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AD8004AR-14-REEL データシート(PDF) 10 Page - Analog Devices |
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AD8004AR-14-REEL データシート(HTML) 10 Page - Analog Devices |
10 / 17 page REV. C AD8004 –9– THEORY OF OPERATION The AD8004 is a member of a new family of high speed current- feedback (CF) amplifiers offering new levels of bandwidth, distortion, and signal-swing capability vs. power. Its wide dynamic range capabilities are due to both a complementary high speed bipolar process and a new design architecture. The AD8004 is basically a two stage (Figure 30) rather than the conventional one stage design. Both stages feature the current-on-demand property associated with current feedback amplifiers. This gives an unprecedented ratio of quiescent current to dynamic performance. The important properties of slew rate and full power bandwidth benefit from this performance. In addition the second gain stage buffers the effects of load impedance, significantly reducing distortion. A full discussion of this new amplifier architecture is available on the data sheet for the AD8011. This discussion only covers the basic principles of operation. DC AND AC CHARACTERISTICS As with traditional op amp circuits the dc closed-loop gain is defined as: A V = G = 1 + R F R N noninverting operation A V = G =− R F R N inverting operation The more exact relationships that take into account open-loop gain errors are: A V = G 1 + 1 − G A O ( s ) + R F T O ( s ) for inverting (G is negative) A V = G 1 + G A O ( s ) + R F T O ( s ) for noninverting (G is positive) In these equations the open-loop voltage gain (AO(s)) is common to both voltage and current-feedback amplifiers and is the ratio of output voltage to differential input voltage. The open-loop transimpedance gain (TO(s)) is the ratio of output voltage to inverting input current and is applicable to current-feedback amplifiers. The open-loop voltage gain and open-loop transim- pedance gain (TO(s)) of the AD8004 are plotted vs. frequency in TPCs 15 and 18. These plots and the basic relationships can be used to predict the first order performance of the AD8004 over frequency. At low closed-loop gains the term (RF/TO(s)) dominates the frequency response characteristics. This gives the result that bandwidth is constant with gain, a familiar property of current feedback amplifiers. An RF of 1 k has been chosen as the nominal value to give optimum frequency response with acceptable peaking at gains of +2/–1. As can be seen from the above relationships, at higher closed-loop gains reducing RF has the effect of increasing closed- loop bandwidth. Table I gives optimum values for RF and RG for a variety of gains. VP Q1 Q2 IPP IPN INP IPN VN ZI IQ1 Q3 Q4 IE CP1 CP1 A2 CL RG RF VO CD ICQ + IO VO´ IQ1 AD8004 A2 CP2 CD A3 RL A1 A1 Z2 Figure 5. Simplified Block Diagram |
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