JFET Application Current Source

Transcription

1 MOSFET Op mps, 5 Imperfections Opamp applications Daylight sags cknowledgements: on oscoe, Neamen, Donald: Microelectronics Circuit nalysis and Design, rd Edition JFET pplication Current Source 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 JFET I DSS current with GS 0 P pchoff oltage i GS D I DSS P N Sprg 08 i D 0 Quiz Design Question Design a circuit such that when a momentary push button switch is closed, switched 9 will be supplied to your design under test for exactly two mutes. You hae aailable a N000 (n channel MOSFET) and a ZP0 which is a pchannel complement to the N000. You hae a 00uf capacitor that is exactly 00uf with negligible leakage current. ased on measurements, your N000 has a GS (th).0 olts and the ZP0 has a GS (th).0. The 9 battery is a constant 9 durg the test. ln(.).098 ln(.).50 ln(.).9 [one or more of these constants may be required for your design]

2 Opmps ctie deice: 0 a( ); note that it is the difference of the put oltage! aopen loop ga ~ Most applications use negatie feedback. Comparator: no feedback 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) o LF5 Opmp Packagg LM 5 5 JFET Input Opamp JFET Differential Pair Small Signal Model.0 Sprg 08 8

3 Circuit Discrete transistors, resistors, capacitor, diode 9 0 Discrete Differential (Emitter Coupled) Pair JT Diff Pair Small Signal Model.0 Sprg 08

4 Differential Pair Common Mode oltage Differential Pair Differential Mode oltage Small Signal Model.0 Sprg 08.0 Sprg 08 MOSFET 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 β x β x β aβ a ( a β ) a x β! $! aβ $ x # &# & " aβ % " aβ % Typical alues: a~00,000 & a β >> ok for aa(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) a β is the loop ga.0 Sprg Sprg 08 Lecture

5 Op mps irtual Node With negatie feedback, put will drie the put oltage difference to zero > Input current 0 enefits of Feedback 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 Feedback Loss of ga; need more stages Greater tendency for stability (oscillations) Idiot proof a x aβ aβ 8 Electrical Characteristics LF5 lmost zero not rail to rail.0 Sprg 08 Lecture 9 0

6 LM ail to ail Output ail to ail Input Decibel (d) Open Loop Frequency Ga o d 0 log i Po d 0 log Pi log 0 ().0 00 d 00, d 0,000 0 d pot half power pot 0 d, d 00 0

7 s NonInertg mplifer 5 5 β (not to be confused with β of a JT) for fite Zero put current; therefore.0 Sprg 08 5 so or β β but ; 5 s 5 Open Loop Frequency Ga Examples at Hz, 000 Hz, and 0kHz oltage ga 0d 00; 00kΩ, kω; [0 0.d!] β0.0 t Hz, ol 00 d x , d 5 β t 000 Hz, ol 0 d d β t 0 khz, ol d. x d β.0.. β is the loop transfer function aβ is the loop ga s 5 Comparison So Why JT Opamps? 5 Input deice JT JFET 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 FETs. Like most JFET op amps, the LF5 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 08 8

8 Ga andwidth Product Constant (No free lunch) Opmp Imperfections eal World Ga: 0d 0 andwidth 5x0 Ga andwidth product 5x0 Input offset oltage Input Current ias Input Offset Current Fite Output oltage Swg Fite Current Fite Ga, ga bandwidth product oltage Noise Johnson Noise Phase Shifts Slew ate 9 0 Input Offset oltage * Offset djustments : 000µ 5: 0,000µ Current technology: 0µ * nalog Deices MT0 Tutorial

9 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. F I I F I 5 I 5 OUT OFF but with no put signal, and we want I thus : OFF I I OFF OFF ; F I I I F OFF 0 0 I I OUT 0, so : F asa condtion for no offset at o F F OUT : 00na 5: 0.05na Current technology: 00pa LT0 F // 5 I DIFF OL I 5 OUT OUT OUT [ I F // I ] OUT 0 if // F [ I F // I ] OUT 0 if // F OL OL * nalog Deices MT08 Tutorial 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 0 log 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. >> f i i f f f 0 5

10 Schmitt Trigger 05 Quiz Question Digital Clock Usg 0Hz The 0 Hz waeform crosses the reference oltage multiple times because of the 00m noise. o Schmitt trigger hae different triggers pots for risg edge and fallg edge. Design a circuit with a Schmitt trigger ( Your design ) usg a LM with the thresholds greater than 00m to fix the problem. LM805 oltage regulator IC is used to proide a 5 put to power the digital clock circuit (modeled as a resistor) as well as to supply 5 for the LM Your design. Can be used to reduce false triggerg This is NOT a negatie feedback circuit. 8 Schmitt Trigger C Feedback Oscillator High Pass Filter HPF (d) 0K opamp. 0k,.k, 0K, C. uf C 0 d slope d / octae slope 0 d / decade 0.uf 0K Display and on the scope. Set.k. Predict what happens to the frequency. j ωc s C j ωc s C j ωc 90 o Degrees f LO or f d PHSE LED 5 o 0 o.k 5 o f LO or f d 9 0

11 Differentiator Insights Low Pass Filter LPF (d) (d) 0 d slope d / octae slope 0 d / decade C 0 d slope d / octae slope 0 d / decade sc ; s jω; sc sc at low frequency sc << sc Degrees 90 o 5 o 0 o 5 o f LO or f d f LO or f d PHSE LED j X C j X sc C j ωc j ωc j ωc 0 o 5 o 90 o Degrees PHSE LG f HI or f d f HI or f d multiplyg by s equals differentiation differentiation works only at f << f lo Integrator Insights (d) Why? 0 d Degrees slope d / octae slope 0 d / decade f HI or f d With, any DC bias current will saturate sce the DC ga is the open loop ga 0 o PHSE LG ; sc s jω; 5 o 90 o at high frequency sc >> f HI or f d sc sc tegration works only at f >> f HI sc >> diidg by s equals tegration

12 Integration and differentiation easy to understand time doma Frequency Doma Insight In frequency doma, difference between square wae and triangle wae is amplitude and phase same harmonics. Integrator (LPF) rolls off harmonics and phase shift to create a triangle wae Differentiator (HPF) amplifies harmonics and phase shift to create a square wae. oltage Follower (buffer) Differential Input asic Opmp Circuits Nonertg Integrator C 5 ( ) C t dt Crossoer Distortion (hole) Diode iasg 0kΩ 0kΩ 0kΩ 0.µF LF5 0.µF?kΩ [a] 5 5 N90 5 0kΩ N90 L 5 LF5?kΩ [b] 0.µF 0.µF N N L C [From Preamplifier]?opamp D 9 D N9 N9 E 5.Ω / watt E 5.Ω / watt N905 N00 N00 L Why is [b] better? F Lecture.0 Sprg 08 Lecture.0 Sprg 08 8