Transistors in Switch and Amplifier Configurations Essay Sample
- Pages: 6
- Word count: 1,584
- Rewriting Possibility: 99% (excellent)
- Category: physics
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Introduction of TOPIC
Transistors are electronic devices usually used as amplifiers or oscillators in communications and computer systems. They consist of a tiny piece of semiconducting material, usually silicon or germanium, to which three or more electrical connections are made. The basic components of the transistor are similar to those of a triode vacuum tube and include an emitter, which corresponds to the heated cathode of the triode tube as the source of electrons. The transistor was developed by American physicists John Bardeen, Walter Houser Brattain and William Bradford Shockley.
This assignment requires us to carry out a number of tasks. For the first stage of the assignment, I will be performing four tests relating to the BC147 transistor to determine its characteristics. Graphs will be plotted with the results gained from this. Then I will carry out an experiment relating to a transistor switch circuit, answering a few questions relating to the experiment.
Another experiment will be performed for the third part of the assignment, this time relating to the common emitter amplifier. This will require several calculations to be made, such as resistor values. These values will be needed once the construction of the circuit begins. The building of the circuit is one of tasks required for this assignment. The circuit will be tested during construction and once complete. D.C. testing will check that the calculated values match the measured values. A.C. testing will be carried out using the oscilloscope, once the circuit is complete.
Extra research is needed to fully complete the assignment.
SECTION B (2e) – Transistor Switch Circuit Experiment
To calculate the base resistance (RB), in a simple transistor circuit.
1. 2 digital voltmeters 6. locktronics 10K potentiometer
2. 2 analogue student meters 7. locktronics 1K resistor
3. 1 50/100mA shunt 8. locktronics special BFY51 resistor
4. locktronics base board 9. links & connection leads
5. 1 fairy light 10. 12V power supply
Measure the HFE of the selected transistor and write it on the line below together with the cradle number.
1 (HFE = 96)
Construct the circuit shown below on the locktronic base board. Make sure that the potentiometer and the 1k are placed as shown since this will provide protection to the base-emitter circuit. RB(a) is not provided since its value is determined in response to the calculation.
Calculate the sum of RB(a),(b),(c) for the component configuration you have assembled so that the transistor just saturates and switches on the lamp. Refer to the above diagram and previous notes. Show all-working below.
Ic (sat) = (VCC – VCE) / RL
Ic (sat) = (12 – 0.35) / 170 = 0.069A
IB = IC / HFE = 0.069 / 96 = 7.19 x 10-4A
R = (VCC – VBE) / IB
R = (12 – 0.74) / 7.19 x 10-4 = 15666?
RB(a) = 15666 – 1000 – 10000 = 4666?
Since RB(a),(b),(c) are known values, the calculation will reflect this before your final choice for RB is made.
Now complete the following tests:
TEST 1: Adjust the potentiometer (RB(c)) to achieve the calculation as measured on
IC (sat) = 68.1mA
IB (sat) = 0.705mA
VCE (sat) = 0.74v
VBE (sat) = 0.24v
TEST 2: State the practical value of HFE using the recorded test figures. State the percentage error between this and the original value of HFE.
HFE = IC / IB
HFE = 68.1 / 0.705 = 96.6
% Error = (Calculated HFE / Measured HFE) x 100%
% Error = (96.6 / 96) x 100%
% Error = 100.625 – 100 = 0.625%
TEST 3: Determine the maximum value that VCE may rise to, before there is a significant fall in the collector current.
VCE = 1.90v
QUESTION 1: When RB(c) was adjusted, at what point would a power rating for the transistor need consideration.
More current flows as the voltage is decreased. Therefore, the lamp will brighten. However, as the current increases, the transistor starts to heat 