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TL441-EP Fiches technique(PDF) 8 Page - Texas Instruments |
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TL441-EP Fiches technique(HTML) 8 Page - Texas Instruments |
8 / 19 page TL441EP LOGARITHMIC AMPLIFIER SGLS268 − OCTOBER 2004 8 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 APPLICATION INFORMATION Although designed for high-performance applications such as infrared detection, this device has a wide range of applications in data compression and analog computation. Basic Logarithmic Function The basic logarithmic response is derived from the exponential current-voltage relationship of collector current and base-emitter voltage. This relationship is given in the equation: m • VBE = In [(IC + ICES)/ICES] where: IC = collector current ICES = collector current at VBE = 0 m = q/kT (in V − 1) VBE = base-emitter voltage The differential input amplifier allows dual-polarity inputs, is self-compensating for temperature variations, and is relatively insensitive to common-mode noise. Logarithmic Sections As can be seen from the schematic, there are eight differential pairs. Each pair is a 15-dB log subsection, and each input feeds two pairs, for a range of 30-dB per stage. Four compensation points are available to allow slight variations in the gain (slope) of the two individual 15-dB stages of input A2 and B2. By slightly changing the voltage on any of the compensation pins from their quiescent values, the gain of that particular 15-dB stage can be adjusted to match the other 15-dB stage in the pair. The compensation pins also can be used to match the transfer characteristics of input A2 to A1 or B2 to B1. The log stages in each half of the circuit are summed by directly connecting their collectors together and summing through a common-base output stage. The two sets of output collectors are used to give two log outputs, Y and Y (or Z and Z), which are equal in amplitude, but opposite in polarity. This increases the versatility of the device. By proper choice of external connections, linear amplification, and linear attenuation many different applications requiring logarithmic signal processing are possible. Input Levels The recommended input voltage range of any one stage is given as 0.01 V to 1 V. Input levels in excess of 1 V may result in a distorted output. When several log sections are summed together, the distorted area of one section overlaps with the next section and the resulting distortion is insignificant. However, there is a limit to the amount of overdrive that can be applied. As the input drive reaches ±3.5 V, saturation occurs, clamping the collector-summing line and severely distorting the output. Therefore, the signal to any input must be limited to approximately ±3 V to ensure a clean output. Figure 10 INPUT A1 CA2 INPUT A2 CA2’ Log Log Log Log Log Log Log Log −15 dB −15 dB −15 dB −15 dB ΣΣ YY Z Z Outputs INPUT B1 CB2 INPUT B2 CB2’ Functional Block Diagram |
Numéro de pièce similaire - TL441-EP |
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Description similaire - TL441-EP |
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