electronics assignment corrections needed
dirk_diggler
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Student Declaration on Submission I confirm that all the work in this coursework is my own. |
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Date.
25-07-2016 |
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General Feedback by Tutor
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Sampled by IV: Sign |
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Tasks for Assessment 1
All answers must be your own worK
Note: Copy and paste these cover and task sheets to the front of your written work as you must sign (electronic signature) and date these in order to have your assessment marked and graded
Task 1 [P1]
(a) State the two types of diodes being utilised in the schematic diagram shown below
Rectifier/Power Diode
Zener Diode
(b) Explain the application of each diode in the above diagram
Power diode – it is used to rectify the ac power supply to dc voltages in the bridge circuit.
Zener diode – it is used to produce a stabilized voltage on the output side for a varying conditions of load current. In operation, it will conduct an enough current in order to maintain the output voltage (V)
(c) With the use of diagrams/graphs explain the operation of each diode
Power Diode
It is a semiconductor device with two P-N terminals
When there is a higher potential at terminal A as compared to K, the diode is said to be forward biased and forward current will flow through it. Otherwise, it is reverse biased and it does not conduct in this state but small current known as leakage current flows. If the reverse voltage is increased further, then current will conduct. This voltage is known as breakdown voltage or avalanche breakdown and the diode will turn permanently to a conductor in any direction. This is shown by the graph below.
In our circuit the four power diodes are arranged to form a bridge rectifier. This allows us to convert the ups and downs of AC current into just ups. This is the first stage of converting AC power to a smooth and steady DC power supply, the process is known as full wave rectification.
Zener Diode
Zener diodes are special diodes designed to break down at a low voltage and is used as a voltage regulator. It has the following symbol
In forward bias, the diode behaves normally like any diode passing the rated current. In reverse biasing, as soon as the reverse voltage exceeds the rated voltage, there will be an avalanche breakdown. This will limit any increase of voltage. The current will increase dramatically to a value that is at its maximum as determined by the circuit. This current is known as saturation current and remains fairly constant over a wide range of input voltages. Voltage across the zener diode becomes stable at a voltage called the zener voltage. At this voltage, current through the diode can be accurately controlled.
From the graph above, we can see a region of fairly constant negative voltage regardless in the amount of current that passes though the diode. This shows its ability to control the output voltage.
Task2 [P2]
(a) With the aid of diagrams/graphs explain the operation of the transistor in the diagram shown below.
The transistor is an enhancement type MOSFET and it requires a gate pulse to switch it ON. MOSFETs have three terminals, Gate, Drain and Source. They can either be a P-MOS or an N-MOS.
The Metal Oxide Gate of the MOSFETs is electrically insulated from the n and p channels of the main semiconductor using SiO2. This acts as a plate capacitor and makes the input resistance a high value in Mega-ohm ranges. This isolation and high resistance does not allow current to flow through the gate, hence current flows through the main channel which is proportional to the amount of voltage that is applied to the input.
Here, the conducting channel is made non-conductive by lightly doping it. This makes the device to be normally off when bias gate voltage, VGS = 0. For an N-MOS, current will start to flow through the device on application of a gate voltage grater than the threshold gate voltage VTH.
If we connect a positive (+ve) Gate voltage to a n-Mos eMOSFET it pulls in more electrons towards the oxide layer around the gate hence enhancing the thickness of the channel permitting more flow of current. Increasing gate voltage increases the drain current and also decreases the channel resistance. This transistor becomes more useful because it has a low on resistance. A graph of its characteristics is shown below
(b) From diagram shown below, state the name of the circuit and explain how it operates including the purpose of the resistors.
The figure is an N-MOS amplifier
Resistors R1 and R2 will form the voltage divider network of the circuit. This voltage will be used to bias the transistor making it ON. RD and Rs will determine the amplification of the circuit. When a signal is applied through the capacitor, it is amplified depending on the values of the drain and source resistors to give an amplified signal an Vo.
Task 2 (contd.)
(b) With the aid of diagrams/graphs explain the operation of the transistor in the diagram shown below.
The transistor is a N-P-N Bipolar transistor. It consists of three layers N, P and N. the transistor has three terminals they are the base, emitter and collector.
This is a current controlled device. When a small current (base current) is applied at the base. It flows from the emitter to the base then a large controlled current (collector current) will flow to the collector. The amount of the collector current is proportional to the base current.
If the is no application of the base current, the transistor will go off therefore controlled current will not go to the collector.
(c) From the diagram shown below, state the name of the circuit and explain how it operates including the purpose of the resistors.
The circuit is called Voltage Divider in common emitter configuration.
When switch SW1 is closed,
Resistors R3 and R1 are used to divide the voltage. Their values are chosen in a way to give the correct bias at the base.
