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The Operational Amplifiers Dr. Farahmand

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Opamps Properties IdealPractical ArchitectureCircuits Open Loop Parameters Modes of operation Frequency Response Closed Loop Frequency Response Negative Feedback Inverting Non- inverting overview

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Operational Amplifiers Historically built using vacuum tubes and used for mathematical operations Today, opamps are linear integrated circtuis (ICs) Terminal – Inverting and non-inverting inputs – Dc supplies – Single output

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Opamps Ideal opamps – Infinite BW – Infinite voltage gain – Infinite input impedance – Zero output impedance Practical opamps – wide BW – Very high voltage gain – Very high input impedance – Very low output impedance

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Architecture 3 stages Differential amplifier input stage: -Take the difference between the input signals -If the input base voltage is different: -Vb1 > Vb2 -Ic1 > Ic2 -VRc1 > VRc2 -Vc1 < Vc2

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Modes of Operations Differential amplifiers can be connected in difference ways – Single-ended mode Single input – Differential mode Out of phase inputs Unwanted noise on both inputs is cancelled – Common mode In phase inputs

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Parameters Common mode input voltage – Input voltage range limitation – Typically +/- 10 V with dc voltages of +/- 15 V Input offset voltage (in mV) – Differential dc voltage required between the inputs to force the output to zero volt Input bias current (in nA) – Dc current required by the inputs of the amplifier to properly operate the first stage ( Ibias = (I1 + I2)/2 ); I1 and I2 are the current into inverting and non-inverting inputs Input impedance (in Mega ohm) – Total resistance between the inverting and non- inverting inputs Output impedance (in ohm) – Total resistance at the output Slew rate (in V/usec) – How fast the output voltage changes in response to the input voltage change

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Parameters Common mode input voltage – Input voltage range limitation – Typically +/- 10 V with dc voltages of +/- 15 V Input offset voltage (in mV) – Differential dc voltage required between the inputs to force the output to zero volt Input bias current (in nA) – Dc current required by the inputs of the amplifier to properly operate the first stage ( Ibias = (I1 + I2)/2 ); I1 and I2 are the current into inverting and non-inverting inputs Input impedance (in Mega ohm) – Total resistance between the inverting and non- inverting inputs Output impedance (in ohm) – Total resistance at the output Slew rate (in V/usec) – How fast the output voltage changes in response to the input voltage change ( t) Refer to Table 12-1

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CMRR Common-mod-rejection ratio (CMRR) – The measurement of how the amplifier can reject common more signals – CMRR = Open loop voltage gain / Common mode gain – Often expressed in dB – The larger the better From data sheet Ideally zero/ indicate how much of input noise is passing through

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Open Loop Frequency Response Aol(OL) : Open loop gain In practice Vmid = Vin x AOL(mid) A OL(mid )

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Open Loop Frequency Response Phase response: = -arctan (R/Xc) = -arctan (f/fc) Delay = Period x Phase shift / 360 Frequency response: Aol(OL) = Aol(mid) Critical frequency is the roll-off point

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Open Loop Frequency Response For multiple stages total = ...... Av(dB) = Av1 + Av2 + Av3 + ….

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Closed Loop Frequency Response Non-inverting – Source is connected to the non- inverting input – Feedback is connected to the inverting input – If Rf and Ri are zero, then unity feedback used for buffering – Vo= Inverting – Feedback and source are connected to the inverting input

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Comparators Determines which input is larger A small difference between inputs results maximum output voltage (high gain) Zero-level detection Non-zero-level detection Max and minimum

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Example Vref = Vin(max).R2/(R1+R2)=1.63 V

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Comparator – Impact of noise (unwanted voltage fluctuation) No NoiseWith Noise Inaccuracy!

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Hysteresis (Schmitt triggers) Making the comparator less sensitive to the input noise – Effectively higher reference level – Upper Trigger Point – Lower Trigger Point VUTP = Vout(max).R2/(R1+R2) VLTP = -Vout(max).R2/(R1+R2) VHYS= VUTP – VLTP

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Zener Bounding The output voltage can be limited using Zener diodes – Vout >0 Vz – Vout < 0 Forward biased (0.7) Note that the output signal is inverted Virtual Ground

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Zener Bounding Combined effect ? Bounding the negative values /

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Resources Applets – http://www.chem.uoa.gr/Applets/AppletOpAmps/A ppl_OpAmps2.html http://www.chem.uoa.gr/Applets/AppletOpAmps/A ppl_OpAmps2.html – http://www.falstad.com/circuit/directions.html http://www.falstad.com/circuit/directions.html

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