Improving Amplifier Noise Figure High Intercept Amplifiers Michae

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Improving Amplifier Noise Figure High Intercept Amplifiers Michae

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Improving Amplifier Noise Figure High Intercept Amplifiers
Michael Steffes
January 1993
Abstract Wide spurious-free dynamic range certainly goal amplifier. This particularly true radar well other systems using high resolution digitizers. Recently introduced current feedback amplifiers offer exceptional order intermodulation intercepts very quiescent power levels, have been plagued relatively poor noise figures. Teaming these amps with simple transformer input coupling yields noise figures less than with order intercepts greater than 40dBm (for frequencies 10MHz). Although commonly considered amplifiers, wideband, coupled operational amplifiers offer considerable performance advantages lower over standard coupled amplifiers. Particularly suitable from distortion standpoint family recently introduced monolithic current feedback operational amplifiers. Similar more common voltage feedback amps, these parts offer very high non-inverting input impedance, very output impedance, very high open loop gain that controlled through external resistors well controlled closed loop gain. These amplifiers unique that inverting node presents impedance through which amplifier senses feedback current opposed more common feedback voltage. (Reference current feedback topology, implemented National's CLC400 CLC401 amplifiers, also exceptionally symmetric. This yields intrinsically distortion mechanisms internal amplifier which then divided loop gain yield closed loop distortion. described Reference loop gain current feedback amplifier principally feedback resistor value. loop gain will, course, show frequency dependence yielding continued improvement distortion down dominant open loop pole frequency approximately 350kHz these parts). Conversely, distortion will worsen moving higher frequencies open loop gain rolls off. Measuring order intermodulation intercept 10MHz yields between 45dBm these parts. Although theory indicates continued improvement below this frequency, accurate measurements difficult perform intercepts above 45dBm output power levels within capability these devices.
Taking advantage this exceptional intercept performance has, however, been impaired noise figures ranging from 20dB depending device gain setting used. Reflecting noise sources non-inverting input typically yields equivalent input spot noise voltage frequencies above noise corner) that range from 2.4nV/Hz well over 5nV/Hz (for CLC401 operated gains.) Aside from intrinsic noise voltage non-inverting input, effect inverting noise current also contributes strongly this result. (See appendix Application Note OA-12 discussion calculating equivalent input noise voltage.) suggested literature (Reference transformer coupling sometimes used reduce amplifier's noise figure. This possible when equivalent input noise voltage much greater than noise voltage generated input noise current through source impedance. Reference suggests optimum source impedance noise figure given ratio noise voltage noise current. this ideal impedance much greater than typical source impedance seen strips amplifiers considered here), significant improvement noise figure achieved using transformer coupling. Conceptually, transformer will provide noiseless voltage gain expense increasing source impedance input noise current. Using this technique with current feedback amps will sacrifice coupling, with transformer setting frequency limit operation. Depending amplifier high frequency limit will yield poor distortion performance near amplifier's -3dB frequency. amplifier's -3dB point largely determined frequency which loop gain dropped unity. With negligible loop gain, internal distortion mechanisms longer corrected yielding poor distortion performance. Hence, preferable have transformer also limit high frequency performance. Both amplifiers considered here offer -3dB bandwidths exceeding 100MHz. transformer offering good performance through 50MHz maximum down frequency desired would probably suitable choice.
1993 National Semiconductor Corporation
Printed U.S.A.
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Topology Description Figure shows topology considered.
