Cadence OrCAD PCB Designer

Figure 1. A simple, one-transistor amplifier that can be simulated using Spice.
you will waste a lot of time and may lose large portions of your work. Please ask for help when
you encounter problems – you are likely to make matters worse if you try to fix things yourself.
You should know this by now, but a reminder is never a bad idea: Save your work frequently
and take regular backups of important circuits.
2 One-transistor amplifier: simple analysis
Figure 1 shows the circuit of a simple amplifier with one transistor. It is intended to amplify
audio signals, say from 20 Hz – 20 kHz. I have used international standard symbols for the
components, such as the zigzag line for a resistor rather than the anonymous rectangle preferred
by the IET.
An engineer always makes a rough design using pencil and paper before simulating it. You
will not learn how to design this particular circuit until the Analogue Electronics 2 course later
in year, so we shall ‘reverse engineer’ it instead. The calculations are simple and should be
done in your laboratory book. You should be pleasantly surprised to find that you don’t need to
know much about transistors to analyse this. In fact you did most of it in Electronic Engineering
1Y. Make sure that you recognise the emitter, base and collector.
We first find the bias point, operating point or quiescent point. This means the conditions
when no signal is applied and the circuit is ‘resting’. There is no input and you can therefore
ignore the 10 mVac source.
1. Resistors R 1
and R 2
form a simple potential divider if we ignore the other components
attached to them. (We should go back and check this assumption when the currents are
known.) Calculate the voltage on the base of the transistor Q 1
.
Hint for demonstrators.
2.7 V ❦
4