Electrical Engineering ⇒ Topic : Logarithmic Amplifier

Gaurav
 
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 nonlinear voltampere relationship of the PN junction itself. This relationship is given by ............(1) where V_{f} is the forward voltage across the diode, V_{T} 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 V_{1} is assumed to be e^{(V}_{f}^{/mV}_{T}^{)} > 1, the logarithmic relationship may be expressed as:
Let m, V_{1}, and I_{1} be taken as constants ignoring the temperature effects. The schematic diagram is shown in Figure (a). The noninverting terminal is grounded. Therefore, there is a virtual ground at inverting terminal. So the current I through R_{1 }is equal to I_{f}. Thus,
................ (2)
Figure (a) Basic logarithmic amplifier.  
 
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