Passive filters university of california, san diego. Measure the changes in the transfer function with temperature, the effects of input offsets, and apply offset correction. The pseudologarithmic amplifier accepts a singleended. 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. The overall design consists of two majors partsa cascade of limiting current amplifiers and a difference amplifier. Baseband log amps are used to compress ac signals such as in audio and video circuits. Log amps can respond to negative inputs in three different ways. When a diode is connected in the feedback loop of an inverting amplifier, the output voltage is a logarithmic function of the input voltage.
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. This article derives the overall dc log amp transfer function from the v betoic transfer characteristic of bipolar transistors. Transfer function the ideal transfer function is vout k log where. Two ad640s in cascade can provide up to 95 db of dynamic range at reduced bandwidth. A linear systems frequency response and plotting it in. Logarithmic amplifier by inserting a bipolar transistor in the loop, an amplifier with logarithmic characteristic can be constructed. A multistage logarithmic converter of the successivedetection or progressivecompression type including circuitry providing an accurate, temperaturestabilized logarithmic transfer function. The simple logarithmic amplifier uses a junction diode as a nonlinear element. Logarithmic amplifiers are available at mouser electronics. Wideband amplifier with logarithmic output characteristic. Integrated logarithmic amplifiers for industrial applications.
Logarithmic functions log b x y means that x by where x 0, b 0, b. In this technical note, we discuss some general principles relating to logarithmic amplification. Among the classes of logarithmic amplifiers, the dc log amp continues to be a very powerful, costeffective solution for compressing widedynamicrange sensor signals. Parallelsummation logarithmic amplifier transfer function. 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.
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. 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. Apr 18, 2019 an operational amplifier can be configured to function as a logarithmic amplifier, or simply log amplifier. Typical indication of the performance of a logarithmic amplifier is the measurement of the log transfer function. Logarithmic and antilogarithmic amplifiers electronics. Logarithmic amplifier chip design and application considerations. Transfer function is eo a log where i1 is a negative input current and ir is the resistorprogrammed internal reference current see. For a forwardbiased diode, the forward current is larger by far than the reverse current i f i s. 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. Adjusting the scale factor k allows alteration of the transferfunction slope, while adjusting the reference current, i ref, allows shifting the logarithmic intercept.
The applications of log amplifiers extend beyond their use in radar and radio. As can be seen, the input is a step function that is 1. 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. This page was last edited on 23 august 2018, at 17. Consumption dccoupled dlvas a detector log video amplifier dlva is a type of logarithmic amplifier in, output of the detector is compressed in the video amplifier section, making inputoutput relationship, voltage followed by a video amplifier with a logarithmic transfer function.
A single ad640 provides up to 50 db of dynamic range for frequencies from dc to 120 mhz. Integrated dc logarithmic amplifiers application note maxim. 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. The scale of the horizontal axis the input is logarithmic, and the ideal transfer characteristic is a straight line. 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. For a given input current, i in, the corresponding output. 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. This is because the current to voltage conversion of a bipolar transistor is a natural logarithm.
Log amplifier is a linear circuit in which the output voltage will be a constant times the natural logarithm of the input. Logarithmic amplifier an overview sciencedirect topics. 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. Logamps are useful in applications where the input signal may vary by a large factor. All structured data from the file and property namespaces is available under the creative commons cc0 license. The main parameter is the voltage transfer function in the frequency domain, hvj. 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 constant, vy, has the dimensions of voltage, because the output is a voltage. The maximum allowable power dissipation at any allowable ambient temperature is. 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. How many parameters describe a log transfer function. By utilizing the log function, you can compress the dynamic range of signals encountered by a system. Amplifier, a true logarithmic if amplifier integrated circuit was developed by plessey research caswell limited funded by mod dcvd.
The duts response to a sweep generated by an analogue generator is rectified and smoothed by a lowpass filter. 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. Logarithmic amplifier telecommunications research laboratories. A graph of the transfer characteristic of a log amp is shown in figure 2. Theory of operation of logarithmic amplification anadyne inc.
