Frequent Faux Applying Wideband Current Feedback Amplifiers opera
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Frequent Faux Applying Wideband Current Feedback Amplifiers
Frequent Faux Applying Wideband Current Feedback Amplifiers
operating speeds have moved ever higher frequencies, whole design concerns have come into play linear applications. With development current feedback topology, design concerns unique that topology must also considered optimal performance achieved from devices offering over bandwidths. This discussion will review some considerations common wideband linear applications well topics unique current feedback topology pioneered Comlinear Corporation. These design guidelines intended help designer full potential Comlinear's high performance, current feedback, operational amplifiers. Since there quite disparate areas consider here, approximate order discussion will follow perceived frequency occurrence ranking. Those considered first seem impact every designer, with more particular concerns dealt with later.
National Semiconductor OA-15 Michael Steffes August 1990
Parasitic Capacitance Effects What About Ground Power Planes Board Layout.
sensitivity Comlinear amplifiers parasitic capacitance arises solely from their wide bandwidth characteristics from current feedback aspect their design. With parts showing loop gain that does drop unity until region, picofarad capacitance ground loop have profound effect phase margin unity gain crossover frequency. Figure shows typical non-inverting gain amp, including internal structure current feedback topology (Note along with most critical external parasitic capacitances.
01278301
FIGURE Non-inverting Gain with Internal Current Feedback Topology
Note Application Note OA-13 OA-31 development current feedback transfer function.
Parasitic, loading, capacitance directly output particularly effective transforming amplifiers into oscillators. Closed loop stability negative feedback amplifier determined open loop phase margin. tracing signal around loop always desirable have significantly less than degrees phase shift around loop unity gain crossover frequency. Adding capacitor directly output will cause additional signal phase shift
OA-15
2002 National Semiconductor Corporation
AN012783
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OA-15
Parasitic Capacitance Effects What About Ground Power Planes Board Layout. (Continued)
additional pole introduced open loop output impedance, driving output capacitance, Cpo. Even small capacitances Ro's cause significant phase shifts with unity gain crossovers region typical unity gain crossover frequency Comlinear amplifiers). Several design test guidelines suggested keep this sensitivity under control. Always clear ground power planes away from output net. This includes opening little wider than standard clearance ground power inner planes through hole trace carrying output signal. Never probe directly with high impedance probe output (passive divider probes okay). probing, always probe through series resistor since this will decouple effect probe capacitance from output pin. This also holds adding board test points. needed, connect test points through series resistor located close possible device being brought out. capacitive load must driven, such flash ADC's most Comlinear amplifier data sheets include plot recommended series resistor output prior load capacitance. Adding resistor prior load parasitic) capacitance, changes load's effect from pole pole-zero pair. This causes phase loop phase response that recovered prior unity gain crossover. Driving into another amplifier stage, actually almost other high input impedance active device, also present enough capacitance cause problems. Again, small series resistor right output prior going this device will defuse situation. Figures show effect output capacitance small signal frequency response CLC205, hybrid current feedback intended higher gains. plot shows SPICE simulated gain, linear frequency under several loading conditions. Similar plots generated using small signal macromodels Application Note OA-09. Initially, load driven, then just load, finally, load with series resistor Figure Clearly, getting into series resistor prior capacitive load dramatically improve frequency response flatness, hence pulse response capabilities amplifier simulated here. Recall that this approach applicable both current voltage feedback amplifiers. Generally, suggested output voltage through resistor soon possible before running over significant length trace cable. matched impedance load driven, source match right output with discrete resistor equal transmission line's characteristic impedance, terminate line similarly. Although short trace runs need impedance matched, using just series resistor will isolate trace capacitance when terminating resistor used.
