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TPA3122D2 데이터시트(PDF) 11 Page - Texas Instruments |
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TPA3122D2 데이터시트(HTML) 11 Page - Texas Instruments |
11 / 20 page www.ti.com INPUT RESISTANCE Ci IN Zi Zf Input Signal f= 1 2 Z C p i i (1) INPUT CAPACITOR, CI f = c 1 2 Z C p i i –3dB fc (2) C = i 1 2 Z f p i c (3) TPA3122D2 SLOS527A – DECEMBER 2007 – REVISED DECEMBER 2007 For design purposes, the input network (discussed in the next section) should be designed assuming an input impedance of 8 k Ω, which is the absolute minimum input impedance of the TPA3122D2. At the higher gain settings, the input impedance could increase as high as 72 k Ω Table 1. Gain Setting AMPLIFIER GAIN (dB) INPUT IMPEDANCE (k Ω) GAIN1 GAIN0 TYPICAL TYPICAL 0 0 20 60 0 1 26 30 1 0 32 15 1 1 36 9 Changing the gain setting can vary the input resistance of the amplifier from its smallest value, 10 k Ω ±20%, to the largest value, 60 k Ω ±20%. As a result, if a single capacitor is used in the input high-pass filter, the -3 dB or cutoff frequency may change when changing gain steps. The -3-dB frequency can be calculated using Equation 1. Use the ZI values given in Table 1. In the typical application, an input capacitor I) is required to allow the amplifier to bias the input signal to the proper dc level for optimum operation. In this case, CI and the input impedance of the amplifier (ZI) form a high-pass filter with the corner frequency determined in Equation 2. The value of CI is important, as it directly affects the bass (low-frequency) performance of the circuit. Consider the example where ZI is 20 kΩ and the specification calls for a flat bass response down to 20 Hz. Equation 2 is reconfigured as Equation 3. In this example, CI is 0.4 µF; so, one would likely choose a value of 0.47 µF as this value is commonly used. If the gain is known and is constant, use ZI from Table 1 to calculate CI. A further consideration for this capacitor is the leakage path from the input source through the input network I) and the feedback network to the load. This leakage current creates a dc offset voltage at the input to the amplifier that reduces useful headroom, especially Copyright © 2007, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s) :TPA3122D2 |
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