Hardware Reference
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resistors have the same value. Now suddenly Vin is changed to 1V so the difference between
the two inputs is also 1V. So the output sets off to amplify this difference into 100,000V.
However, as the output rises to one volt then the voltage on the negative input will have
risen to half a volt, because the two resistors act as a potential divider and feed half the volt-
age of the output back into the negative input. At this stage the difference between the two
voltages is only half a volt, so the output tries to amplify this by 100,000 to give an output of
50,000V. But the higher the voltage gets on the output the more of it is fed back to the nega-
tive input. Eventually a balance point is reached when the voltage on the two inputs is exactly
the same and so the amplifier's output will not get any higher. This balance point, in this
case, happens when the output is exactly twice the input, so in effect the whole circuit has a
gain of 2. You can make the gain anything you want, within reason, by simply altering the
ratio of the two resistors. So if you feed a tenth of the output back into the negative input,
you will have a gain of ten. The actual formula for calculating the gain is shown in Figure 3-6.
So armed with that information you can set about to design the beat extracting circuit whose
schematic is shown in Figure 3-7.
Figure 3-7:
The schematic of
the beat driver.
This uses two op amps which conveniently come in one package. The signal passes through
C1 to remove any DC component and then into a pot so that you can set the level into the
amplifier. This first op amp is configured just like the previous example as a non-inverting
amplifier, only this time there is a capacitor across the feedback resistor. This means that for
low frequencies the gain is determined by the value of resistor R2. But as the frequency
increases, the capacitive reactance of the capacitor shorts out the feedback resistor to lower
the gain. So in this section you have combined a low-pass filter with an amplifier.
The output of this amplifier is passed through a diode. This is a component that will only let
electricity flow in one direction, in this case from the amplifier into capacitor C3. So as the
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