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FIGURE Simulation Circuit Capacitive Loading Test
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FIGURE Frequency Response Various Loadings Parasitic capacitance inverting node considerably more complex, nearly troublesome, phenomena. Capacitance this input affects both non-inverting signal gain, appearing shunt with gain setting resistor, loop gain phase margin order sense. types pulse response characteristics observed this parasitic. dominant effect simply shunting ideal gain expression, neglecting loop gain effects now, single overshoot with decay will observed. this capacitance large enough effect phase margin, considerable ringing pulse response will also observed. Again, minimize ground power planes around inverting node net. single overshoot decay will most often observed when current feedback part intended higher gains, hence designed relatively high feedback resistor value, used non-inverting gains. relatively high required non-inverting gain will bring impact whatever parasitic present down frequency into passband amplifier. solution here, beyond simply limiting Cpi, inverting mode possible, switch part intended lower gain
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OA-15
Parasitic Capacitance Effects What About Ground Power Planes Board Layout. (Continued)
operation, hence designed lower resistor values. Given fixed Cpi, operating with lower will move zero frequency beyond amplifier passband. Application Note OA-14 discusses more detail gain range considerations current feedback amplifiers. Figures shows test circuit simulation results demonstrating effect inverting node capacitance CLC205 operated relatively non-inverting gain. effect increasing from circuit Figure seen considerable peaking frequency response Figure This not, this case, loss phase margin peaking, simply zero coming into non-inverting transfer function shunting This zero frequency (Cpi (Rg||Rf) pi)) Note pulse response Figure that causes single overshoot with negligible ringing.
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FIGURE Frequency Response
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FIGURE Pulse Response ringing observed pulse response, this more likely capacitance output pin. However, larger parasitic capacitances inverting node
FIGURE Simulation Circuit Inverting Input Capacitive Test
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OA-15
Parasitic Capacitance Effects What About Ground Power Planes Board Layout. (Continued)
also cause loop gain phase margin problems, particularly parts intended high gains. Again, discussion about avoiding parasitic capacitances output applies equally well here. Figure shows difference pulse response behavior between effects. upper trace, which plotted with offset clarity, shows ringing pulse response most peaked response Figure This typical output capacitance effects. lower trace repeat pulse response Figure Parasitic capacitance ground non-inverting input, including capacitance high impedance non-inverting input itself, will generally only introduce additional response pole, depending source impedance driving input capacitance. source impedances, this pole comes well beyond passband amplifier. However, when parasitic capacitance non-inverting inverting nodes approximately equal, intentionally adding non-inverting source impedance equal Rg||Rf very effective cancelling response zero coming through shunting
pling capacitors connecting into this ground plane. layout drawing 730013 evaluation board product accessories section catalog), shows good high frequency layout 8-pin monolithic amplifier products offered Comlinear.
01278308
FIGURE Simulation Circuit Peaking Cancellation
01278309
FIGURE Frequency Response Demonstrating Zero Cancellation
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FIGURE Contrasting Pulse Responses Figure shows this approach with case considered earlier Figure Note that have intentionally matched capacitance non-inverting input added bring frequency response back flatness. signal gain changed addition This approach simply cancels zero apparent upper trace Figure significantly decreasing pulse overshoot shown Figure Although critical remove ground plane from signal input output nodes, good, inductance, ground return path must provided load current. This typically provided putting small-valued ceramic capacitors directly power supplies, connected good adjacent ground plane. These conflicting goals good power supply grounding with parasitic capacitance pins achieved opening window around part ground power planes with high frequency decou-
01278310
FIGURE Pulse Response with Zero Cancellation
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OA-15
Parasitic Capacitance Effects What About Ground Power Planes Board Layout. (Continued)
Figure shows same amplifier Figure with suggestions handling parasitic capacitances incorporated. circuit Figure includes every possible problems arising from parasitic capacitance discussed thus far. Very rarely would these required same application. test points brought out, always come through least resistors with body those resistors close possible amplifier pins. Probing these test points still radically alter signal path frequency response. amplifier should, however, remain stable with least isolating resistors. inverting node parasitic capacitance seems problem, very effective cancelling (except when large cause phase margin problems). parasitic load capacitance must driven, used very effectively improve frequency response flatness. And, always high frequency capacitors power supplies close part possible into good ground plane.
