Operational Amplifiers

Book: Op-amp by Ramakant Gayakwad

Introduction

• The operational amplifier (also known as Op-Amp) is a multi-terminal direct coupled high gain amplifier which consisting of one or more differential amplifiers and a level transistor and an output.
• The Op-Amp is a versatile device which can be used to amplify both DC and AC signals and these are mainly designed for performing mathematical operations like addition, subtraction, multiplication etc.

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Block Diagram of Operational Amplifier

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Block Diagram of Operational Amplifier

• Input Stage − The input stage is the dual input, balanced output differential amplifier. This stage provides most of the voltage gain and introduces the input resistance of operational amplifier.
• Intermediate Stage − This stage is dual input, unbalanced output differential amplifier, which is driven by the output of first stage.

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Block Diagram of Operational Amplifier

• Level Shifting Stage − Since direct coupling is used, therefore the DC voltage at the output of intermediate stage is above the ground potential. Hence, the level shifting transistor circuit is used after intermediate stage to shift the DC level at intermediate stage output downward to zero volts with respect to ground.
• Output Stage − The output stage is a push-pull complementary amplifier. The output stage increases the output voltage. The output stage also provides low output resistance.

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Pin Diagram

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Pin Diagram

• Pin4 & Pin7 (Power Supply): Pin7 is the positive voltage supply terminal and Pin4 is the negative voltage supply terminal. The 741 IC draws in power for its operation from these pins. The voltage between these two pins can be anywhere between 5V and 18V.
• Pin6 (Output): This is the output pin of IC 741. The voltage at this pin depends on the signals at the input pins and the feedback mechanism used. If the output is said to be high, it means that voltage at the output is equal to positive supply voltage. Similarly, if the output is said to be low, it means that voltage at the output is equal to negative supply voltage.

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Pin Diagram

• Pin2 & Pin3 (Input): These are input pins for the IC. Pin2 is the inverting input and Pin3 is the non-inverting input. If the voltage at Pin2 is greater than the voltage at Pin3, i.e., the voltage at inverting input is higher, the output signal stays low. Similarly, if the voltage at Pin3 is greater than the voltage at Pin2, i.e., the voltage at non-inverting input is high, the output goes high.

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Pin Diagram

• Pin1 & Pin5 (Offset Null): Because of high gain provided by 741 Op-Amp, even slight differences in voltages at the inverting and non-inverting inputs, caused due to irregularities in manufacturing process or external disturbances, can influence the output. To nullify this effect, an offset voltage can be applied at pin1 and pin5, and is usually done using a potentiometer.
• Pin8 (N/C): This pin is not connected to any circuit inside 741 IC. It’s just a dummy lead used to fill the void space in standard 8 pin packages.

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Characteristics of Op-Amp

• Input Offset Voltage
• It is the voltage, that must be applied between the two terminals of opamp to null the output. Maximum it can be 6mv for 741 IC.

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Characteristics of Op-Amp

• Input Offset Current
• The algebraic difference between the currents into the inverting and non inverting terminals is referred to as input offset current. Max 200 nA.

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Characteristics of Op-Amp

• Input Bias Current
• It is the average current that flows into the inverting and non inverting input terminals of op-amp. Max 500nA.

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Characteristics of Op-Amp

• Differential Input Resistance
• It is the equivalent resistance that can be measured at either inverting or non inverting input terminal with other terminal connected to ground. Max it can be 2 MΩ.
• Input capacitance
• It is the equivalent capacitance that can be measured at the inverting or non inverting input terminal with other terminal connected to ground. Max it can be 1.4 pF.

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Characteristics of Op-Amp

• Offset Voltage Adjustment Range
• Pin no 1 and 5 are used for offset null purpose. 10K resistor is connected between 1 and 5 and its variable arm is connected with pin 4, -VEE. By varying this potentiometer output offset voltage can be reduced to zero. Max range ± 15mv

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Input Voltage Range

• When same voltage is applied to both input and output terminals, the voltage is called common mode voltage. This is the voltage that can be applied without disturbing operation of opamp.
• Maximum +13v and Minimum -13V
• The input voltage range is range of common mode voltages over which the offset specifications apply.

