Op Amp Packaging. Op Amps. JFET Application Current Source

Transcription

1 JET pplication Current Source Op mps, 5 Imperfections Op amp applications Household application: battery charger (car, laptop, mp players) Differential amplifier current source amp waeform generator High Speed D conerter usg capacitors Simple circuit: N559 Nchannel JET I DSS = current with GS =0 P = pchoff oltage N559 5 i D cknowledgements: on oscoe, Neamen, Donald: Microelectronics Circuit nalysis and Design, rd Edition i D I DSS GS P.0 Sprg 0 Lecture.0 Sprg 0 Op mps Op mp Packagg ctie deice: 0 = a( ); note that it is the difference of the put oltage! a=open loop ga ~ Most applications use negatie feedback. Comparator: no feedback o ctie deice requires power. No shown for simplicity. Classics opamps:, 5 ~ $0.0; one, two or four a package. Newer opamps operate at <. (OP9.8) L5 LM.0 Sprg 0 Lecture.0 Sprg 0 Lecture

2 5 JET Input Op amp JET Differential Pair Small Signal Model.0 Sprg 0 Lecture 5.0 Sprg 0 Circuit Differential (Emitter Coupled) Pair JT Diff Pair Small Signal Model transistors, resistors, capacitor, diode.0 Sprg 0 Lecture.0 Sprg 0 8

3 Differential Pair Common Mode oltage Differential Pair Differential Mode oltage Small Signal Model.0 Sprg Sprg 0 0 MOSET Differential Pair irtual Node nalysis Small Signal Model = a( ) a = ga β = feedback or loop function a If a>> and a>> β then ~ Current to put termals zero by design x x x a a a a x a x a a Typical alues: a~00,000 & a β >> ok for a=a(s) and β = β(s) as long as a(s) β(s) >> β is the loop transfer function (not to be confused of β of a JT) aβ is the loop ga.0 Sprg 0.0 Sprg 0 Lecture

4 Op mps irtual Node With negatie feedback, put will drie the put oltage difference to zero => = Input current = 0 enefits of eedback Stabilize ga agast deice ariations, temperature, agg Input and put impedances adjusted by (aβ) educe distortion by the feedback factor [(aβ)] Ga determed by passie components Op mp Max atgs common mode oltage appears at both puts Need cc, ee for operation Disadantages of eedback Loss of ga; need more stages Greater tendency for stability (oscillations) Idiot proof a x a a.0 Sprg 0 Lecture.0 Sprg 0 Lecture Electrical Characteristics L5 lmost zero not rail to rail.0 Sprg 0 Lecture 5.0 Sprg 0 Lecture

5 LM ail to ail Output ail to ail Input.0 Sprg 0 Lecture.0 Sprg 0 Lecture 8 Decibel (d) Open Loop requency Ga d 0log o i Po d 0log Pi log 0 ()=.0 00 d = 00,000 = d = 0,000 = 0 d pot = half power pot 0 d =,000 = 0 0 d = 00 = 0.0 Sprg 0 Lecture 9.0 Sprg 0 Lecture 0

6 s Non Inertg mplifer 5 5 β (not to be confused with β of a JT) for fite Zero put current; therefore so.0 Sprg 0 or but ; 5 s 5 Open Loop requency Ga Examples at Hz, 000 Hz, and 0kHz oltage ga =0d = 00; = 00k, = k; [0 = 0.d!] β=0.0 t Hz, ol = 00 d = x 0 5 = 00, d t 000 Hz, ol = 0 d = 0 = d t 0 khz, ol = d =. x 0 = d.0.. β is the loop transfer function aβ is the loop ga.0 Sprg 0 Lecture s 5 Comparison So Why JT Op amps? 5 Input deice JT JET Input bias current 0.5u 0.000u Input resistance 0. MΩ 0 MΩ Slew rate* 0.5 /μs.5 /μs Ga andwidth product Mhz 5 Mhz Output short circuit duration contuous contuous JTs hae higher transconductance (ga), better consistency spec between pieces, and some applications, lower noise than ETs. Like most JET op amps, the L5 has a relatiely high offset oltage, and relatiely high drifts. JT opamps tend to hae much lower offset oltage and drifts. Identical p * comparators hae >50 /μs slew rate.0 Sprg 0 Lecture.0 Sprg 0

7 Ga andwidth Product = Constant (No free lunch) Op mp Imperfections eal World Ga: 0d = 0 andwidth = 5x0 Ga andwidth product = 5x0 Input offset oltage Input Current ias Input Offset Current ite Output oltage Swg ite Current ite Ga, ga bandwidth product oltage Noise Johnson Noise Phase Shifts Slew ate.0 Sprg 0 Lecture 5.0 Sprg 0 Lecture Input Offset oltage * Offset djustments : 000μ 5: 0,000μ * nalog Deices MT0 Tutorial.0 Sprg 0 Lecture.0 Sprg 0 Lecture 8

8 Input ias Current * Inertg mplifier ias Current Compensation The put offset current, I OS, is the difference between I and I, or I OS = I I. I I I 5 I 5 O but with no put I thus : O I O signal, I O I ; as I I 0 and we want O I a condtion 0 I 0, so : for no offset at o : 00na 5: 0.05na // 5 I DI OL I 5 I // 0 if I // I // I OL OL * nalog Deices MT08 Tutorial 0 if //.0 Sprg 0 Lecture 9.0 Sprg 0 Lecture 0 Common Mode ejection atio CM CM: ratio of the commonmode ga to differentialmode ga. Example, if a differential put change of Y olts produces a change of at the put, and a commonmode change of X olts produces a similar change of, then the CM is X/Y. CM often expressed d: CM 0log OL CM Inertg mplifer irtual Ground nalysis i i f 5 5 f ssumptions Infite put impedance: i 0; i 0 0 because is grounded. i f i f f f 0.0 Sprg 0 Lecture.0 Sprg 0 Lecture

9 Schmitt Trigger Schmitt Trigger C eedback = Oscillator o Schmitt trigger hae different triggers pots for risg edge and fallg edge. 0K op amp. =0k, =.k, =0K, C=.uf Can be used to reduce false triggerg This is NOT a negatie feedback circuit. 0.uf 0K Display and on the scope. Set =.k. Predict what happens to the frequency..k.0 Sprg 0 Lecture.0 Sprg 0 Lecture High Pass ilter HP Differentiator Insights (d) (d) C 0 d slope = d / octae slope = 0 d / decade 0 d slope = d / octae slope = 0 d / decade j C s C j C s C j C Degrees f LO or f d Degrees f LO or f d 90 o 5 o 0 o PHSE LED sc sc ; sc s j ; 90 o 5 o 0 o PHSE LED 5 o at low frequency sc 5 o f LO or f d sc f LO or f d multiplyg by s equals differentiation differentiation works only at f << f lo.0 Sprg 0 Lecture 5.0 Sprg 0 Lecture

10 Low Pass ilter LP Integrator Insights (d) (d) 0 d C 0 d slope = d / octae slope = 0 d / decade slope = d / octae slope = 0 d / decade f HI or f d sc j X C j X C j C Degrees j C j C 0 o 5 o 90 o PHSE LG f HI or f d ; sc at high frequency s j; sc Degrees 0 o 5 o 90 o PHSE LG f HI or f d f HI or f d sc sc tegration works only at f >> f HI sc diidg by s equals tegration.0 Sprg 0 Lecture.0 Sprg 0 Lecture 8 Why? requency Doma Insight With, any DC bias current will saturate sce the DC ga is the open loop ga Integration and differentiation easy to understand time doma In frequency doma, difference between square wae and triangle wae is amplitude and phase same harmonics. Integrator (LP) rolls off harmonics and phase shift to create a triangle wae Differentiator (HP) amplifies harmonics and phase shift to create a square wae..0 Sprg 0 Lecture 9.0 Sprg 0 Lecture 0

11 oltage ollower (buffer) Differential Input asic Opmp Circuits Nonertg Integrator C C t dt 0k?k 0k 0. L5 0. [a] Crossoer Distortion (hole) 5 5 N90 5 N90 5 L Lecture.0 Sprg 0 0k Why is [b] better??k L5 [b] k N N L Diode iasg N9 N00 E =5. / watt D 9 C [rom Preamplifier]?opamp D N9 E =5. / watt N905 N00 L Lecture.0 Sprg 0