Operational amplifiers

Operational AmplifiersOperational AmplifiersAnOperational Amplifier, or op-amp for short, is fundamentally a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. These feedback components determine the resulting function or operation of the amplifier and by virtue of the different feedback configurations whether resistive, capacitive or both, the amplifier can perform a variety of different operations, giving rise to its name of Operational Amplifier.Symbol and Pin configuration

AnOperational Amplifieris basically a three-terminal device which consists of two high impedance inputs, one called theInverting Input, marked with a negative or minus sign, (-) and the other one called theNon-inverting Input, marked with a positive or plus sign (+).The third terminal represents theOperational Amplifiersoutput port.

Op Amp Equivalent Circuitvd = v2 v1

A is the open-loop voltage gainv2v1Voltage controlled voltage sourceIC Product

DIP-741Dual op-amp 1458 deviceApplicationsAudio amplifiersSpeakers and microphone circuits in cell phones, computers, mpg players, boom boxes, etc.Instrumentation amplifiersBiomedical systems including heart monitors and oxygen sensors.Power amplifiersAnalog computersCombination of integrators, differentiators, summing amplifiers, and multipliers

Mode of OperationRef:080114HKNOperational Amplifier4Single-Ended Input

+ terminal : Source terminal : Ground 0o phase change + terminal : Ground terminal : Source 180o phase change

Mode of OperationRef:080114HKNOperational Amplifier5Double-Ended Input

Differential input 0o phase shift change between Vo and Vd

Ref:080114HKNOperational Amplifier6Distortion

The output voltage never excess the DC voltage supply of the Op-AmpMode of OperationRef:080114HKNOperational Amplifier7Common-Mode Operation

Same voltage source is applied at both terminals Ideally, two input are equally amplified Output voltage is ideally zero due to differential voltage is zero Practically, a small output signal can still be measured

Note for differential circuits:Opposite inputs : highly amplifiedCommon inputs : slightly amplified Common-Mode Rejection Ref:080114HKNOperational Amplifier11Common-Mode Rejection Ratio (CMRR)Differential voltage input :

Common voltage input :

Output voltage :

Gd : Differential gainGc : Common mode gain

Common-mode rejection ratio:Note:When Gd >> Gc or CMRR Vo = GdVd

Ref:080114HKNOperational Amplifier12Op-Amp PropertiesInfinite Open Loop gain The gain without feedbackEqual to differential gainZero common-mode gainPratically, Gd = 20,000 to 200,000(2) Infinite Input impedanceInput current ii ~0AT- in high-grade op-amp m-A input current in low-grade op-amp(3) Zero Output Impedanceact as perfect internal voltage sourceNo internal resistanceOutput impedance in series with loadReducing output voltage to the loadPractically, Rout ~ 20-100

Ref:080114HKNOperational Amplifier13Frequency-Gain RelationIdeally, signals are amplified from DC to the highest AC frequencyPractically, bandwidth is limited741 family op-amp have an limit bandwidth of few KHz.Unity Gain frequency f1: the gain at unityCutoff frequency fc: the gain drop by 3dB from dc gain Gd

GB Product : f1 = Gd fc20log(0.707)=3dBRef:080114HKNOperational Amplifier14GB ProductExample: Determine the cutoff frequency of an op-amp having a unit gain frequency f1 = 10 MHz and voltage differential gain Gd = 20V/mV

Sol:Since f1 = 10 MHzBy using GB production equationf1 = Gd fcfc = f1 / Gd = 10 MHz / 20 V/mV = 10 106 / 20 103 = 500 Hz

10MHz? HzRef:080114HKNOperational Amplifier15Ideal Vs Practical Op-AmpIdealPracticalOpen Loop gain A105Bandwidth BW10-100HzInput Impedance Zin>1MOutput Impedance Zout0 10-100 Output Voltage VoutDepends only on Vd = (V+V)Differential mode signalDepends slightly on average input Vc = (V++V)/2 Common-Mode signalCMRR10-100dB

Ref:080114HKNOperational Amplifier16Ideal Op-Amp ApplicationsAnalysis Method :

Two ideal Op-Amp Properties:The voltage between V+ and V is zero V+ = VThe current into both V+ and V termainals is zero

For ideal Op-Amp circuit:Write the kirchhoff node equation at the noninverting terminal V+ Write the kirchhoff node eqaution at the inverting terminal V Set V+ = V and solve for the desired closed-loop gainRef:080114HKNOperational Amplifier17Non-inverting AmplifierKirchhoff node equation at V+ yields,

Kirchhoff node equation at V yields,

Setting V+ = V yields

or

Ref:080114HKNOperational Amplifier18Inverting AmplifierKirchhoff node equation at V+ yields,

Kirchhoff node equation at V yields,

Setting V+ = V yields

Notice: The closed-loop gain Vo/Vin is dependent upon the ratio of two resistors, and is independent of the open-loop gain. This is caused by the use of feedback output voltage to subtract from the input voltage.

Ref:080114HKNOperational Amplifier19Multiple InputsKirchhoff node equation at V+ yields,

Kirchhoff node equation at V yields,

Setting V+ = V yields

Ref:080114HKNOperational Amplifier20Op-Amp Integrator

Ref:080114HKNOperational Amplifier21Op-Amp Differentiator

AssignmentMake a research on the following:Op-amp:Maximum RatingsElectrical CharacteristicsInput ParametersOutput ParametersDynamic ParametersHistory timeline of Op-ampTo be hand-written on short bond paper and be submitted next meeting!End!!!