Input Reference Point; CLCXXX Output Reference Point; So,No
noise comers amplifiers. (Refer individual data sheets Application Note OA-12 detailed noise data). Noise Figure Computation develop expression noise figure circuit Figure most elementary definition shown Equation will used
Figure transformer will provide noiseless voltage gain from voltage applied input non-inverting input amplifier. This done expense increasing source impedance looking back amplifier's non-inverting input. Increasing turns ratio transformer (i.e. picking voltage gain) will decrease noise figure until noise term non-inverting input noise current times source impedance equals equivalent non-inverting input noise voltage. Constraints Assumptions Input impedance matching source impedance (Rs) transformer input desired. Therefore, n2Rs With this assumption, going output side transformer, source impedance non-inverting input noise current will equal R1/2 terms will n2Rs/2. will also introduce noise voltage term into analysis. Since offers output impedance, separate matching resistor must added drive into matched load would typical application (normally, ohms). assume resistor noise output matching network negligible compared noise output, change ratio will seen going from output amplifier load point. Therefore, noise figure gain will calculated load point neglecting noise added output matching resistor, various noise contributors amplifier considered uncorrelated. This allows equivalent total noise powers developed separate noise powers. noise voltage currents taken spot noise values yielding spot noise figure value. transformer frequency rolloff comers 100kHz, some increase spot noises frequencies will observed
This definition states that noise figure times ratio signal/noise ratio input signal/noise ratio output. These ratios signal noise powers available input output. noise power available input taken that delivered conjugate matched load where noise that load separated being added system. Since some noise will always added, signal/noise ratio output will degraded from that input yielding noise figure always evaluate noise figure expression, circuit Figure redrawn more idealized form Figure
Figure this circuit, transformer been replaced equivalent elements; input terminating impedance (Rs), noiseless voltage gain given turns ratio (n), equivalent output impedance taken parallel combination reflected through transformer. (R1/2). Note that been reflected input side noiseless terminating resistor, R1's noise contribution retained output side transformer since this needs considered part noise added system. amplifier been replaced infinite input impedance gain block (Av) with equivalent noise sources brought Note that includes noise contributions inverting input noise current feedback gain setting resistor noises. (This analysis described appendix.) Although gain noise terms Figure have thus been expressed voltage gains with noise voltage current terms, noise figure development deals only with power gains noise powers. Therefore, gains noises shown Figure will modified power gain from input output noise powers delivered input output.
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Looking separate parts argument Equation separate them inverse power gain through path
Pulling expression, ratio input output noise power rewritten
in2n2
output noise power over input noise power output noise power developed taking each contributing noise voltage term through output then developing power that voltage across adding terms. separate noise voltage terms output are: Source noise voltage 4KTRs
Combining expressions noise power ratios inverse power gain through channel, developed above, yields:
in2n2
in2n2
Terminating resistor noise 4KTR1 where Boltzman' constant 1.38E Joules °Kelvin °Kelvin 290° this analysis then Amplifier voltage noise Amplifier current noise
Multiplying fraction through, bottom, going back form noise figure yields:
10log KTRs
Note that both noise voltages intrinsic attenuated impedance matching present both sides transformer (i.e. reflects source side ground, reflects secondary side driving impedance non-inverting input terminating impedance, R1). Substituting with n2Rs, adding each noise voltage term squared divided output terminating impedance, will yield total output noise power.
KTRs KTn2R
Looking component parts this expression, argument arises from terminating with discrete (noisy) matching resistor, This increases minimum achievable noise figure from 3dB.
parts fraction
input noise power derived power delivered source matching resistor from source resistor noise voltage. This
4KTRs
represents noise voltages (the total equivalent input noise voltage voltage generated noise current through source impedance) input amplifier reflected transformer input added powers across term denominator simply noise power available from source input network. From this, turns ratio increases, contribution noise current increases while that noise voltage decreases reported Reference minimum value will occur when these terms equal. Solving optimum turns ratio minimize noise figure: nopt
