Datasheet AD8392A (Analog Devices) - 7

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Data Sheet AD8392A THEORY OF OPERATION
The AD8392A is a current feedback amplifier with high Of course, for a real amplifier there are additional poles that (500 mA) output current capability. With a current feedback contribute excess phase, and there is a value for RF below which amplifier, the current into the inverting input is the feedback the amplifier is unstable. Tolerance for peaking and desired signal, and the open-loop behavior is that of a transimpedance, flatness determines the optimum RF in each application. dVO/dIIN or TZ.
RF
The open-loop transimpedance is analogous to the open-loop
R
voltage gain of a voltage feedback amplifier. Figure 16 shows a
G R
simplified model of a current feedback amplifier. Because R
IN
IN is
TZ V
proportional to 1/g
I OUT IN
m, the equivalent voltage gain is TZ × gm,
R
where g
N
m is the transconductance of the input stage. Basic analysis of the fol ower with gain circuit yields
VIN
022 V T (S) 06477- O Z = G× Figure 16. Simplified Block Diagram V T (S)+ × + IN Z G RIN RF The AD8392A is capable of delivering 500 mA of output where: current while swinging to within 2 V of either power supply RF rail. The AD8392A also has a power management system G =1+ included on-chip. It features four user-programmable power G R levels (three active power modes as well as the provision for 1 R = ≈ IN 50 Ω complete shutdown). gm Because G ×  RIN << RF for low gains, a current feedback amplifier has relatively constant bandwidth vs. gain, the 3 dB point being set when |TZ| = RF. Rev. A | Page 7 of 12 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Maximum Power Dissipation ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS SUPPLIES, GROUNDING, AND LAYOUT POWER MANAGEMENT THERMAL CONSIDERATIONS TYPICAL ADSL/ADSL2+ APPLICATION MULTITONE POWER RATIO OUTLINE DIMENSIONS ORDERING GUIDE NOTES