Measure the changes in the transfer function with temperature, the effects of input offsets, and the offset correction. Wideband amplifier with logarithmic output characteristic. Replaces several discrete log amp ics product description the ad640 is a complete monolithic logarithmic amplifier. A simple log amp whose transfer function satisfies equation 3 is shown in figure 3.
Amplifier, a true logarithmic if amplifier integrated circuit was developed by plessey research caswell limited funded by mod dcvd. Pdf document tags logarithmic if amplifier abstract. The op amp operates as an inverting amplifier with a feedback diode. The input, vin, is divided by a voltage, vx, because the argument of a logarithm must be a simple dimensionless ratio. The figure shows the transfer function at 900 mhz, and over temperature, of the ad83, a 100mhzto2. Figure 1 shows an amplifier that provides a logarithmic output for a linear. Theory of operation of logarithmic amplification anadyne inc.
An operational amplifier can be configured to function as a logarithmic amplifier, or simply log amplifier. Adjusting the scale factor k allows alteration of the transferfunction slope, while adjusting the reference current, i ref, allows shifting the logarithmic intercept. Another reason for using a logarithmic scale for magnitudes is that the output, input, and complex transfer function amplitudes are connected in a product form as yj. Log amplifier is a nonlinear circuit configuration, where the output is k times the logarithmic value of the input voltage applied. The graph below shows the response of the ad8307 logarithmic amplifiers with similar input conditions varying only the intensity of the input signal. Log amplifiers find the applications in computations such as multiplication. A wideband amplifier of electrical signals comprising, a plurality of amplifier gain stages having a logarithmic output transfer function, each gain stage having an input, an output and a characteristic offset voltage, said plurality of amplifier gain stages arranged in a series, x1, x2. The slope and intercept are the two specifications that define the transfer function of the log amp, that is, the relationship between output voltage and input signal level. Transfer function is eo a log where i1 is a negative input current and ir is the resistorprogrammed internal reference current see.
Properties of logarithmic functions exponential functions an exponential function is a function of the form f xbx, where b 0 and x is any real number. Logarithmic amplifier goal design a logarithmic amplifier built with operational amplifiers and discrete devices, and verify its operation. Pdf this paper describes the theory of operation, mathematical analysis, modeling and. How many parameters describe a log transfer function. The pair of traces marked 2v represents the a high and a low outputs of a single channel to an input of 2 volts in intensity. With the diode being virtually anodegrounded, the op amp needs to generate a negative output voltage to forward bias the diode. Adjusting the scale factor k allows alteration of the transfer function slope, while adjusting the reference current, i ref, allows shifting the logarithmic intercept. The overall design consists of two majors partsa cascade of limiting current amplifiers and a difference amplifier. Design of a 120db pseudologarithmic amplifier implemented in. The duts response to a sweep generated by an analogue generator is rectified and smoothed by a lowpass filter. Pdf analysis and design of the true piecewise approximation.
The basic output equation of a log amplifier is v vout k ln vinvref. Abstract a highspeed logarithmic amplifier circuit for a pds microdensitometer is discussed. The maximum allowable power dissipation at any allowable ambient temperature is. Passive filters university of california, san diego. By utilizing the log function, you can compress the dynamic range of signals encountered by a system. Answers to some questions about logarithmic amplifiers. All structured data from the file and property namespaces is available under the creative commons cc0 license. Logarithmic and antilogarithmic amplifiers electronics. We see that the slope of the characteristic,and hence the. The applications of log amplifiers extend beyond their use in radar and radio.
The circuit is designed around a 757p logamp module which replaces. Computing the logarithm of a signal is a very useful signal processing technique to compress a large dynamic range into a more manageable smaller range. In some situations, especially in rf domain, monolithic logarithmic amplifiers are also used to reduce number of components and space used, as well improve bandwidth and. The pseudologarithmic amplifier accepts a singleended. Properties of logarithms shoreline community college. When a diode is connected in the feedback loop of an inverting amplifier, the output voltage is a logarithmic function of the input voltage.
Since the log function is nonlinear, it is clear that a log amplifier is not linear in the sense discussed in chapter 3. Frequency response analysis of amplifiers and filters. Logarithmic amplifiers are available at mouser electronics. This is accomplished by the use of computer controlled test equipment, in which the output of an rf signal generator is stepped over the input dynamic range of the unit under test. A logarithmic transfer characteristic is shown in figure 1524. The simple logarithmic amplifier uses a junction diode as a nonlinear element. First of all, no amplifier can produce a logarithmic transfer function over a dynamic range which includes an input signal level of zero, since log 0.
Log amplifier is a linear circuit in which the output voltage will be a constant times the natural logarithm of the input. Transferfunction measurement with sweeps four audio. As implied by the name, a logarithmic amplifier expresses an output that is related to its input by the mathematical log function the logarithmic base is not important, as the different logbased functions are related by constants. The ideal transfer function of such a log amp is v ideal k x log 10 i log i ref.