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Substituting this Equation yields minimum noise figure: NFmin Recognizing that transformer turns ratios actually only available integer steps, optimum turns ratio somewhat academic. However, given recognized that anything that will reduce will improve noise figure. Little done reduce noise current amplifier's non-inverting input. equivalent input noise voltage can, however, reduced amplifier operated higher gains. results appendix show that equivalent input noise terms inverting noise current resistor noises reduced gain increases. However, once these noise terms have been reduced below intrinsic non-inverting input noise voltage, further improvements through increased gain minimal. Design Procedure Test Results illustrate design procedure resulting performance using this input transformer coupling, possible designs using CLC400 CLC401 will developed. designs will proceed with assumption that maximum gain consistent with broad bandwidth good order intermod intercept desired. Enough information presented allow design proceed from targeted gain well. CLC400 broadband coupled monolithic amplifier intended relatively gain operation. Typical specifications show 200MHz bandwidth (-3dB) gain Both parts pull nominal load current 15mA when operated from their recommended volt power supplies. current feedback topology, gain part corresponds part that been optimized lower value feedback resistor opposed high gain part such CLC401. Hence, nominal gain CLC400 shown data sheet ohms, while CLC401 optimized 1.5k feedback gain +20. Most requisite information design found National Application Notes OA-12 (Noise Analysis) OA-13 (Current Feedback Loop Gain Analysis). Non-inverting input intrinsic noise voltage Inverting input noise current Non-inverting input noise current Inverting input impedance Nominal feedback transimpedance maximally flat frequency response 2.5nV/Hz 14pA/Hz 3.2pA/Hz
Using these numbers, equation appendix, maximum amplifier gain reduced equivalent input noise voltage derived (this assumes 1/9) Rounding this gain yields feedback resistor value (Eq. Appendix)
Note that taking gain high will eventually yield very values from this equation. very values significant degradation both bandwidth order intercept will observed added output stage loading presented feedback network. Generally, should taken lower limit Computing equivalent input noise voltage using appendix yields:
(2.5nV)2 [(14pA)(37.5)]
21(37.5) 2.67
hoped, this total equivalent input noise voltage nearly equal intrinsic noise voltage listed above. From these results, assuming source impedance, optimum transformer turns ratio would nopt 6.67nV 5.78 3.2pA(25) This yields best case noise figure equal
2.67nV NFmin 6.2dB however, difficult maintain broadband performance through transformer with turns ratio this high. test, turns ratio transformer from Mini-Circuits selected reasonable compromise between best noise figure broadband performance (part #T16-6T) resulting test circuit CLC400 shown Figure
Input point gain noise figure R1~800 1.21k Output point gain, noise figure order intercept Supply de-coupling shown
CLC400
Mini-Ckts T16-6T transformer
Figure
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(4(3.2pA)25)2 2.67nV 6.2dB (50)
Frequency
10MHz 20MHz 30MHz 40MHz
Noise Figure
6.8dB 6.8dB 7.1dB 7.1dB
Order Intercept
44dB 38dB 33dB 30dB
Using this test circuit, anticipated performance calculated Overall gain (18dB) Noise Figure (from Eq.2) test, first step tune input impedance matching provide good match over frequency range interest, after which transfer function (S21) measured. These results shown Figure
Figure measured noise figure shows excellent agreement with predicted value, while order intercept parallels CLC400 data sheet plots. Note that data sheets typically show intercept defined power level output opposed lower value defined matched load. Adding results shown above gets back data sheet plots. This indicates that intercept been degraded transformer input coupling. Note that noise figure just CLC400, configured shown Figure without transformer, derived simply letting noise figure equation (Equation Doing this yields noise figure 15.8dB CLC400 itself (assuming only non-inverting input impedance matching resistor). Hence, transformer only provides with more gain with greatly improved noise figure. summary, this circuit shows in/50out, 18dB gain block with very flat frequency response from 60kHz 30MHz offering approximate noise figure with order intercept greater than 33dBm over that frequency range, while dissipating only 150mW quiescent power! Design Test Results CLC401 CLC401 monolithic, coupled, wideband current feedback amplifier optimized higher closed loop gains. Typical specifications show 150MHz -3dB bandwidth gain using 1.5k feedback resistor while drawing only 15mA load current from specified volt supplies. Getting requisite design information from Application Notes OA-12 OA-13, Non-inverting input intrinsic noise voltage 2.4nV/Hz Inverting input noise current 7pA/Hz Non-inverting input noise current 2.SpA/Hz Inverting input impedance Nominal feedback transimpedance 2.5k maximally flat frequency response Using these numbers, Appendix, yields maximum amplifier gain minimal equivalent input noise voltage (assuming 1/9) 32.5 Building circuit this gain using same transformer CLC400 test circuit resulted response peaking higher frequency limits.
Magnitude
Magnitude (5/div)
Phase (36°/div)
Phase
Frequency (MHz)
Figure Input Impedance
Gain
Magnitude (1dB/div)
Phase (50°/div)
Phase
0.01
Frequency (MHz)
Figure Transfer Function These results show excellent input impedance matching over broad frequency range with very flat passband gain from about 60kHz 30MHz. noise figure this circuit measured using HP8970A with HP346B noise source. Figure tabulates those results along with order intercept.
