Chapter2-Log Amplifiers.pdf

21SELE322: Chapter 2 0 Fundamentals of Log and Antilog Amplifiers There are several applications of log and antilog amplifiers. Antilog computations math require functions such as ln x, log x, sinh x etc. these are performed with log-antilog amplifiers. Log amplifiers can p...

21SELE322: Chapter 2 0 Fundamentals of Log and Antilog Amplifiers There are several applications of log and antilog amplifiers. Antilog computations math require functions such as ln x, log x, sinh x etc. these are performed with log-antilog amplifiers. Log amplifiers can perform direct dB display on digital voltmeter and spectrum analyzer. 1 The circuit diagram of a basic log amplifier using diode is shown in figure. The diode is used in the feedback path. 2  VD VT I D = I o (e − 1) where ID is the diode current, Io is the reverse saturation current , VD is the voltage across the diode, VT is the thermal voltage, and η is the ideality factor, also known as the quality factor or sometimes emission coefficient. The ideality factor η typically varies from 1 to 2. where k is the Boltzmann constant [(1.38 × 10−23 joule per kelvin (K)], T is the absolute temperature of the p–n junction, and q is the magnitude of charge of an electron (1.6 X 10-19) Coulombs. By virtual ground VA=VB= 0. As the op-amp draws zero input current, from the input side, we write The current I also flows through the feedback path. Hence we write, Voltage across the diode is VB −VO = −VO. Hence using the diode current equation we write, 4  Substituting for I, we get As IOR is constant dc voltage, it is referred as Vref. Then 5 It is seen that the output voltage is a function of logarithm of the input voltage. The basic log amplifier can also be constructed by replacing diode by a transistor. The output is proportional to the logarithm of the input given by 6  Antilog Amplifier The basic antilog amplifier is shown. The positions of diode and resistor are interchanged w.r.t log amplifier. By virtual ground VA=VB=0. Let Vin be the drop across the diode. From the diode equation, 7 As the op-amp draws zero input current If = I. Hence we write, As IOR is constant dc voltage, it is referred as Vref. becomes, Thus from the above equation it is seen that the output voltage is exponential function of the input. The basic antilog amplifier can also be constructed by replacing diode by a transistor. The output is proportional to the antilogarithm of the input. This equation holds good for both the cases. This equation is temperature dependent and hence as temperature changes the equation varies. So the antilog amplifier also suffers from the same problem as that of log amplifier. Hence some sort of temperature compensation is required. The antilog amplifier is subjected to noise, bias currents, offset voltages, drifts and frequency stability problems. Transistorized circuits give accuracy, reduced bulk resistance and high operating ranges.