Hardware Reference
In-Depth Information
volt power supply and you could ind an op amp that would work at that level. What happens
in practice is that the output will only go as high as the power supply. Also the open loop gain
is too high to be useful most of the time, and so when you design a circuit that you want to
use as an ampliier some negative feedback is applied, as shown in Figure 11-6. Don't con-
fuse this with positive feedback, sometimes known just as feedback or howl around, when an
ampliier's output is fed into an input, like a microphone picking up the ampliied sound.
Figure 11-6:
A non-inverting
ampliier.
Negative feedback means feeding a proportion of the output back into the - (negative) input so
that the input in efect gets turned down. Consider the circuit in Figure 11-6 and assume that
there is zero volts on Vin, and also zero volts on the output. Also imagine that the two resistors
have the same value. Now suddenly Vin is changed to 1V so the diference between the two
inputs is also 1V. So the output sets of to amplify this diference 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 voltage of the output back
into the negative input. At this stage the diference 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 negative input. Eventually a balance
point is reached when the voltage on the two inputs is exactly the same, and so the ampliier's
output will not get any higher. his balance point, in this case, happens when the output is
exactly twice the input, so in efect 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 10. he actual
formula for calculating the gain is shown in Figure 11-6.
So armed with that information you can set about to design the beat extracting circuit whose
schematic is shown in Figure 11-7.
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