Operational AmplifiersDigital Simulation

Nate Moehring

Operational Amplifiers

• Used in amplifiers, attenuators, and filters

• Ideal Properties of an Op Amp:• Infinite open-loop gain, Aol = infinity

• Infinite input impedance, Rin = infinity

• Zero output impedance, Rout = 0

• Zero noise contribution

• Zero DC output offset

• Infinite bandwidth, infinite frequency response

• Both differential inputs stick together

Why use op amps?

• Extremely high gain (10,000x)

• Infinite input resistance good for adding multiple sources (0 current = 0 voltage drop)

• Develop high pole filters

Example

• Passive Band Pass

Example

• Active Band Pass

Example

• Active Band Pass w/ amplifier

Op Amp configurations

• Rails +/- to higher input

• (Takes difference

• Multiplies by 10,000)

Comparator

Op Amp configurations

• Vo = -Vi(Rf/Ri)

• (Feedback loop creates

180o out of phase signal

that eliminates large Aol)

Inverting Gain

Op Amp configurations

• Vo = Vi(1 + Rf/Ri)

• (Feedback line

must always go to negative

input. Otherwise, we

would be summing infinitely.)

Non - Inverting Gain

Op Amp configurations

• Vo = Vi(1 + 0/) = 1

• Also called “follow me”

amplifier.

Buffer – Isolation Amplifier

Op Amp configurations

• fc = 1/(2RC)

• “Active”, although this

filter is no better than a

passive filter.

Filter

Op Amp configurationsDifferential Amplifier

• Vo = R2/R1(V2-V1)

• “Subtractor”

Op Amp configurationsSumming Amplifier

• Vo = V1(R3/R1) + V2(R3/R2)

• “Adder”

Parametric resultsw/ transient

Parametric resultsw/ AC Sweep

TransistorsBipolar Junction Transistors BJT (npn)

• Uses a small amount of current to allow a large amount of current to pass through it. (Amplifier) (Flood gate)

• Also used as “on/off” switch. (Transistor Transistor Logic (TTL))

• Bipolar.olb

Base

Collector

Emitter

NOT Gate

These TTL representations are

greatly simplified for logical interpretation and are not meant for

actual implementation as seen here.

AND Gate

OR Gate

Digital Simulation

• Use Bias Point calculations manually changing VDCs

• Use Bussed STIM sources to perform transient analysis.

• Get digital components from 74ac (pro) or Eval library (student)

Using STIM sources

• Use STIM4 from source library• Draw bus wire• Place “Net Alias” ON bus, ie. A[0:3]• Use “Bus Entry” tool to extract lines• Name lines using Place “Net Alias”• Double click STIM4 to create pattern• Probe input and outputs• Run transient simulation

Voter Problem

• Build using 74xx logic gates

• Input manually using VDC sources and view outputs using Bias Voltages

• Input using STIM4 providing all possible input combinations.