Chapter 4The Operational Amplifier

Ckts W/ Operational AmplifiersWhy Study OpAmps At This Point?OpAmps Are Very Useful Electronic ComponentsWe Have Already Developed The Tools To Analyze Practical OpAmps CircuitsThe Linear Models for OpAmps Include Dependent SourcesA PRACTICAL Application of Dependent Srcs

OpAmp Symbol & ModelThe Circuit Symbol Is a Version of the Amplifier TRIANGLEThe Linear ModelTypical Values

-Vcc, when A(Vp-Vn)+VccVo =

2 Ways of Using Op-AmpsOpen Loop: very high gain amplifierUseful for comparing 2 voltagesFixed gain, always at MAX OUTPUT!!Closed Loop with negative feedbackUseful for amplifying, adding, subtracting, differentiation and integration (using capacitors)Variable gain, controlled by resistor selection

Open-Loop examplesIdeal Comparator and Transfer CharacteristicZero-Cross Detector Heart of Solid State Relay Cnrtl

Closed Loop Example: Unity Gain Buffer Controlling Variable = Solve For Buffer GainThus The Amplification

Sheet1

Op-AmpBUFFER GAIN

LM3240.99999

LMC64920.9998

MAX42400.99995

PA050.999992

Consequences for Vp-VnNormally, A is 10,000 or more, so to avoid saturation, abs(Vp-Vn) must be < Vcc/10000, or, if Vcc = 20V, about 2 mV which is negligible for most circuits

With an Ideal Op-amp, A = infinity, so Vp = Vn to avoid saturation

Negative Feedback resistors force Vp = Vn i.e. if Vp-Vn gets large, A(Vp-Vn) pulls back toward zero (more on this later)

Butalways remember Vcc/-Vcc limitsif Vo saturates Vp does not = Vn !!!Terminal Characteristics for an ideal op amp in its Linear Range:Ip = In = 0 (very high input resistance=1MOhm)Io can be very large (coming from Vcc/-Vcc)

Inverting Amplifier Vo = -RfVs Rs

When in linear region

Inverting Amplifier with Feedback Path brokenVo= -AVs

Saturation(unless Vs

Summing Amplifier Vo = - Rf (Va + Vb + Vc) (in linear region) Rs

Non-Inverting AmplifierVo=(Rs+Rf) Vg Rs

In linear region

Difference AmplifierVo= Rb (Vb Va) in linear region AND Ra IFF Ra/Rb = Rc/Rd

Common Mode Rejection RatioIF Ra/Rb =(1-e) Rc/Rc (e=very small)CMMR = abs(1 + Rb/Ra) e

Comparator, used in digital circuits

Application, the Flash ConverterComparators quantize

Encoder converts to binary