used risk lower phase margin greater frequency response peaking. Higher values used expense lower amplifier bandwidth. fact, increasing feedback resistor value very effective means overcompensating amplifier. Unlike voltage feedback amplifiers, unity gain follower application requires recommended feedback resistor place from output inverting input. Although having influence frequency signal gain unity follower application, feedback element still needed determine loop gain current feedback topology. Using reactive elements feedback path, either intentionally unintentionally, play havoc with loop gain phase margin. Generally, this should avoided unless done with extreme care. small signal macromodels Application Note OA-9 very useful predicting what will happen with different feedback configurations. Using direct capacitive feedback, implement integrator, will generally cause oscillations with current feedback amplifier. Integrators implemented, however, using alternative topologies shown Application Note OA-7. Also, CLC420, wideband voltage feedback amp, used implement classical integrator topologies. Returning feedback resistor itself, never wirewound type this, other, resistor broadband application. Also, trying compensate amplifier using shunt capacitance across will typically yield oscillations with current feedback topology. much more fruitful compensate increasing value feedback resistor, although feedback path recommended.
Non-Inverting Source Impedance Considerations
impedance seen looking non-inverting input play strong role determining amplifier's overall performance. very broadband applications, significant resistive source impedance, conjunction with part's input capacitance, become bandlimiting point system. This normally problem terminations driven from source. When running amplifier inverting mode, non-inverting input would typically grounded, either directly, through approximately resistor. attempt source impedance matching inputs bias current cancellation should made since bias currents current feedback amplifier totally unrelated both magnitude polarity. Hence, unlike voltage feedback amp, there meaning offset current specification. Generally, taking non-inverting input ground through resistor (for inverting amplifier applications) will eliminate oscillations that might seen negative input impedance effects very high frequencies non-inverting input. oftentimes sufficient simply ground non-inverting input. careful check level oscillations above should made, particularly faster amplifiers, direct grounding desired. oscillations observed, going higher, resistor ground will kill this self oscillation non-inverting input transistors. When desired couple non-inverting input signal, shown Figure particular attention must paid effect terminating resistor operating point amplifier. Oftentimes, effort achieve very pole frequencies coupling (with5 www.national.com
01278311
FIGURE Non-Inverting Amplifier Featuring Several Suggested Protections from Parasitic Capacitance Effects
Importance Feedback Resistor
feedback resistor value becomes paramount importance current feedback topology used most Comlinear amplifiers. discussed detail Application Note OA-14, feedback resistor single most important element setting overall frequency response current feedback amplifier topology. Briefly, since looking feedback current from output voltage inverting input, feedback impedance plays dominant role determining what this will This, turn, will determine amplifier's loop gain phase margin. Achieving adequate phase margin critical success operational amplifier application. Every current feedback amplifier optimized particular value feedback resistor. This value typically noted heading specifications listing. Always select value near this starting point design. Lower values
OA-15
Non-Inverting Source Impedance Considerations (Continued)
inordinately large coupling capacitor, Ct), made very large some cases, included.
cept devices including output limiting clamping function (CLC500, CLC501, CLC502), exceeding output voltage range will result saturation internal amplifier. cases, this will result very slow recovery time from overdrive. Since error signal, current feedback, current back inverting input, saturating output voltage that longer fully supplies current being gain setting resistor will cause current build inverting input. This analogous voltage developing across inputs voltage feedback amplifier when overdriven. This inverting input current also limit recovery time from saturation effects internal amplifier. monolithic amplifiers from Comlinear handle this saturation without damage. Extreme overdrives inputs can, however, exceed current handling capability inverting node which point voltage will start build across inputs. This can, large enough, break down some internal junctions leading increase noise possibly shift characteristics amplifier. Unless specifically indicated overdrive protected (CLC205, CLC206, CLC207, CLC560, CLC561), special care should taken drive hybrid amplifiers into output saturation. Intended widest band, high power operation, these parts have enough internal drive capability potentially damage themselves under saturated output condition. Although noted data sheets, CLC231 CLC232 gain hybrid amplifiers also output stage saturated without damage. Even with output saturable capability, hybrid amplifiers need careful analysis junction temperatures ensure that they exceed rated maximum 175°C. From these considerations, recommended that these unprotected hybrid amps used comparators with output intentionally forced from supply rail supply rail. (The fast recovery clamping CLC501 does, however, offer excellent opportunity very flexible high speed comparator function.) also recommended increase value output stage collector resistors, those parts bringing output transistor collectors separately, current limit since this will only saturate output stage sooner. Generally, these resistors intended only de-couple high speed load current transients from rest amplifier enhance high speed settling times. possible, however, these resistors output short current limit those parts indicated being overdrive protected. finally, when using adjustable gain circuits, particularly with switching FETs, take care keep amplifiers open loop situation during gain adjust. situations requiring robust output overdrive capability, clamping amplifiers best choice.
