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AD8391 Datasheet(PDF) 10 Page - Analog Devices |
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AD8391 Datasheet(HTML) 10 Page - Analog Devices |
10 / 20 page REV. A AD8391 –10– VO BIAS VN VP Figure 3. Simplified Schematic G = 1 IT = IIN CT RT IIN VOUT RG RF RIN + VIN – VO + – Figure 4. Model of Current Feedback Amplifier Feedback Resistor Selection In current feedback amplifiers, selection of the feedback and gain resistors will impact distortion, bandwidth, noise, and gain flatness. Care should be exercised in the selection of these resistors so that the optimum performance is achieved. Table I shows the recommended resistor values for use in a variety of gain settings for the test circuits in TPC 1 and TPC 19. These values are only intended to be a starting point when designing for any application. Table I. Resistor Selection Guide Gain RF ( )RG ( ) –1 909 909 –2 909 453 –3 909 303 –4 909 227 –5 909 178 GENERAL INFORMATION Theory of Operation The AD8391 is a dual current feedback amplifier with high output current capability. It is fabricated on Analog Devices’ proprietary eXtra Fast Complementary Bipolar Process (XFCB) that enables the construction of PNP and NPN transistors with fT’s greater than 3 GHz. The process uses dielectrically isolated transistors to eliminate the parasitic and latch-up problems caused by junction isolation. These features enable the construction of high frequency, low distortion amplifiers. The AD8391 has a unique pin out. The two noninverting inputs of the amplifier are connected to the VMID pin, which is internally biased by two 5 k Ω resistors forming a voltage divider between +VS and –VS. VMID is accessible through Pin 7. There is also a 10 pF internal capacitor from VMID to –VS. The two inverting pins are available at Pin 1 and Pin 8, allowing the gain of the amplifiers to be set with external resistors. See Page 1 for a connection diagram of the AD8391. A simplified schematic of an amplifier is shown in Figure 3. Emitter followers buffer the positive input, VP, to provide low input current and current noise. The low impedance current feedback summing junction is at the negative input, VN. The output stage is another high gain amplifier used as an integrator to provide frequency compensation. The complementary common- emitter output provides the extended output swing. A current feedback amplifier’s bandwidth and distortion perfor- mance are relatively insensitive to its closed-loop signal gain, which is a distinct advantage over a voltage-feedback architecture. Figure 4 shows a simplified model of a current feedback amplifier. The feedback signal is an error current that flows into the inverting node. RIN is inversely proportional to the transconductance of the amplifier’s input stage, gmi. Circuit analysis of the pictured follower with gain circuit yields: V V GTz s Tz s R G R OUT IN F IN = × () () ++ × where: G R R F G =+ 1 Tz s R sC R F TT () = + 1( ) R g IN mi = ≅ 1 125 Ω Recognizing that G × RIN << RF , and that the –3 dB point is set when Tz(s) = RF, one can see that the amplifier’s bandwidth depends primarily on the feedback resistor. There is a value of RF below which the amplifier will be unstable, as the amplifier will have additional poles that will contribute excess phase shift. The optimum value for RF depends on the gain and the amount of peaking tolerable in the application. For more information about current feedback amplifiers, see ADI’s high speed design techniques at www.analog.com/technology/amplifiersLinear/ designTools/evaluationBoards/pdf/1.pdf. |
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