Opamp Pesentation

Definition: An opamp is a direct coupled high gain differential amplifier. It is a versatile device. It can be used as an AC and DC input signal. It is basically used for mathematical operations like addition, subtraction, differentiation, integration etc…

Block diagram of Op-Amp [µa 741]: Non inverting i/p Input stage

Intermediate stage

Level transistor

Output stage

Inverting i/p Dual input balanced output differentiator amplifier

Dual input unbalanced output differentiator amplifier

Complementary Emitter follower

push pull amplifier

output

An opamp is a direct coupled high gain differential amplifier. It consists of one or two difference amplifier followed by level transistor and an output stage. The input stage is a dual input balanced. Output differential amplifier which provides voltage gain of establishes high input impedance of the opamp. The intermediate stage is another dual input unbalanced output differential amplifier which provides the output which is parallel to the first stage. This stage is driven by output of the first stage. Both the input and intermediate stage are direct coupled, due to this DC voltage at output of intermediate stage is above ground level. Therefore a level transistor is used to shift the Dc level and the output of the intermediate stage to zero volts with respect to ground level, for this an emitter follower is used.

SYMBOL OF Op-Amp Inverting terminal

+Vcc Output

Non inverting terminal

-Vee

An opamp has two inputs and one output terminal. The –ve terminal is called inverting terminal and +ve terminal is called non inverting terminal. The output is the difference between two input terminal Vα=V0=V2-V1

Characteristics of opamp {ideal}: • • • • • •

input impedance = ∞{Ri=∞} output impedance = 0 { Ro= 0} infinite voltage gain = ∞ {Av=∞} Characteristics do not drift with temperature. perfect balance when V1=V2 when V0=0 Infinite common mode rejection ratio

Parameters of Op-Amp • • • • • • • • •

Input offset voltage {Vio} Input offset current {Iio} Output offset voltage Common mode rejection ratio Gain Power supply rejection ratio bandwidth product Output impedance Input impedance Zi or Ri

Input offset voltage {Vio}: Input offset voltage is that voltage which must be applied between input terminals to balance the amplifier. Input offset current {Iio}: Input offset current is the difference between the currents enters the input terminal of the balanced amplifier. Output offset voltage: It is the difference between two DC voltages present at two output terminals when two input terminals are grounded. Common mode rejection ratio: It is defined s the ratio of differential voltage gain ‘Ad’ to the common mode voltage gain ‘Ac’. It is denoted by ‘∫’ {Et}

Gain: It is the ratio of output voltage to difference of two input voltage. Av=V0/Vd Av=V0/ (V2-V1) Power supply rejection ratio: It is the ratio of change in an opamp input offset voltage. ∆Vio to variation in supply voltage is called power supply rejection ratio. It is expressed in µv/Ώ (micro volts/ohm} PSRR=∆Vio/∆V µv/Ώ Bandwidth product: It is the product of bandwidth of the opamp when voltage gain is unity.

Input impedance Zi or Ri: It is the total resistance between inverting and non inverting inputs. Output impedance: It is the equivalent resistance i.e. measured between output terminal and ground.

Applications of Op-Amp • • • • • • • • • • • •

Inverting amplifier Non inverting amplifier Voltage follower Summing amplifier Difference amplifier Differentiator Integrator Comparator Zero crossing detector and peak detector Window comparator Multivibrator Active filter

Inverting amplifier Inverting amplifier is one in which the output is 180˚ out of phase with the input. Here the input signal is applied to inverting terminal and non inverting terminal is grounded. Rf is called as feed back resistor. Since the voltage gain and input impedance are infinite virtual ground exists between inverting and non inverting terminal therefore junction ‘A’ is called summing point. Apply kcl at point ‘A ‘ i+if=0 i = -if (Vi-0) /Rs=(0-V0) /Rf V1/Rs = -Vo/Rf V0/V1= -Rf/Rs Av= -Rf/Rs The –ve sign implies that the output is 80˚out phase with input.

Circuit & Wave Form Of Inverting Amplifier

Vi t Vo

t

Non invertintg amplifier A Non inverting amplifier is one in which the output is in phase with the input. Here the input signal is applied to non inverting terminal and the inverting terminal is fed with the feed back resistor Rf. Apply kcl at point A -i-if=0 -i=if i=-ifVi/Rs= (V1-V0)/Rf V1/Rs= (-Vi/Rf) + (V0/Rs) V1/Rs+V1/Rf=V0/Rf V1 [1/Rs+1/Rf] =V0/Rf V1/V0= [Rs+Rf]/Rf V1/V0=Rs/Rs+Rf/Rf V0/V1=1+ (Rf/Rf)

Circuit & Wave Form Of Non Inverting Amplifier

Vi

t

Vo t

Summing amplifier A summing amplifier using opamp it is one which adds two or more inputs. The input voltage are applied to inverting terminal and non inverting terminal is gnd. Due to virtual gnd i.e. because of infinite input impedance and infinite voltage gain point a is held at zero potential. Apply kcl at node A I1+I2-If=0 I1+I2=If V1/R1+V2/R2= (0-Vf)/Rf V1/R1+V2/R2=-Vf/Rf Assume R1=R2=RF=R (V1+V2)R=-V0/R V0=-(V1+V2) The –ve sign implied that the input voltage is fed to inverting terminal

