Datasheet ADA4311-1 (Analog Devices) - 10

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ADA4311-1 THEORY OF OPERATION
The ADA4311-1 is a dual-current feedback amplifier with high Because G × RIN << RF for low gains, a current feedback amplifier output current capability. With a current feedback amplifier, the has relatively constant bandwidth vs. gain, the 3 dB point being current into the inverting input is the feedback signal, and the set when |TZ| = RF. open-loop behavior is that of a transimpedance, dVO/dIIN or TZ. For a real amplifier, there are additional poles that contribute The open-loop transimpedance is analogous to the open-loop excess phase, and there is a value for RF below which the amplifier voltage gain of a voltage feedback amplifier. Figure 21 shows a is unstable. Tolerance for peaking and desired flatness determines simplified model of a current feedback amplifier. Because RIN is the optimum RF in each application. proportional to 1/gm, the equivalent voltage gain is TZ × gm,
RF
where gm is the transconductance of the input stage. Basic analysis of the follower with gain circuit yields
RG R
V T (s)
IN
OUT Z = G ×
TZ V I OUT IN
V T (s) + G × R + R IN Z IN F
RN
where:
VIN
-018 R 40 F G = 1+ 69 0 R Figure 21. Simplified Block Diagram G 1 R = ≈ 50 Ω IN g m Rev. 0 | Page 10 of 16 Document Outline FEATURES APPLICATIONS PIN CONFIGURATION TYPICAL APPLICATION GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Maximum Power Dissipation ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATION INFORMATION FEEDBACK RESISTOR SELECTION POWER CONTROL MODES OF OPERATION EXPOSED THERMAL PAD CONNECTIONS POWERLINE APPLICATION BOARD LAYOUT POWER SUPPLY BYPASSING OUTLINE DIMENSIONS ORDERING GUIDE