up and could well blow. The power rating needs to be considered before the transistor can heat up.
QUESTION 2: Apart from the meter tolerances, which part of this practical work has the potential to give rise to significant differences between calculated and practical results.
The temperature in the room can increase or decrease and this will have an effect on the transistor’s HFE value. As the circuit is handled, the transistor will have an increase in temperature as a result of the body heat from the handler. Bad connections on the circuit will also effect the results. If the circuit is assembled inaccurately, there will be a greater resistance.
SECTION C (3d & e) – Measuring the Gain Characteristics and the Bandwidth – AC Testing
Once the circuit amplifier had been assembled and all measurements recorded, calculations to add capacitors were done. The calculations were as follows:
Lowest frequency, f = 50Hz
= 1 / T
Period, T = 1 /f
= 1 / 50
= 20 ï¿½ 10-3
As T = CR
The time constant is usually taken as five times the period of the lowest frequency.
So 5T = CR
5 ï¿½ 20 ï¿½ 10-3 = C ï¿½ 2000
Therefore C = 50ï¿½F (a 47ï¿½F was used instead)
As R3 = 200?
5 ï¿½ 20 ï¿½ 10-3 = C ï¿½ 200
Therefore C = 500ï¿½F (a 470ï¿½F was used instead)
The following capacitors were used:
A 50ï¿½F (a 47ï¿½F was used instead),
A 500ï¿½F (a 470ï¿½F was used instead),
A 10nF capacitor
Once the capacitors had been calculated and assembled, the gain and bandwidth needed to be obtained. A signal generator was set to produce a sine wave. It was connected to channel 1 of the oscilloscope to obtain a reading of 10mV peak-to-peak. The power supply was adjusted to apply 10V to the constructed amplifier. The input lead from the circuit amplifier was also connected to the signal generator. Channel 2 of the oscilloscope was attached to ground and to the 10nF capacitor to measure the output of the amplifier.
Input Voltage: 10mV
Output Voltage: 1.2V
The amplifier gain is equal to the ratio of the input to the output voltages.
e.g. gain = output / input
= 1.2/10 ï¿½ 10-3
The bandwidth for current or voltage amplifiers is defined as the range of frequencies at which the gain does not drop down further than 70.7%. The cut-off frequencies are the points at which fulfil the requirements for the amplifier gain. Below is a typical frequency response curve:
The bandwidth is calculated using the formula:
Bandwidth = Upper cut-off frequency – Lower cut-off frequency
Upper cut-off frequency: 21kHz
Lower cut-off frequency: 29Hz
Bandwidth = 21000 – 29
= 20971Hz or 20.97kHz
The bandwidth of the circuit amplifier is 20.97kHz
SECTION B (2a) – Specific function of two different types of transistor within a genuine circuit
1. Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
Below is a diagram of a typical MOSFET common-source amplifier. Low power MOSFETs are useful for radio frequency circuits. This is because they have a rapid response time. Amplifiers based on power MOSFETs are useful for power-control circuits. They have better thermal stability than bipolar transistors (less subject to thermal runaway).
2. 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’.