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Common Mode Rejection Ratio

• It is the ratio of differential mode gain to common mode gain.
• CMRR= Ad / Acm
• Here Ad is large signal voltage gain and Acm is common mode voltage gain
• Acm is very small, so CMRR is very high it is expressed in decibel(db)
• Typically it is 90 db.

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Supply Voltage Rejection Ratio

• It is the change in opamp input offset voltage caused by variations in supply voltage, is called the supply voltage rejection ratio (SVRR).
• SVRR = ΔVio/ΔV
• It is also known as PSRR - Power supply rejection ratio and PSS - Power supply sensitivity.

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Large signal voltage gain

• Opamp amplifies output between two input terminals, the voltage gain of the amplifier is given by
• Voltage gain = (Output voltage)/(differential input voltage)
• A= Vo/Vid
• Typically 200000

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Output Voltage Swing

• Output voltage swing is indicates positive and negative saturation voltages of the opamp.
• Typically it is +13v and -13V. For RL >= 2KΩ
• It gives 26V peak to peak undistorted sine wave for ac input signals.
• The output voltage never exceed these limits of +Vcc and -Vee

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Output Resistance

• It is the equivalent resistance that can be measured between the output terminal of the opamp and the ground.
• Typically it is 75Ω for741 IC.

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Output Short Circuit Current

• No one connects output terminal to ground intentionally.
• But by mistake , if it is connected to ground, current more than offset current or bias current will flow through ground.
• It may damage opamp.
• If datasheet shows Isc=25mA, it means opamp has protection circuit to withstand 25mA current.

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Slew Rate

• It is defined as the maximum rate of change of the output voltage per unit of time and is expressed in voltage/micro seconds.
• SR=dVo/dt|maximum Unit is V/us
• It indicates how rapidly the output of an op-amp can change in response to changes in the input frequency.
• 741 IC has 0.5V/us

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Supply Current

• It is the current drawn by the opamp from power supply.
• It is typically 2.8 mA

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Power Consumption

• It is the amount of quiescent power that must be consumed by the op-amp in order to operate properly.
• The amount of power consumed by the 741 IC is 85 mW.

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Transient Response

• The response of any practically useful network to a input is composed of two parts, the transient and steady state response.
• Transient response is that portion of the response before the output attains fixed value.
• Once reached this fixed value remains at that level and is therefore called as steady state value.
• Response after this fixed value is independent of time and called steady state response.

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Other Parameters

• Gain-Bandwidth product
• Average Temperature coefficient of input offset voltage and current
• Long Term input offset voltage and current stability
• Equivalent input noise voltage and current
• Channel separation

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Ideal Opamp Characteristics

• Infinite voltage gain A
• Infinite input resistance Ri, so that almost any signal source can drive it and no loading of preceding stage
• Zero output resistance Ro, so that the output can drive an infinite number of other devices
• Zero output voltage when input voltage is zero

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Ideal Opamp Characteristics

• Infinite bandwidth so that any frequency signal from 0 to ∞ Hz can be amplified without attenuation.
• Infinite common mode rejection ratio so that the output common mode noise voltage is zero.
• Infinite slew rate so that output voltage changes occur simultaneously with input voltage changes.

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Ideal Opamp Characteristics

• Infinite bandwidth so that any frequency signal from 0 to ∞ Hz can be amplified without attenuation.
• Infinite common mode rejection ratio so that the output common mode noise voltage is zero.
• Infinite slew rate so that output voltage changes occur simultaneously with input voltage changes.

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Op-Amp equivalent circuit

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Op-Amp equivalent circuit

• Here AVid is equivalent Thevenin’s voltage source and Ro is the thevenin’s equivalent resistance looking back into the output terminal of op-amp.
• The equivalent circuit is useful in analysing basic operating principles of opamp.
• Here output voltage is given by
• Vo=A Vid=A(V1-V2), A is large signal gain, Vid is differential input voltage , V1 and V2 are voltages of non inverting terminal and inverting terminal with reference to ground.

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Op-Amp equivalent circuit

• As per above equation, opamp amplifies algebraic difference between the two input voltages.
• It does not amplify individual voltages.
• For this reason polarity of output voltage depends on the polarity of the difference voltage.

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Ideal Voltage Transfer Curve

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