An introduction to electronics
An introduction to electronics

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An introduction to electronics

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.

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

The gain of the amplifier is defined to be

cap g equals cap v sub out divided by cap v sub in

where cap v sub in is the input voltage and cap v sub out is the output voltage. For this kind of circuit, the gain is given by the formula

cap g equals cap r sub one plus cap r sub two divided by cap r sub one

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.

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

SAQ 18

  • a.What is the gain for the circuit shown in Figure 37?
  • b.What would the resistor cap r sub two have to be to make the gain 100?

Answer

  • a.
     
    equation sequence cap g equals open 100 plus 900 close normal cap omega divided by 100 postfix times normal cap omega equals 10
  • b.Rearranging the equation for cap g in terms of cap r sub two gives

    equation sequence cap r sub two equals cap g times cap r sub one minus cap r sub one equals cap r sub one times open cap g minus one close

    So for a gain of 100, with cap r sub one = 100 Ω,

    equation left hand side cap r sub two equals right hand side 100 postfix times equation left hand side normal cap omega multiplication open 100 minus one close equals right hand side 9900 postfix times normal cap omega

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

Described image
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)] .

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