Electrical Engineering ⇒ Topic : Logarithmic Amplifier
It is possible to produce logarithmic and antilogarithmic or exponential amplifiers having good linearity with the help of a high performance OP Amp and simple circuitry. These amplifiers use the non-linear volt-ampere relationship of the P-N junction itself. This relationship is given by
where Vf is the forward voltage across the diode, VT is the terminal voltage, m is a parameter ≈ 2 for small currents in silicon devices, and Is is the saturation current of the diode.
If the operating range of V1 is assumed to be e(Vf/mVT) > 1, the logarithmic relationship may be expressed as:
Let m, V1, and I1 be taken as constants ignoring the temperature effects. The schematic diagram is shown in Figure (a).
The non-inverting terminal is grounded. Therefore, there is a virtual ground at inverting terminal. So the current I through R1 is equal to If. Thus,
Figure (a) Basic logarithmic amplifier.
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