Logarithmic amplifier chip design and application considerations. The main parameter is the voltage transfer function in the frequency domain, hvj. The gain stages are dccoupled throughout, though each also employs a demodulator comprising a fullwave rectifier, allowing operation in both. Nov 18, 2015 a logarithmic log amplifier produces an output that is proportional to the logarithm of its input. How can op amp be used to get nonlinear transfer functions. For a forwardbiased diode, the forward current is larger by far than the reverse current i f i s. The resulting voltage is input to a differential amplifier whose other input is the voltage derived from a discrete. Figure 1 shows an amplifier that provides a logarithmic output for a linear input current or voltage. Log amps can respond to negative inputs in three different ways. A linear systems frequency response and plotting it in logarithmic scale hossein taheri school of electrical and computer engineering georgia institute of technology you work with a circuits transfer function all the time in this course and make extensive use of the logarithmic scale. Abstract a highspeed logarithmicamplifier circuit for a pds microdensitometer is discussed. An311theory and applications of logarithmic amplifiers. Investigation of logarithmic amplifier output response. A linear systems frequency response and plotting it in logarithmic scale hossein taheri school of electrical and computer engineering georgia institute of technology you work with a circuits transfer function all the time in this course and make extensive use of.
For a given input current, i in, the corresponding output. Design of memristor model based logarithmic amplifier article pdf available in international journal of research 71. This article derives the overall dc log amp transfer function from the v betoic transfer characteristic of bipolar transistors. Logarithmic functions log b x y means that x by where x 0, b 0, b. Integrated logarithmic amplifiers for industrial applications. Logamps are useful in applications where the input signal may vary by a large factor. The amplifier gains are chosen to provide a logarithmic transfer function for the overall structure, as will be shown in the preferred novel design procedure given later in this section. Files are available under licenses specified on their description page. A dc4ghz true logarithmic amplifier semantic scholar. Two ad640s in cascade can provide up to 95 db of dynamic range at reduced bandwidth. However, if the linear part of the transfer function is extrapolated until it crosses the horizontal axis 0 v theoretical output, it passes through a point.
The logarithm function is indeterminate for negative values of x. One of the oldest methods of bringing a transfer function onto paper already involved sweeps as excitation signal. In this technical note, we discuss some general principles relating to logarithmic amplification. Transfer function the ideal transfer function is vout k log where. Specifications design an amplifier with logarithmic transfer function between. This page was last edited on 23 august 2018, at 17. Kuhn march 24, 20 a log converter is a circuit that converts an input voltage to an output voltage that is a logarithmic function of the input voltage. The circuit is designed around a 757p logamp module which replaces the fmi 531 negative logantilog device of the original. A multistage logarithmic converter of the successivedetection or progressivecompression type including circuitry providing an accurate, temperaturestabilized logarithmic transfer function. As can be seen, the input is a step function that is 1. Logarithmic amplifier telecommunications research laboratories. A linear systems frequency response and plotting it in. Integrated dc logarithmic amplifiers application note maxim. In a forwardbiased diode, the voltage drop across the diode varies proportionally to the log of the current through it.
Measure the changes in the transfer function with temperature, the effects of input offsets, and apply offset correction. The schottky diode detector rectifies the incoming rf signal and produces a video voltage and the logarithmic amplifier transforms the scale of the video voltage from linear scale to logarithmic scale in order to observe the rf signals with a wide amplitude range. This notice is taken from the ieee pspb operations manual, section 8. Among the classes of logarithmic amplifiers, the dc log amp continues to be a very powerful, costeffective solution for compressing widedynamicrange sensor signals. A log amp must satisfy a transfer function of the form. This is because the current to voltage conversion of a bipolar transistor is a natural logarithm. Thus, equation 2 simplifies to 3 a simple log amp whose transfer function satisfies equation 3 is shown in figure 3.
Logarithmic amplifier by inserting a bipolar transistor in the loop, an amplifier with logarithmic characteristic can be constructed. A single ad640 provides up to 50 db of dynamic range for frequencies from dc to 120 mhz. K the scale factor with units of voltsdecade i1 numerator input current i2 denominator input current. Stage a samples wxcl06 have already been evaluated by the industrial sponsor, plessey radar limited, who constructed experimental 6 and 8stage amplifiers with a purposebuilt 50 ohm output buffer.
Typical indication of the performance of a logarithmic amplifier is the measurement of the log transfer function. Baseband log amps are used to compress ac signals such as in audio and video circuits. A logarithmic log amplifier produces an output that is proportional to the logarithm of its input. An opamp, diode, and resistor can together be used to create a logarithmic amplifier, where the output voltage is proportional to the log of the input voltage. Singlesupply, highspeed, precision logarithmic amplifier datasheet. As a result of this transfer function, the output voltage swing of a logarithmic amplifier is proportional to the. Accuracy accuracy considerations for a log ratio amplifier are some. Log amplifiers find the applications in computations such as multiplication and division of signals.
Parallelsummation logarithmic amplifier transfer function. The constant, vy, has the dimensions of voltage, because the output is a voltage. The scale of the horizontal axis the input is logarithmic, and the ideal transfer characteristic is a straight line. The frequency response of a device or a circuit describes its operation over a specified range of signal frequencies by showing how its gain, or the amount of signal it lets through changes with frequency. To design a logarithm amplifier circuit, high performance opamps like lm1458, lm771, lm714 are commonly used and a compensated logarithm amplifier may include more than one.
Logarithmic amplifier an overview sciencedirect topics. In general, the purpose of this class of amplifier is to condense a large input dynamic range into a much smaller, manageable one through a logarithmic transfer function. A graph of the transfer characteristic of a log amp is shown in figure 2. This article derives the overall dc log amp transfer function from the vbetoic transfer characteristic of bipolar transistors.
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