4.9  Amplifying signals

Most of the electrical signals that record sounds are very weak. For example, a guitar pickup generates electricity as the metal string vibrates in a magnetic field. The amount of electricity that can be generated in this way is limited and typical voltages are in the order of 100 mV. To become useful, this signal needs to be amplified. This means, literally, that the amplitude of the signal has to be increased.

One way to amplify a signal is to use an operational amplifier (op-amp) with two resistors connected to form an amplifying feedback circuit, as shown in Figure 37.

Figure 37  Circuit to generate and amplify a weak signal (screenshot from Multisim Live)

Show description|Hide description

The electrical circuit to create a weak wave has three alternating current generators vertically in series on the left. From bottom to top, these are labelled V1, V2 and V3. They have frequencies of 400, 1200 and 2000 hertz, and corresponding amplitudes of 0.07 volts, 0.02 volts and 0.01 volts. These waves combine to form a more complicated wave that approximates a square wave.

On the right is an op-amp in a non-inverting amplifier circuit, with the wave created by the alternating current generators providing V_in. The feedback resistors are set to R1 equals 100 ohms and R2 equals 900 ohms. The output of the amplifier, V_out, is on the right with a green voltage probe attached. The input to the amplifier circuit has a blue voltage probe attached after the three AC voltage generators and before the op-amp input.

Figure 37  Circuit to generate and amplify a weak signal (screenshot from Multisim Live)

The gain of the amplifier is defined to be

where is the input voltage and is the output voltage. For this kind of circuit, the gain is given by the formula

The circuit in Figure 37 is made using the Multisim Live simulation package. The three circular objects on the left are here used to generate a signal. V1 generates a voltage sine wave with amplitude 0.07 V and frequency 400 Hz. V2 generates a voltage sine wave with amplitude 0.02 V and frequency 1200 Hz. V3 generates a voltage sine wave with amplitude 0.01 V and frequency 2000 Hz. Together, they generate the very weak signal shown in Figure 38.

Maximise

Figure 38  The waveform to be amplified (screenshot from Multisim Live)

Show description|Hide description

This is a Multisim Live graph showing the input to the op-amp, V_in, as recorded by the blue voltage probe in Figure 37. The wave approximates a square wave and has a very small amplitude, oscillating around zero volts.

Figure 38  The waveform to be amplified (screenshot from Multisim Live)

The original circuit shown in Figure 37 gives the result shown in Figure 39.

Maximise

Figure 39  The signal (blue) and the amplified signal (green) (screenshot from Multisim Live)

Show description|Hide description

This is a Multisim Live graph showing the input and output of the op-amp, V_in and V_out, as recorded respectively by the blue and green voltage probes in Figure 37. The blue wave is the same as the one shown in Figure 38. It approximates a square wave and has a very small amplitude, oscillating around zero volts. The green (amplified) wave has a much greater amplitude. It approximates a rectangular wave, having small wiggles on the top and bottom where it should be flat.

Figure 39  The signal (blue) and the amplified signal (green) (screenshot from Multisim Live)

Op-amps are able to amplify signals many thousands of times. Here the amplification is by a gain factor of 10 so that the signal and the amplified signal can both be seen on the same scale. Typically a signal of 100 mV would be amplified by a factor of 50 or more to bring the result to the order of magnitude ±5 V. Some signals from sensors are much weaker and may require a gain in the order of hundreds or even thousands.

This concludes your lightning visit to the world of electronic signals and filtering. You have seen how effectively noise can be removed from a signal, and how different sounds (such as ‘yes’ and ‘no’) appear in both the time and the frequency domain. As well as investigating the theoretical properties of signals, you have seen how they can be amplified using op-amps. If you are interested in going further, a version of the Multisim Live simulator can be used online at no cost – see www.multisim.com [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .