Operational Amplifiers High Speed Operational Amplifiers

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1 F Electrnics: Operatinal Amplifiers Page 11.1 Operatinal Amplifiers High Speed Operatinal Amplifiers Operatinal amplifiers with 3 db bandwidths f up t 1.5 GHz are nw available, such peratinal amplifiers are used in cnsumer applicatins like hard disk drives and cmputer mnitrs, industrial and medical applicatins, such as CAT scanners. In additin ne can use these peratinal amplifiers fr IF amplifiers in many cmmunicatin receiver applicatins. These high speed amplifiers fall int three categries: ltage Feedback Amplifiers Current Feedback Amplifiers Dual Feedback Amplifiers The ltage Feedback amplifiers have the same prperties as the LF356 r 741 peratinal amplifiers, used at lw frequencies. The ltage feedback amplifiers have lwer nise, better dc perfrmance and mre freedm in feedback cnfiguratin. The Current Feedback amplifiers have faster slew rates, lwer distrtin but restrictins n feedback cnfiguratins. If an peratinal amplifier has input vltages 1 and 2 at the input terminals, then fr a vltage feedback amplifier the utput vltage is: ut = A( ( 1-2 ), where A( is the pen lp gain, which is frequency dependent. Cnsider the nninverting amplifier cnfiguratin: Figure 1. Nninverting Amplifier (Natinal Semicnductr Appl Nte OA-30) f g If ne makes: G then g in G G 1 A( Since G( decreases with frequency as shwn in figure 2, the bandwidth f the amplifier will depend n the gain as is als shwn in figure , C. J. Kikkert, James Ck University

2 F Electrnics: Operatinal Amplifiers Page 11.2 Figure 2. Open lp and Clsed lp gain f ltage Feedback Amplifier (Natinal Semicnductr Appl Nte OA-30) The Current feedback amplifier Figure 3 Current Feedback amplifier cnfiguratin (Natinal Semicnductr Appl Nte OA-30) Fr the inverting amplifier cnfiguratin, the equatins fr the amplifier are: i errr in f in g Since Z( I, substituting fr Ierrr in the abve equatin after manipulatin leads t: errr g Z( f in Z( The current feedback amplifier will thus have a transfer functin f: G in cmpared with a transfer functin f : f 1 Z( G in fr the vltage feedback amplifier. G 1 A( , C. J. Kikkert, James Ck University

3 F Electrnics: Operatinal Amplifiers Page 11.3 f g Where G is the ideal amplifier gain, which is the same fr bth amplifiers. g The difference is thus in the way G/A( and f /Z( behave with frequency. Halving bth f and g will duble the bandwidth and keep the gain the same fr the current feedback amplifier, but nt change the gain fr the vltage feedback amplifier. Dubling the gain by halving g will halve the bandwidth f the ltage feedback amplifier but will nt change the bandwidth f the current feedback amplifier. The use f lw impedances will allw current amplifiers with high bandwidths t be btained. Current feedback amplifers have higher input currents and that makes them less suitable fr integratrs. There is basically n slew rate limit fr the current feedback amplifier, s that much higher slew rates can be btained. Current feedback amplifiers generally result in have a lwer distrtin than ltage Feedback amplifiers, as can be seen in the table 1. Current and ltage Feedback Amplifier Cmparisn Current Feedback ltage Feedback Device LMH 6702 LMH6624 3dB Bandwidth 720 MHz 1.5 GHz Slew rate 3100 /s 350 /s Nise ltage 1.83 n/hz 0.92 n/hz Nise Current 3.0 pa/hz 2.3 pa/hz Distrtin (HD2) -79 dbc (20 MHz, 2pp) -63 dbc (10 MHz, 1pp) Distrtin (HD3) -88 dbc (20 MHz, 2pp) -80 dbc (10 MHz, 1pp) Input Offset ltage 1 m 0.1 m Input Offset Current 13 A 0.1 A Lad esistr test circ Feedback esistr test circ Feedback esistr range Narrw ery wide Supply ltage 5 t t 6 Supply Current 12.5 ma 12 ma Cst (US$/1000) $1.49 $1.67 Table 1. Current and ltage Feedback perfrmance cmparisn , C. J. Kikkert, James Ck University

4 F Electrnics: Operatinal Amplifiers Page 11.4 Lw Nise designs using Operatinal Amplifiers: The nise assciated with a resistr is E N 4kTB Where: K = Bltzman s cnstant 1.38E-23 Jules/ Kelvin T = Abslute temperature Kelvin B = Bandwidth in Hz = esistance in In a circuit each resistr is then assciated with its wn nise surce which has a randm nise vltage f E N. In additin the peratinal amplifier prduces nise which can mst accurately be represented as vltage (e ni ) and current nise surces (i ni+ and i ni- ) at the inputs, as shwn in figure 4. Figure 4 Nise surces in an peratinal amplifier cnfiguratin (Natinal Semicnductr Appl Nte OA-11) The values fr e ni and fr i ni+ and i ni- are nrmally btained frm manufacturers data sheets. The table 2 shws the nise perfrmance f sme lw nise amplifiers frm different manufacturers. The nise f peratinal amplifiers will rise at lw frequency, as shwn in figures 5 t 8, and is inversely prprtinal t frequency at lw frequencies, due t flicker nise r 1/f nise. The crner frequency f that 1/f nise may be imprtant in a design. Fr example is a micrphne amplifier is required, the nise perfrmance at less than 1 khz is imprtant and the LMH6622 has a better nise perfrmance at thse frequencies than the LMH6624, which has a better nise perfrmance at higher frequencies nly , C. J. Kikkert, James Ck University

5 F Electrnics: Operatinal Amplifiers Page 11.5 Manufacturer Part n/sqrthz 1/f Hz GBWP MHz inoffset m Texas Instruments TLE Texas Instruments OPA Analg Devices AD Hz Analg Devices AD Analg Devices AD Hz Natinal Instruments LMH kHz Natinal Instruments LMH kHz Natinal Instruments LF Hz 5 3 Table 2. Lw Nise Operatinal Amplifiers. Figure 5. Nise f LMH6624L versus frequency Figure 6. Nise f LMH6622 versus frequency , C. J. Kikkert, James Ck University

6 F Electrnics: Operatinal Amplifiers Page 11.6 Figure 7. Nise f LF356 and AD8027/AD8028 versus frequency Figure 8. Nise and Distrtin f AD797 Nte the lwer 1/f pint f the AD8027/8028, but the higher nise vltage f the AD8027. Example A micrphne has a 100 Ω surce impedance and requires a lad resistance larger than 1 kω fr the micrphne t perate crrectly. The utput frm the micrphne is 3m. An amplifier with a vltage gain f 1000 is required. The nise btained frm an peratinal amplifier circuit fr a micrphne amplifier using an inverting and nninverting amplifier cnfiguratin is shwn in table , C. J. Kikkert, James Ck University

7 F Electrnics: Operatinal Amplifiers Page 11.7 Operatinal amplifer Nise calculatin 4kT 1.60E-20 Micrphne, 10m ut, l 1k, Bandwidth (Hz) Op Amp LMH6624 L s g f Surce resistance Input resistance series with inverting input Feedback resistr Nn Inverting Op Amp alue Nise Pwer/Hz Output Nise Pwer/H z en 1.00E E E-12 Inverting Amp alue Nise Pwer/Hz Output Nise Pwer/Hz 1.00E E E-12 in+ A 2.3E E E E E E+00 in- A 2.3E E E E E E-11 s E E E E+00 g E E E E-11 f E E-16 Gain E E Ttal Pwer/Hz 1.27E E-10 Ttal Nise Pwer 2.54E E-06 Nise ltage m 5.04E E+00 Mic Output m SN Nise figure Table 3. Nise fr inverting and nninverting amplifier cnfiguratins , C. J. Kikkert, James Ck University

8 F Electrnics: Operatinal Amplifiers Page 11.8 eferences: Nise: Natinal data sheet: LMH6624/LMH6626 Single/Dual Ultra Lw Nise Wideband Operatinal Amplifier Current vs. ltage Feedback Amplifiers Natinal Semicnductr Applicatin Nte OA , C. J. Kikkert, James Ck University