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AD8011ANZ Fiches technique(PDF) 9 Page - Analog Devices |
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AD8011ANZ Fiches technique(HTML) 9 Page - Analog Devices |
9 / 16 page REV. C AD8011 –9– THEORY OF OPERATION The AD8011 is a revolutionary generic high speed CF amplifier that attains new levels of BW, power, distortion, and signal swing capability. If these key parameters were combined as a figure of ac merit performance or [(frequency VSIG)/(distortion power)], no IC amplifier today would come close to the merit value of the AD8011 for frequencies above a few MHz. Its wide dynamic performance (including noise) is the result of both a new com- plementary high speed bipolar process and a new and unique architectural design. The AD8011 uses basically a two gain stage complementary design approach versus the traditional “single stage” complementary mirror structure sometimes referred to as the Nelson amplifier. Though twin stages have been tried before, they typically consumed high power since they were of a folded cascade design much like the AD9617. This design allows for the standing or quiescent current to add to the high signal or slew current induced stages much like the Nelson or single-stage design. Thus, in the time domain, the large signal output rise/fall time and slew rate is controlled typically by the small signal BW of the amplifier and the input signal step amplitude respectively, not the dc quiescent current of the gain stages (with the exception of input level shift diodes Q1/Q2). Using two stages versus one also allows for a higher overall gain bandwidth product (GBWP) for the same power, thus lower signal distortion and the ability to drive heavier external loads. In addition, the second gain stage also isolates (divides down) A3’s input reflected load drive and the nonlinearities created resulting in relatively lower distortion and higher open-loop gain. Overall, when high external load drive and low ac distortion is a requirement, a twin gain stage integrating amplifier like the AD8011 will provide superior results for lower power over the traditional single-stage complementary devices. In addition, being a CF amplifier, closed-loop BW variations versus external gain variations (varying RN) will be much lower compared to a VF op amp, where the BW varies inversely with gain. Another key attribute of this amplifier is its ability to run on a single 5 V supply due in part to its wide common-mode input and output voltage range capability. For 5 V supply operation, the device obviously consumes half the quiescent power (versus 10 V supply) with little degradation in its ac and dc performance characteristics. See Specifications. DC GAIN CHARACTERISTICS Gain stages A1/A1B and A2/A2B combined provide negative feedforward transresistance gain (see Figure 6). Stage A3 is a unity gain buffer that provides external load isolation to A2. Each stage uses a symmetrical complementary design. (A3 is also complemen- tary though not explicitly shown.) This is done to reduce second order signal distortion and overall quiescent power as discussed previously. In the quasi dc to low frequency region, the closed- loop gain relationship can be approximated as G = 1 + RF/RN noninverting operation G = –RF/RN inverting operation These basic relationships are common to all traditional opera- tional amplifiers. Due to the inverting input error current (IE) required to servo the output and the inverting IE RI drop VP Q1 Q2 IPP IPN IPN VN ZI IQ1 Q3 Q4 IE IR – IFC IR + IFC CP1 CP1 Z2 A2 CL RL ICQ – IO RF VO CD ICQ + IO –VI –VI VO IQ1 AD8011 A2 CP2 Z1 Z1 = R1 || C1 Z1 CD RL A1 A1 INP A3 Figure 6. Simplified Block Diagram |
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