R2 is the coupling resistor with the transistor
R4 prevents shorting the terminals when the transistor is in operation.
So when the switch is closed, the correct bias current will flow through R2. This will put the transistor on. Therefore, collector emitter current will flow through R4. Hence, the LED will switch ON.
Task 3 [P3]
(a) Identify and explain the operation of 3 logic gates shown in the diagram below including their truth tables
AND Gate – Gives a High output when all its inputs are High
Y=AB where Y is the output
OR Gate – Gives a High output when one or all of its inputs are High.
Y = A + B
NOT Gate (Inverter) – Inverts the input from Low to High and vice versa
Y=Ā
(b) From the diagram shown below, explain how it operates by giving the truth table and Boolean expression at each stage marked with a box
Ā
AḊ
ĀBCD
ĀD/AḊ
A
ĀBC
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Task 4 [P7]
(a) Use a computer simulation package such as Yenka to build the BJT (BC108) amplifier circuit, with an AC input sine wave Vi = 50 mV @ 1 kHz and DC blocking capacitor C = 10µF, as shown below:
(b) Using the simulator’s Oscilloscope or Graph function to display and measure the gain Av of the amplifier. Clearly show both Vi and Vo on your graph. Indicate the X (time) and Y (voltage) scales used.
(c) Confirm your measurement by also calculating the gain for the circuit.
(d) Briefly explain the role of each component in your circuit (150-200 words)
R1 and R2 are used to divide the voltage so that the correct bias current can be obtained. The bias voltage can be calculated as shown below.
R3 and R4 are used in the determination of the gain of the BJT amplifier. Their values are chosen in a way to achieve the desired gain.
The capacitor is used to block the dc element of the signal from the amplification circuit which may lead to noise or errors in the circuit.
The transistor is used as the amplifier.
(e) As part of your evidence, print-screen a copy of your computer based circuit layout and graph. Save your simulator file as ‘U4A2T1_amp1’ and attach a copy.
Task 5 [D1]
(a) Modify the circuit in Task 4 to achieve a gain of 10 with an AC input Vi =10mV @1 KHz. Save your simulator file as ‘U4A2T2_amp2’ and attach a copy.
(b) Modify the circuit in Task 5a to find the maximum (i.e. open) gain of the BJT amplifier. Save your simulator file as ‘U4A2T2_amp3’ and attach a copy.
(c) Show evidence of using the simulator’s Oscilloscope or Graph function, by displaying and measuring the gain Av of the amplifier for both task parts.
(d) As part of your evidence, print-screen copies of your computer based circuit layouts and graphs.
Task 6 [P8]
A logic system is required that will sound a buzzer if the conditions within a PCB etching tank are incorrect. The buzzer must operate if the state of three sensors is as follows:
· Sensor A is ON and B is ON but C is OFF
· Sensor A is ON but B and C are both OFF
· Sensor B is ON but A and C are both OFF
· Sensors B and C are both ON but A is OFF
Under all other conditions the buzzer must not operate.
a) Complete the truth table for the above conditions
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Q(Buzzer) |
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b) Derive the Boolean Expression for the Truth Table shown above and draw the circuit required to implement the Boolean expression.
It can be minimized as
c) Use a computer simulation package such as Yenka to build your design, with the logic latch switches and indicators connected for each combination of inputs which can verify your design.
d) Briefly explain the role of each component in your circuit (150-200 words)
NOT Gate – it is a digital gate that is used to invert the input. In the circuit it changes the input from High to Low or Low to High.
AND Gate – it is a digital gate that is used to give a logic High output when all its inputs are logic High. Otherwise, it will give logic Low output.
OR Gate – It is a digital gate used to give a High output when one of its inputs is logic High. When all the inputs are logic Low, it will give a logic Low output.
e) As part of your evidence, print-screen a copy of your computer based circuit layout. Save your simulator file as ‘U4A2T3_logic1’ and attach a copy.
References
· Electronics a first course- Owen Bishop.
· Electronics for dummies-Dickon Ross
· Electrical circuit theory and technology- John Bird
· Www.howstuffworks.com
Assessment A1
S
W
1
R
4
R
2
B
C
1
0
8
R
1
R
3
9
V
0
V
L
E
D
6.00
()120
50
Vo
GainAv
Vim
===
2
12
2.2*12
*2.2
2.29.8
0.7
2.20.7
1.5
1
5050
33.33
1.5
//33.33//132.25
3.9
120.93
30.30
Bcc
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BBE
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Av
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===
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=
-
-
===
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===
2
12
*
Bcc
R
VV
RR
=
+
FABCABCABCABA
=+++
FABAC
=+
