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AD8307AR 데이터시트(PDF) 9 Page - Analog Devices |
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AD8307AR 데이터시트(HTML) 9 Page - Analog Devices |
9 / 20 page AD8307 –9– REV. A Note that only two design parameters are involved in determin- ing VY, namely, the cell gain A and the knee voltage EK, while N, the number of stages, is unimportant in setting the slope of the overall function. For A = 5 and EK = 100 mV, the slope would be a rather awkward 572.3 mV per decade (28.6 mV/dB). A well designed log amp will have rational scaling parameters. The intercept voltage can be determined by using two pairs of transition points on the output function (consider Figure 22). The result is: VX = EK A N +1/ A−1 () () (5) For the case under consideration, using N = 6, we calculate VZ = 4.28 µV. However, we need to be careful about the inter- pretation of this parameter, since it was earlier defined as the input voltage at which the output passes through zero (see Fig- ure 19). But clearly, in the absence of noise and offsets, the output of the amplifier chain shown in Figure 21 can be zero when, and only when, VIN = 0. This anomaly is due to the finite gain of the cascaded amplifier, which results in a failure to maintain the logarithmic approximation below the lin-log transition (point in Figure 22). Closer analysis shows that the voltage given by Equation 5 represents the extrapolated, rather than actual, intercept. Demodulating Log Amps Log amps based on a cascade of A/1 cells are useful in baseband applications, because they do not demodulate their input signal. However, baseband and demodulating log amps alike can be made using a different type of amplifier stage, which we will call an A/0 cell. Its function differs from that of the A/1 cell in that the gain above the knee voltage EK falls to zero, as shown by the solid line in Figure 23. This is also known as the limiter func- tion, and a chain of N such cells is often used to generate a hard-limited output, in recovering the signal in FM and PM modes. SLOPE = A SLOPE = 0 AEK 0 EK INPUT A/0 TANH Figure 23. A/0 Amplifier Functions (Ideal and Tanh) The AD640, AD606, AD608, AD8307 and various other Analog Devices communications products incorporating a logarithmic IF amplifier all use this technique. It will be apparent that the output of the last stage can no longer provide the logarithmic output, since this remains unchanged for all inputs above the limiting threshold, which occurs at VIN = EK /A N–1. Instead, the logarithmic output is now generated by summing the outputs of all the stages. The full analysis for this type of log amp is only slightly more complicated than that of the previous case. It is readily shown that, for practical purpose, the intercept voltage VX is identical to that given in Equation 5, while the slope voltage is: VY = AEK log10 A () (6) Preference for the A/0 style of log amp, over one using A/1 cells, stems from several considerations. The first is that an A/0 cell can be very simple. In the AD8307 it is based on a bipolar- transistor differential pair, having resistive loads RL and an emitter current source, IE. This will exhibit an equivalent knee- voltage of EK = 2 kT/q and a small signal gain of A = IERL /EK. The large signal transfer function is the hyperbolic tangent (see dotted line in Figure 23). This function is very precise, and the deviation from an ideal A/0 form is not detrimental. In fact, the rounded shoulders of the tanh function beneficially result in a lower ripple in the logarithmic conformance than that obtained using an ideal A/0 function. An amplifier built of these cells is entirely differential in struc- ture and can thus be rendered very insensitive to disturbances on the supply lines and, with careful design, to temperature variations. The output of each gain cell has an associated transconductance (gm) cell, which converts the differential out- put voltage of the cell to a pair of differential currents, which are summed simply by connecting the outputs of all the gm (detec- tor) stages in parallel. The total current is then converted back to a voltage by a transresistance stage, to generate the logarith- mic output. This scheme is depicted, in single-sided form, in Figure 24. IOUT VIN A4VIN VLIM A/0 gm A/0 gm A3VIN A/0 gm A2VIN A/0 gm AVIN gm Figure 24. Log Amp Using A/0 Stages and Auxiliary Sum- ming Cells The chief advantage of this approach is that the slope voltage may now be decoupled from the knee-voltage EK = 2 kT/q, which is inherently PTAT. By contrast, the simple summation of the cell outputs would result in a very high temperature coef- ficient of the slope voltage given by Equation 6. To do this, the detector stages are biased with currents (not shown in the Fig- ure) which are rendered stable with temperature. These are derived either from the supply voltage (as in the AD606 and AD608) or from an internal bandgap reference (as in the AD640 and AD8307). This topology affords complete control over the magnitude and temperature behavior of the logarithmic slope, decoupling it completely from EK. A further step is yet needed to achieve the demodulation response, required when the log amp is to convert an alternating input into a quasi-dc baseband output. This is achieved by altering the gm cells used for summation purposes to also implement the rectification function. Early discrete log amps based on the progressive compression technique used half-wave rectifiers. This made post-detection filtering difficult. The AD640 was the first commercial monolithic log amp to use a full-wave rectifier, a practice followed in all subsequent Analog Devices types. |
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