using an input as a square wave, draw the output waveforms of the following operational amplifiers.

5.1.1. the intergrator

5.1.2. the non-inverter

5.1.3. the differentiator

5.1.4. the inverting

5.2. Give the name of an operational amplifier used without feedback.

6.1. Show with the aid of a neat, labelled diagram how a quadrac can be used to control the speed of a small AC motor. Also show the input and output waveforms.

6.2. Draw a neat, labelled block diagram of a closed-loop motor speed control.

6.3. Draw the characteristic curve of a light activated silicon controlled rectifier.

7.1. State three factors which influence the capacitance of capacitive transducer.

7.2. Draw and explain the operation of a transducer that is used to monitor the speed of rotation of a shaft.

8. an oscilloscope displays a sine wave over six vertical divisions and six horizontal divisions. The vertical amplifier is 2V per a division, and the time base is at 20 µs/division.

Calculate the following:

8.1. the amplitude

8.2. the peak to peak value

8.3. the frequency

8.4. the average DC value

1. A 12V battery is connected in series with an R1=5Ω resistor. Two parallel resistors R2 and RL, with values 10 Ω and 8 Ω respectively are also included in the circuit. Use Kirchhoffs method to calculate the voltage drop across the load resistor RL.

2. study the figure 1 and calculate the following:

2.1. the resonance frequency

2.2. the impedance of the circuit

2.3. the total current flow

2.4. Draw the phasor diagram

3. give one use of the following diodes:

3.1.1. the junction diode

3.1.2. the zener diode

3.1.3. the varactor diode

3.1.4. the tunnel diode

3.2. state three advantages of an LC filter

3.3. explain the term voltage regulation.

3.4. the following information is given for a capacitor filter.

- input = full wave rectifier

- frequency = 50Hz

- capacitor value = 150µF

- maximum secondary voltage = 10 V

4.1. true or false

4.1.1. the common emitter amplifier produces at 0 deg C phase shift on the output.

4.1.2. in the FET the output current controls the input voltage

4.1.3. MOSFETs are widely used in digital applications

4.1.4. in an N-Channel JFET the majority carriers are electrons

4.1.5. UJT is a transistor with two

4.2. explain by means of a neat, labelled block diagram what you understand by negative feedback in amplifiers.

4.3. calculate the values of IC and RB if the following information is given:

- VCC=10V

- Vbe=0,6V

- RC=1kΩ

- Rin=0,12MΩ

- B = 200

5.1. state four properties of an operational amplifier.

5.2. calculate the input voltage of an operational amplifier whose output voltage is in phase with input voltage, if the following information is available.

- ouput voltage = 4V

- Feedback resistance = 10kΩ

- Input resistance=1kΩ

Also draw the circuit diagram of this amplifier

5.3. draw the symbol, input and output wave forms of a comparator.

6.1. name three methods that can be used to switch off an SCR.

6.2. Demonstrate, with the aid of a labelled characteristic curve, the principle of operation of the following thyristors:

6.2.2. LASCR

6.3. match each of the following:

 Column A Column B 6.3.1. Triac A A multilayer, three-terminal, bi-directional, semi-conductor device 6.3.2. Diac B A four layer p-n-p-n device 6.3.3. Scr C Basically a triac and diac combined together in a single package 6.3.4. Lascr D A bi-directional trigger diode used as an AC Switch 6.3.5. quadrac E A four-layer, p-n-p-n device turned on by light

6.4. boiling water in a kettle is an example of a open-loop system. Only state True or False.

7.1. explain, with the aid of a labelled sketch, how the pressure of a process can be observed by means of a differential transformer transducer.

7.2. name four points that should be considered before a transducer is selected for a particular application.

8. for the wave-form shown in figure 2, the amplifier gain setting is 0,6 V/div and the time-base setting is 2µsec/div.

Calculate the following:

8.1. the amplitude

8.2. the peak-to-peak value

8.3. the period

8.4. the frequency

8.5. the RMS value