Optimum performance current feedback amplifiers general HA-5020 partic

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HA5020 Operational Amplifier Feedback Resistor Selection
Optimum performance current feedback amplifiers general HA-5020 particular depends upon careful selection feedback resistor, benefit higher usable bandwidth (compared with conventional voltage feedback amplifiers) ability control frequency response choosing value carries expense that design process becomes more complicated. This particularly true intuitive knowledge device will behave application lacking. purpose this Note provide conceptual foundation which this intuitive knowledge built. choice optimum resistor value depends upon design goals application subject conditions closed loop gain, source impedance, load. point reference, typical curves provided data sheet that show frequency response affected closed loop gain, feedback resistor value, load resistance. Source impedance, large, becomes factor only conjunction with capacitance inputs. data sheet curves generated with source impedance. illustrate might approach problem selecting feedback resistor based closed loop gain, consider simple model Figure Between inputs unity gain voltage buffer with non-zero output impedance indicated transimpedance gain, function frequency having high value that forces zero. model's behavior influenced external elements consisting feedback network RG), source load impedances RL), stray capacitance amplifier's inputs (CS).
VOUT
gain peaking particularly gains (intuitively, parallel with causing gain determined feedback network increase with frequency). Gain peaking capacitance inverting input most easily dealt with placing resistor series with positive input. assume that stray capacitance positive input equals stray negative input, choose equal parallel combination This introduces pole positive input which cancels zero negative input, thereby eliminating gain peak. Note that remaining gain peaking result excessive phase shift around loop. Excess phase shift around loop reduced increasing Bandwidth degradation non-zero inverting input resistance also easy deal with long product closed loop gain inverting input resistance does exceed optimum value unity gain. solving transfer function constant bandwidth, arrive following equations (ACL Where, optimum value unity gain (1000), inverting input impedance (75), desired closed loop gain. comparison between actual measured results Figures provides graphic reinforcement utility these equations. Figure illustrates failure consider stray input capacitance inverting input resistance, while Figure incorporates lessons learned from analyzing simple model. Figure family closed loop gain curves obtained representative unit using constant 1000). measured stray capacitance either input 2pF. results Figure were obtained from same unit, except that (within constraints available standard resistor values) were chosen according equations above chosen equal parallel combination limitation above model that does include effects load. general above, response independent load. less than 400, response becomes more damped bandwidth degrades. Here again bandwidth degradation compensated lowering value (EQ. (EQ.
RZ(S)
FIGURE SIMPLE CURRENT FEEDBACK AMPLIFIER MODEL
Derivation transfer function will confirm that nonzero inverting input impedance, causes circuit's bandwidth degrade closed loop gain increases, while stray capacitance negative input gives rise
1-888-INTERSIL 321-724-7143 Copyright
Intersil Corporation 1999
Application Note 9305
NORMALIZED GAIN (dB) FREQUENCY (MHz) NORMALIZED GAIN (dB) FREQUENCY (MHz)
FIGURE FREQUENCY RESPONSE CLOSED LOOP GAIN USING FIXED TABLE RESISTOR VALUES FIGURE (MHz) PEAKING (dB)
FIGURE FREQUENCY RESPONSE CLOSED LOOP GAIN 1000-AV(75), TABLE RESISTOR VALUES FIGURE (MHz) PEAKING (dB)
26.1
90.9 23.7
NOTE: 1000-AV(75),
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