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This seemed arise from gain dependent non-inverting input impedance resonating with transformer. Reducing amplifier gain ameliorated this effect, Since amplifier gain being determined somewhat arbitrarily reduce noise figure, backing away from this gain improve frequency response seemed reasonable. test, amplifier gain selected. Using appendix shows equivalent input noise voltage with again given
(4(2.8pA)25)2 2.70nV 6.7dB (50)
(2.4nV)2 [(17pA)(37.5)]
21(50) 2.70
with CLC400, test sequence input matching impedance network yield good match over wide frequency range possible. After this, input output transfer function measured (S21). Figure show these results:
Magnitude (5/div)
With this result, optimum turns ratio transformer theoretical best noise figure calculated nopt 2.70nV (2.8pA) NFmin 5.9dB Again, high transformer turns ratio required optimum noise figure would result unnecessarily limited bandwidth. Backing turns ratio transformer yielded test circuit shown Figure
Magnitude
Phase (20°/div)
(2.8pA)(25)
2.70nV
Phase
Frequency (MHz)
Figure Input Impedance
Gain
Magnitude (1dB/div)
Phase (50°/div)
Input point gain noise figure 1.21k
Output point gain, noise figure order intercept 53.6 1.27k Supply de-coupling shown
Phase
CLC401
Mini-Ckts T16-6T transformer
0.01
Frequency (MHz)
Figure Transfer Function Figure Note that this test, amplifier's gain setting resistor been coupled with capacitor. This intended reduce gain amplifier's input offset voltage holding output close possible. capacitor value chosen yield transfer function pole well below transformer frequency cutoff. feedback resistor value using Equation Appendix. anticipated midband gain noise figure performance calculated Overall Gain (34dB) Noise Figure (from This circuit doesn't quite well holding input impedance higher frequencies does provide reasonably flat frequency response from 70kHz 50MHz (passband with 0.5dB ripple). measure noise performance obtained using HP3585 spectrum analyzer along with CLC100 noise wideband amplifier preamp analyzer input. Although accurate noise figure measurements difficult achieve this fashion, this approach indicated noise figures between 8dB. Figure tabulates measured order intercept results this estimated noise figure.
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Frequency
10MHz 20MHz 30MHz 40MHz 50MHz
Estimated
7-8dB 7-8dB 7-8dB 7-8dB 7-8dB
Order Intercept
38dB 33dB 29dB 25dB 23dB
This approach noise figure improvement applicable with optimum source resistance greater than actual source resistance. With total equivalent input noise voltage non-inverting input decreasing closed loop gain increased shown appendix), advantageous operate amps high gains. current feedback topology, pioneered National, particularly suitable wideband, high gain applications. described Application Notes 300-1 OA-13, current feedback topology largely eliminates gain-bandwidth performance limitations plaguing earlier voltage feedback designs. Therefore, running amplifiers higher gains, effort drive down non-inverting input voltage noise, will sacrifice broadband performance would using voltage feedback part. Acknowledgments Ralph Carfi, General Electric, Syracuse, N.Y. measuring noise figure CLC400 test circuit many useful discussions this application. Alan Baker, R&D, National Semiconductor Corp., review discussion noise figure equation development. Steve Smith, R&D, National Semiconductor Corp., automating order intercept measurement procedure. References 1."Current-Feedback Amplifiers, "Sergio Franco EDN, Jan. 1989, page Comlinear Corporation 1989/1991 Databook). Also, National Application Note 300-1 Application Note OA-14. 2."Low-Noise Electronic Design," Motchenbacher Fitchen; Wiley 1973, 127. Appendix: Computing equivalent Input noise voltage, gain, feedback resistor values noise figure reduction with current feedback amps. equations determining equivalent input noise voltage noise figure calculations will developed. Since external resistors around amplifier, play large role setting that noise, amplifiers transfer function, which also determined these resistors, will given used gain. Figure shows necessary information develop both transfer function from equivalent input noise voltage expression. described Reference current feedback amplifier uses unity gain buffer from input inverting node, with inverting node current (ierr) acting feedback signal sensed passed output through transimpedance gain,
Figure Calculating noise figure just CLC401 without transformer coupling letting noise figure equation) yields 15.9dB just amplifier itself with non-inverting termination resistor. again, transformer added signal gain while greatly improving noise figure. results Figures show in/50 out, 34dB gain block with reasonably flat frequency response from 70kHz 50MHz offering approximate noise figure with order intercepts greater than 25dBm operation below 40MHz dissipating only 150mW! intercept performance improves rapidly lower frequencies with continued improvement observed below 10MHz. Comparisons Conclusions Clearly, transformer coupling offers potential some real improvement noise figures amplifiers considered here. Having given coupling process, however looking compare these parts more classical coupled amplifiers. Those parts generally Class output stage opposed Class structure used most National's amplifier products. This, along with high loop gain lower frequencies, allows exceptional distortion performance achieved fraction quiescent power dissipation more classical Class output. This advantage narrows move frequencies over 100MHz with amp's loop gain dropping below unity these higher frequencies. Generally, lower frequency applications, circuits described here, similar circuits using different National amplifiers offer considerable advantages areas power dissipation, size, cost. transformer coupling offers additional flexibility through potential signal inversion, reversing convention, output shifting, inserting voltage place ground secondary, potential narrowband filtering. higher output power levels desired, this same approach could used with National's hybrid amps offering higher supply voltages greater output power capability. CLC232, gains, CLC207 higher gains, particularly harmonic distortion parts that would also benefit, from noise figure standpoint, from transformer coupling.
