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Vs |
R1 |
R2 |
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- |
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Vo |
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+ |
R3 |
R4 |
-Vs
• When used in this mode, it is probable that both inputs may have the same voltage that is not zero. The result of this common offset is that the output will drift with the common inputs. The technical measure is the Common Mode Rejection Ratio (CMRR). This is generally measured by the manufacturer, and provided in the device specifications.
5.1.2.9 - Low Pass Filter
• A Low pass filter will enable us to cut off the higher frequency components of an input signal,
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Cf |
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Rf |
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I |
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R1 |
I |
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VI |
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Vo |
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- |
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+ |
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Recognizing that the inverting input is at ground we may write the following expressions for the current,
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VI = IR1 |
Vo |
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1 |
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= –I |
----------------------- |
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1 |
+ jω |
C |
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---- |
f |
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R |
f |
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Next, we can combine these expressions to find the gain of the system,
I |
= |
VI |
= |
–Vo |
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----- |
Rf |
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R1 |
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-------------------------- |
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1 + jω C |
R |
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f |
f |
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Vo |
= |
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–Rf |
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----- |
-----------------------------------R1 + R1jω CfRf |
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VI |
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NOTE: The gain will drop as the frequency rises. Past a certain point the gain will be very low. The corner frequency and gain are,
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Rf |
gain = –----- |
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R1 |
ω c = |
1 |
---------- |
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CfRf |