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Logarithmic Amplifier

Module by: Don Johnson

Summary: Logarithmic amplifiers

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Figure 1
Diode Circuit
Diode Circuit (diode3.png)

Here is a circuit involving a diode that is actually simpler to analyze than the previous one. We know that the current through the resistor must equal that through the diode. Thus, the diode's current is proportional to the input voltage. As the voltage across the diode is related to the logarithm of its current, we see that the input-output relation is

v out =-kTqln v in R I 0 +1 v out k T q v in R I 0 1 (1)
Clearly, the name logarithmic amplifier is justified for this circuit.

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