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ME 6405 Operational Amplifiers 10/2/12

ME 6405 Operational Amplifiers 10/2/12. Alex Ribner  Eric Sanford  Christina Biggs. Outline by: Alex Ribner. What is an Op Amp? Ideal versus Real Characteristics Types of Op Amps Applications. Background.

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ME 6405 Operational Amplifiers 10/2/12

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  1. ME 6405Operational Amplifiers10/2/12 Alex Ribner Eric Sanford Christina Biggs

  2. Outlineby: Alex Ribner What is an Op Amp? Ideal versus Real Characteristics Types of Op Amps Applications

  3. Background Operational amplifiers (op-amps), use an external power source to apply a gain to an input signal. Made of resistors, transistors, diodes and capacitors. Variety of functions such as: mathematical operations, perform buffering or amplify AC and DC signals.

  4. 741 Op-Amp Schematic current mirror current mirror voltage level shifter output stage current mirror high-gain amplifier differential amplifier

  5. Timeline 1946 –patent for an op-amp using vacuum tubes. 1953 –op-amps for sale 1961 – discrete IC op-amp 1965 – successful monolithic op-amps 1968 – uA741

  6. General Schematic Active device! Requires power. Some Op Amps have more than these 5 terminals

  7. Feedback • Closed loop configurations reduce the gain of the amplifier, but adds stability. • Part of the output signal is applied back to the inverting input of the amplifier. • Op amps use negative feedback. • Negative feedback helps to: overcome distortion and non-linearity, tailor frequency response, and stabilize circuit properties from outside influences such as temperature.

  8. Behavior of an Op Amp • Achieves: • Very high input impedance • Very high open loop gain • Very low output impedance. In Three Steps: Differential input stage, draws negligible amounts of input current enables assumption for ideal Op Amp properties. Voltage gain stage, responsible for gaining up input signal and sending it to output stage. Output stage, delivers current to op amp’s load.

  9. ‘Golden Rules’ of Ideal Op-Ampsby: Eric Sanford • These characteristics can be summarized with two ‘golden rules’: 1 - The output attempts to do whatever is necessary to make the voltage difference between the inputs equal to zero (when used in a closed-loop design). 2 - The inputs draw no current.

  10. i- = 0 V- - Zout Zin Vout K + V+ i+ = 0 Ideal Op-Amp • Characteristics: • Gain, K = Vout / (V+-V-) = ∞ • Input impedance, Zin = ∞ • Input currents, i+ = i- = 0 • Output impedance, Zout = 0 • Unlimited bandwidth • Temperature-independent

  11. Real Op-Amp • Characteristics (typical values): • Gain, K = Vout / (V+-V-) = 105 < K < 109 • Input impedance, Zin = 106  (BJT),109  - 1012  (FET) • Input currents, i+ = i- = 10-12 – 10-8 A • Output impedance, Zout = up to 1000  • Finite bandwidth, 1-20 MHz • All parameters change with temperature

  12. Ideal versus Real Op-Amps Ideal Real

  13. Saturation Voltages Note: vd = vin, v0 = vout, vcc = source voltage + saturation:Vout = Vsat+≈ Vcc+ Linear Mode: Vout = K (V+- V-) - saturation:Vout = Vsat-≈ Vcc-

  14. Basic Op-Amp Typesby: Christina Biggs Inverting Non-Inverting Integrating Differential Summing

  15. Three Op Amp Setups Differential Input 2) Inverting Mode 3) Non-inverting Mode

  16. Non-Inverting Amplifier Analysis • Amplifies the input voltage by a constant • Determined by voltage output

  17. Derivation of Non-inverting Amplifier Vout=K(V+-V-) R1/(R1+R2)  Voltage Divider Rule V-=Vout (R1/(R1+R2) ) Vout=[Vin-Vout (R1/(R1+R2))] K Vout=Vin/[(1/K)+ (R1/(R1+R2))] As discussed previously assuming, K is very large, we have: Vout=Vin/(R1/(R1+R2)) Vout=Vin (1+(R2/R1))

  18. Inverting Amplifier • Amplifies and inverts the input voltage • Polarity of the output voltage is opposite to the input voltage • Determined by both voltage input and output virtualground

  19. Derivation of Inverting Amplifier Vout=K(V+-V-) V-=Vout(Rin/(Rin+Rf))+Vin(Rf/(Rin+Rf)) V-=(VoutRin+VinRf)/(Rin+Rf) Vout=K(0-V-) Vout=-VinRf/[(Rin+Rf)/K+(Rin)] Vout=-VinRf/Rin

  20. Op-Amp Integrator • Integrates the inverted input signal over time • Magnitude of the output is determined by length of time voltage is present at input • The longer the input voltage is present, the greater the output

  21. Op-Amp Differentiator • Magnitude of output determined by the rate at which the applied voltage changes. • Faster change, greater output voltage • The resistor and capacitor create an RC network

  22. Op-Amp Summing Amplifier • Scales the sum of the input voltages by the feedback resistance and input to produce an output voltage.

  23. Op-Amp Differential Amplifier • Produces an output proportional to the difference of the input voltages If R1 = R2 and Rf = Rg:

  24. Applications Filters, Strain Gages, PID Controllers, Converters, Etc…

  25. PID Controllers • Goal is to have VSET = VOUT • Remember that VERROR = VSET – VSENSOR • Output Process uses VERROR from the PID controller to adjust Vout such that it is ~VSET

  26. Strain Gages Use a Wheatstone bridge to determine the strain of an element by measuring the change in resistance of a strain gauge (No strain) Balanced Bridge R #1 = R #2 (Strain) Unbalanced Bridge R #1 ≠ R #2

  27. 2nd Order Op-Amp Filters • Three 2nd order filters: low pass, high pass, and bandpass.

  28. Conclusion Questions?

  29. References [1] "What Is an Op Amp?" What Is an Op Amp? National, n.d. Web. 25 Sept. 2012. <http://www.national.com/AU/design/courses/268/the02/01the02.htm>. [2] Student Lecture Fall 2010. Op-Amps… and why they are useful to us. [3] Student Lecture Fall 2011. What is an Op-Amp? [4] "Operational Amplifier." Wikipedia. Wikimedia Foundation, n.d. Web. 25 Sept. 2012. <http://en.wikipedia.org/wiki/Operational_amplifier>. [5] "Op-Amp Basics." Op-Amp Basics. N.p., n.d. Web. 27 Sept. 2012. <http://www.bowdenshobbycircuits.info/opamp.htm>. [6] Jung, Walter G. Op Amp Applications Handbook. Burlington, MA: Newnes, 2006. Web. 26 Sept. 2012. <http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html>. [7] "Operational Amplifiers." Operational Amplifiers. N.p., n.d. Web. 25 Sept. 2012. <http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opamp.html>.

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