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Z(s)i [eo]
Setting gain resistor values needs done context maintaining adequate phase margin closed loop amplifier response. Analyzing circuit Figure Vo/Vi transfer function yields (see Application Note OA-13 more complete development); where: Z(s) Forward transimpedance gain
4kTRg
4kTRf
Where: K-Boltzman's constant =1.38 E-23 J/°K T-Degrees Kelvin 290°K used here
amplifier (frequency dependent) Inverting input impedance (considered noiseless real)
Figure goal here develop equivalent non-inverting input noise voltage source place non-inverting input noise figure calculations. Normally, noise generator non-inverting termination resistor would included this analysis. context using input transformer coupling, however, this resistor will impedance matching concerns removing variable equivalent input noise voltage reduction. effect this resistor's noise included development noise figure. noise sources inverting side circuit must reflected non-inverting side combined with intrinsic noise voltage, eni, already present model. Neglecting ini, which left separate later noise figure equations, each noise voltage current will develop output voltage noise. With non-inverting signal gain defined Rf/Rg), separate output noise voltages are: intrinsic non-inverting input noise voltage inverting input noise current combined resistor noise terms 4KTR Combining terms root squared elements, reflecting this non-inverting input yields equivalent input noise voltage
inverting node current feedback signal with output voltage inverting input current transfer gain given Every current feedback amplifier internal forward transimpedance gain function (Z(s)) optimized certain value Typically, this optimization yields phase margin gain feedback resistor value specified data sheet guaranteed performance specs. first approximation, this held constant (maintaining maximum closed loop bandwidth with peaking) desired closed loop signal gain changed from nominal design point. This done adjusting gain. Solving this from above expression yields:
this expression placed into equivalent input noise expression developed above, get:
4KTR
apparent from this expression, both gain resistor values used reduce input noise voltage. Increasing gain and/or reducing resistor values will both decrease apparent input noise voltage. This effort bounded intrinsic input noise voltage, eni.
only variable left this point desired closed loop gain. absolute resistor values have been removed with assumption that maximally flat frequency response desired closed loop gain changed. Again that increasing gain will decrease equivalent input noise voltage. This approach decreasingly effective those terms involving become less than non-inverting input noise voltage eni. target desired ratio squared terms involving intrinsic non-inverting input noise voltage squared, develop targeted maximum gain beyond which minimal noise reduction achieved through further gain increases. call that ratio noise powers solve for:
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From this expression, knowledge maximum desired gain derived. This yields somewhat arbitrary upper limit amplifier gain that only trying increase gain until negligible improvements noise figure seen. amplifier can, course, operated lower gains, with increase noise, higher gains, with little noise improvement eventual bandwidth limitation. (saying that reflected equivalent noise power terms non-inverting input intrinsic input noise power non-inverting input noise voltage) those terms increase equivalent input noise voltage only This will initial targeted design criteria used example developments. Application Note OA-13 complete development adjusting hold constant loop gain, hence bandwidth, desired signal gain changed.
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