Difference amplifier: It is one whose output is the difference of one or more input. Assume V2=0, A=0, B=0 Apply Kcl at Node A I1-I2=0 V1/R1=-VO/R2 -------->1 Assume V1=0 A=0 B=0 Apply kcl at node A I1-I2=0 V2/R1=0/R2 V2/R1=0 -------->2 Subtract 1 from 2 V2/R1-V1/R1=V0/R2 R1=R2=R V2-V1=0 V2/R1-V1/R1=0-(-V0)/R2

Voltage Follower Fig shows a voltage follower circuit. A voltage follower is one in which the output voltage follower the input voltage. The input voltage is applied to non inverting terminal and inverting terminal is directly shorted to output terminal due to infinite input impedance and infinite voltage gain the inverting terminal is also held at Vi. Therefore output voltage is equal to the input voltage i.e. V0=Vi. Since no resistors are used gain of the voltage follower is equal to unity.

Differentiator A Differentiator produces an output voltage which is directly proportional to derivative of input voltage. Fig shows a Differentiator circuit which uses an RC network. The through the capacitor is given by I=Cdvi/dt and current through the resistor r is given by If=-V0/R due to virtual gnd at the opamp input. I=If Dvi/dt=-V0/R V0=-RCdvi/dt Therefore V0αdvi/dt The –ve sign implies that the signal is connected to inverting terminal. Vi =vc=Q/c Vi =Q/c Vi = (1/c)Q dvi/dt = 1/c (dq/dt) dvi/dt = i/c i=cdvi/dt

Circuit & Wave Form Of Differentiator

Vi t Vo t

Integrator A Integrator is one whose output voltage is directly proportional to the integral of input voltage. Current through the resistor is given by I= (Vin/R) Current through the capacitor is If = -C(dVout/dt) Due to the virtual ground at the input I=If (Vin/R) = -C(dVout/dt) dVout = -(Vin/RC)dt by integrating the above equation we get Vout is directly proportional to the integral of Vin -ve sign indicates that input voltages are fed to inverting terminal

Circuit & Wave Form Of Integrator

Vi

Vo

Comparator It is a circuit which compares a signal voltage at one input of an opamp with a known reference voltage at the other input. Comparator is basically an open loop opamp with output equals to +/-Vsat There are basically two types of comparator s non inverting and inverting comparators.

Inverting comparator with +ve Ref Fig shows an inverting comparator with +ve ref voltage. Here inverting terminal is supplied with input signal and non inverting terminal with known ref voltage. When Vin< Vref then V0 = +Vsat because the non inverting terminal voltage is greater than inverting terminal voltage. When Vin > Vref then V0 = -Vsat because the inverting terminal voltage is greater than non inverting terminal voltage. When Vin = Vref the transistor takes places from +Vsat to -Vsat.

Circuit & Waveforms Of Inverting Comparator With +Ve Ref

Vi t Vo t

Inverting comparator with –Ve Ref Figure shows an inverting comparator with –ve reference voltage. The input signal is applied to inverting terminal and known reference voltage is applied to non-inverting terminal. When Vi<-Vr then Vo=+Vsat Because a non inverting terminal voltage is greater than inverting terminal voltage When Vi>-Vr then Vo=-Vsat because inverting terminal voltage is greater than non inverting terminal voltage. When Vi=Vr the transition takes place from –Vsat to +Vsat

Circuit & Waveforms Of Inverting Comparator With -Ve Ref

Vi

t Vo t

Non Inverting [+ve Ref] The fig shows non inverting comparator with +ve ref. The input is applied to non inverting terminal and reference voltage is applied to inverting terminal. When Vi< Vr, V0 = -Vsat because the inverting terminal voltage is greater then non inverting. When Vi> Vr, V0= Vsat because the non inverting terminal voltage is greater than inverting terminal.

Circuit & Waveforms Of Non Inverting Comparator With +Ve Ref

Vi t Vo t

Non Inverting [-ve Ref] The fig shows non inverting comparator with -ve ref. The input is applied to non inverting terminal and reference voltage is applied to inverting terminal. When Vi< Vr, V0 = Vsat because the non inverting terminal voltage is greater than inverting. When Vi> Vr, V0= Vsat because the inverting terminal voltage is greater than non inverting terminal.

Circuit & Waveforms Of Non Inverting Comparator With -Ve Ref

Vi t

Vo t

Astable multivibrator A multivibrator which generates square wave of its own is called astable multivibrator. An astable multivibrator output frequency depends on charging and discharging of a capacitor. There is no input signal to this multivibrator. Initially assume that the output is in +ve saturation +Vsat. The voltage at non inverting terminal becomes Vlt=(R2/R1+R2)+Vsat

So capacitor starts charging exponentially towards +Vsat as shown by solid arrows. But never reaches to +Vsat because once the capacitor voltage reaches just above Vlt the output switches from +Vsat to –Vsat. Now the –ve voltage is Vlt=(R2/R1+R2) –Vsat It is fed back to the non inverting input. Now the capacitor starts discharging from +Vlt to zero volts completely and recharges towards –Vsat in the reverse direction as shown by the dotted arrows. When the capacitor voltage reaches just below the lower trigger voltage –Vlt then the output switches back from –Vsat to +Vsat. Thus because of continuous charging and discharging of capacitor the output is a square wave with 50% duty cycle.

The time period is given by T = 2Rc ln(1+ß/1-ß) Where ß --->R2/(R1+R2) The frequency of output wave form is given by F = 1/T

Vi t Vo

t

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