Since the log function is nonlinear, it is clear that a log amplifier is not linear in the sense discussed in chapter 3. The figure shows the transfer function at 900 mhz, and over temperature, of the ad83, a 100mhzto2. Singlesupply, highspeed, precision logarithmic amplifier datasheet. Answers to some questions about logarithmic amplifiers. The resulting voltage is input to a differential amplifier whose other input is the voltage derived from a discrete. Transferfunction measurement with sweeps four audio. 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 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. Design of memristor model based logarithmic amplifier article pdf available in international journal of research 71. Measure the changes in the transfer function with temperature, the effects of input offsets, and the offset correction.
The graph below shows the response of the ad8307 logarithmic amplifiers with similar input conditions varying only the intensity of the input signal. Investigation of logarithmic amplifier output response. Pdf design of memristor model based logarithmic amplifier. A logarithmic log amplifier produces an output that is proportional to the logarithm of its input. 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. The circuit is designed around a 757p logamp module which replaces. Log amplifier is a nonlinear circuit configuration, where the output is k times the logarithmic value of the input voltage applied. As a result of this transfer function, the output voltage swing of a logarithmic amplifier is proportional to the. The logarithm function is indeterminate for negative values of x. Logarithmic amplifier goal design a logarithmic amplifier built with operational amplifiers and discrete devices, and verify its operation. A dc4ghz true logarithmic amplifier semantic scholar. Properties of logarithms shoreline community college. Frequency response analysis of amplifiers and filters.
A logarithmic transfer characteristic is shown in figure 1524. A log amp must satisfy a transfer function of the form. Adjusting the scale factor k allows alteration of the transfer function slope, while adjusting the reference current, i ref, allows shifting the logarithmic intercept. With the diode being virtually anodegrounded, the op amp needs to generate a negative output voltage to forward bias the diode. The input, vin, is divided by a voltage, vx, because the argument of a logarithm must be a simple dimensionless ratio. An operational amplifier can be configured to function as a logarithmic amplifier, or simply log amplifier. Nov 18, 2015 a logarithmic log amplifier produces an output that is proportional to the logarithm of its input. In a forwardbiased diode, the voltage drop across the diode varies proportionally to the log of the current through it. 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. Pdf document tags logarithmic if amplifier abstract. An311theory and applications of logarithmic amplifiers.
How can op amp be used to get nonlinear transfer functions. Specifications design an amplifier with logarithmic transfer function between. 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. Log amplifiers find the applications in computations such as multiplication. The ideal transfer function of such a log amp is v ideal k x log 10 i log i ref. The basic output equation of a log amplifier is v vout k ln vinvref. We see that the slope of the characteristic,and hence the. Figure 1 shows an amplifier that provides a logarithmic output for a linear. The gain stages are dccoupled throughout, though each also employs a demodulator comprising a fullwave rectifier, allowing operation in both. 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. Files are available under licenses specified on their description page.
Pdf analysis and design of the true piecewise approximation. Abstract a highspeed logarithmicamplifier circuit for a pds microdensitometer is discussed. Accuracy accuracy considerations for a log ratio amplifier are some. Integrated dc logarithmic amplifiers application note. Design of a 120db pseudologarithmic amplifier implemented in. Abstract a highspeed logarithmic amplifier circuit for a pds microdensitometer is discussed. Figure 1 shows an amplifier that provides a logarithmic output for a linear input current or voltage. K the scale factor with units of voltsdecade i1 numerator input current i2 denominator input current. Pdf this paper describes the theory of operation, mathematical analysis, modeling and. Replaces several discrete log amp ics product description the ad640 is a complete monolithic logarithmic amplifier. This article derives the overall dc log amp transfer function from the vbetoic transfer characteristic of bipolar transistors. 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.
The op amp operates as an inverting amplifier with a feedback diode. Thus, equation 2 simplifies to 3 a simple log amp whose transfer function satisfies equation 3 is shown in figure 3. A simple log amp whose transfer function satisfies equation 3 is shown in figure 3. In this tutorial we have seen how the range of frequencies over which an electronic circuit operates is determined by its frequency response. Log amplifiers find the applications in computations such as multiplication and division of signals. 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. The circuit is designed around a 757p logamp module which replaces the fmi 531 negative logantilog device of the original. This notice is taken from the ieee pspb operations manual, section 8. One of the oldest methods of bringing a transfer function onto paper already involved sweeps as excitation signal.