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Transistors in Switch and Amplifier Configurations.

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Transistors in Switch and Amplifier Configurations


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.

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IB = IC / HFE = 0.069 / 96 = 7.19 x 10-4A

R = (VCC – VBE) / IB

R = (12 – 0.74) / 7.19 x 10-4 = 15666Ω

RB(a) = 15666 – 1000 – 10000 = 4666Ω

        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 an ohmmeter. Apply the power supply and record the readings of the various meters. If VCE (sat) (or a lower value) can’t be achieved, is RB(a) to high a value?

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 = 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.

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  1. Unijunction Transistor (UJT)









This is one of the simplest oscillator circuits based on a unijunction transistor. In the above circuit, the potential difference across the capacitor gradually builds up and is released almost instantly. This type of oscillator is known as a ‘relaxation oscillator’.

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