01278312
FIGURE Effect Input Offset Voltage however, provides current path non-inverting input bias current. Wideband amplifiers with purely bipolar construction, Comlinear's amplifiers typically are, have input bias current ranging into 10's µA's. critical, therefore, consider effect large input offset voltage shown Figure 12). very easy, with large this bias current requirement have driven input output into saturation precluding proper high frequency operation. effect high non-inverting noise current gain should also considered. This noise current will input noise voltage term dependent frequency dependent source impedance looking back non-inverting input.
Input Output Voltage Range Considerations
common mode input voltage range specification (CMIR) shown Comlinear data sheets indicates near specified supply voltages non-inverting input voltage proper operation. When operating properly, inverting node voltage simply follows non-inverting even differential applications. current feedback, this unity gain buffer from while voltage feedback, this feedback loop. Since amplifiers specified with balanced bipolar supply voltages, CMIR output voltage ranges given allowed bipolar swing around ground. Both specifications are, however, indicating required voltage headroom supplies non-inverting input output pins respectively. Recasting these specifications required voltage headroom would allow input output voltage ranges non-standard supply voltages. almost cases, maximum output voltage swing will limiting factor. Only very non-inverting gains, single amplifier differential operation, will CMIR limit operation. crossover non-inverting gain where limiting point will change from input output found dividing output voltage range input voltage range. Operation that cause amplifier exceed output voltage range should handled with special caution.
Cascaded Amplifier Considerations
High gain, cascaded amplifier applications require particular attention number parasitic operational effects. Figure shows example circuit 3-CLC401's configured overall gain 1000 that will used demonstrate suggestions developed here. Several opportunities exist develop oscillator with very high gain, wideband circuits. most common direct output input parasitic coupling. output signal path should physically isolated and, necessary, shielded from input signal path. When final output driving relatively heavy load, either capacitive this case,
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OA-15
Cascaded Amplifier Considerations (Continued)
resistive (100), high frequency load currents through supplies couple back into early stages completing oscillatory feedback loop. High frequency de-coupling directly supply pins each stage required minimum. Interstage ferrite beads supply rails, shown Figure also used attenuate this feedback path. power supply connections Figure bring supplies final gain stage with filter stages used connects into earlier amplifier stages. This provides increasing high frequency attenuation amplifier stages earlier gain path. This very desirable from both PSRR standpoint breaking feedback path through power supplies from output input. close physically coupled amplifier stages, interstage matched impedances probably necessary. interstage resistors Figure intended isolate input capacitance next stage from output previous stage suggested earlier discussion parasitic load capacitance effects.
concern very high gain path build-up errors. circuit Figure couples gain setting resistors which reduces gain each amplifier stage. With only input offset voltage first stage, final amplifier output, (prior matching loss), would this gain 2000 capacitors been used gain setting networks. coupling high gains desired, some sort composite correction loop described Application Note OA-07) should considered. general rule, highest gain stage should used first stage limit impact overall input noise noise contribution succeeding stages. Here, equivalent input noise stages would divided gain first stage adding overall equivalent input noise. total equivalent input noise circuit Figure nV/Hz. Application Notes OA-12 noise calculation discussion OA-14 reducing input noise coupled applications. Figure shows measured broadband gain phase response circuit Figure Note that measured bandwidth, extending from MHz, achieves equivalent Gain-Bandwidth product.
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FIGURE Wideband, High Gain, Cascaded Amplifiers
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Frequent Faux Applying Wideband Current Feedback Amplifiers
Cascaded Amplifier Considerations
(Continued)
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FIGURE Measured Gain Phase High Gain Cascaded Circuit circuits included this application note have been tested with National Semiconductor parts that have been obsoleted and/or replaced with newer products. Please refer conversion table find appropriate replacement part obsolete device.
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OA-15
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