The Transistor Amplifier

Measuring your amplifier

The capacitors in the circuit affect how it responds to a.c. signals. The input and output capacitors,

and

act as ‘d.c. blocks’. They tend to pass through any voltage fluctuations, but stop any external connections (to signal generators, 'scopes, etc) from affecting the d.c. levels in the circuit. Without them, the circuit would be liable to stop working as soon as you connect anything to it!

The 22

, is a ‘shunt’ capacitor. In principle, we could omit

, but if we did the amplifier voltage gain would be low. Can you explain why this would be the case?

Build your amplifier using the values you have been given for the capacitors and the values you have worked out for the resistors. Build the circuit using the transistor and board from the previous experiment.

To see photographs of what your circuit should look like, click on the image of a camera. Make your circuit as similar as you can to the amplifier shown in the photographs, but remember that your resistor values may be different!

As with earlier experiments, you will need to hand in this circuit when you are finished to get the experiment marked, so make sure you also put your name on it.

Switch the amplifier on (i.e. connect the power supply +15V & 0V lines and turn on the power!) and use the DVM to measure

, and

. You should find that

volts and

volts. If they're more than a volt or so away from these values, check you've built the circuit correctly. If not sure, ask a demonstrator.

Make a note on Diagram 6 of the voltages you measure and indicate the resistance values in your circuit.

You can now measure the a.c. properties of your circuit. Use your 'scope to observe the input and output voltages,

. Connect the signal generator to provide an input sinewave signal.

The voltage gain, G, of the circuit can be defined as the ratio

. Plot a graph showing how the gain varies with sinewave frequency. Plot a couple of dozen values over the range from 10Hz up to 50 kHz.

Note. The amplifier gain only means something when the amplifier behaves in a fairly linear manner. If the amplifier's operating point is very wrong — or if you use too large an input — the amplifier will visibly distort the signal. Watch out for this on the 'scope trace. The input should look like a good sinewave. The output should also look like a sinewave. If the output is visibly ‘flattened’ or ‘clipped’ then the amplifier is distorting the signal. Reduce the input level until the output looks OK.

You should find that the gain is quite frequency dependent, so the size of input you can use without distortion will also depend upon signal frequency.

Why is the gain frequency dependent? In particular, why does the gain fall away at low frequencies? What could be done to improve this?

You should also find that the amplifier tends to invert the signal — i.e. the output appears 'upside-down'. Why is this?

(If you don't know the answers to these questions, ask a demonstrator.)

Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.