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Operational Amplifiers

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opamp.html#c4

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Operational Amplifiers

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  1. Operational Amplifiers Brandon Borm Shelley Nation Chloe Milion

  2. Outline • Introduction • Background • Fundamentals of Op-Amps • Real vs. Ideal • Applications

  3. What is an Op-Amp • Low cost integrating circuit consisting of • transistors • resistors • capacitors • Op-amps amplify an input signal using an external power supply

  4. Uses for Op-Amps • Op-Amps are commonly used for both linear and nonlinear applications • Linear • Amplifiers • Summers • Integrators • Differentiators • Filters (High, Low, and Band Pass) • Non-linear • Comparators • A/D converters

  5. Vacuum Tube Op-Amps • First op amps built in 1930’s-1940’s • Technically feedback amplifiers due to only having one useable input • Used in WWII to help how to strike military targets • Buffers, summers, differentiators, inverters • Took ±300V to ± 100V to power http://en.wikipedia.org/wiki/Image:K2-w_vaccuum_tube_op-amp.jpg1

  6. Solid State Discrete Op-Amps • Solid state op amps invented in 1960’s • Possible due to invention of silicon transistors and the IC • Chip and discrete parts • Reduced power input to ±15V to ±10V • Packaging in small black boxes allowed for integration with a circuit

  7. Monolithic Integrated Circuit Op-Amp • First created in 1963 • μA702 by Fairchild Semiconductor • μA741 created in 1968 • Became widely used due to its ease of use • 8 pin, dual in-line package (DIP) • Further advancements include use of field effects transistors (FET), greater precision, faster response, and smaller packaging

  8. +Vs +Vin + Vout -Vin - -Vs Features of Op-Amps • +Vin: non-inverting input • -Vin: inverting input • +Vs: positive source • -Vs: negative source • Vout: output voltage • ON: Offset Null • NC: Not Connected ON NC +Vs -Vin +Vin Vout -Vs ON

  9. Ideal Op-Amp Infinite open loop gain (GOL): Zero common mode gain Infinite bandwidth: Range of frequencies with non-zero gain Real Op-Amp Limited open loop gain: Decreases with increase in frequency Non-zero common mode gain Limited Bandwidth: Gain becomes zero at high frequencies Characteristics of Op-Amps

  10. Ideal Op-Amp Infinite slew rate Infinite input impedance No input current Zero output impedance Infinite output current Real Op-Amp Finite slew rate Large input impedance Small input current Non-zero output impedance Limited output current Characteristics of Op-Amps

  11. Summary of Characteristics

  12. Ideal Op-Amp • Active device • Infinite open loop gain • Infinite input impedance • Zero output impedance +Vs iin = 0A + Vdiff Vout = Vdiff x Gopenloop - -Vs

  13. Negative Feedback • Vout is a linear function of the input voltage • Zin = infinity Iin=0A Vdiff=0V • Modelisation of basic mathematical operation

  14. Non Inverting Circuit +Vs (1) V- - Vout = R2 x i iin = 0A + Vout (2) V- = - R1 x i Vdiff = 0V Vin - V- = V+ = Vin (2) i = -Vin/R1 0A -Vs R1 R2 i (1) Vin – Vout = -Vin x R1/R2 V- V- - Vout Vout = (1 + R1/R2) x Vin

  15. Inverting Circuit +Vs (1) V- - Vout = R2 x i iin = 0A + Vout (2) Vin - V- = R1 x i Vdiff = 0V - -Vs V- = V+ = 0 (1) i = Vin / R1 Vin R1 R2 i Vin – V- V- - Vout Vout = - R2/R1 x Vin

  16. Follower Circuit + Vs Vin Vout - Vs

  17. Summing Op-Amp • Adds analog signals Ohm’s Law: Solving for Vout:

  18. Summing Op-Amp

  19. Difference Op-Amps • Subtracts analog signals • Output voltage is proportional to difference between input voltages:

  20. Difference Op-Amp

  21. Integrator Op-Amps • Similar layout to inverting op-amp, but replace feedback resistor with a capacitor • A constant input signal generates a certain rate of change in output voltage • Smoothes signals over time • Output voltage is proportional to the integral of the input voltage:

  22. Integrator Op-Amp

  23. Differentiating Op-Amp • Similar to inverting op-amp, but input resistor is replaced with a capacitor • Accentuates noise over time • Output signal is scaled derivative of input signal:

  24. Differentiating Op-Amp

  25. Active Filters • Different types of active filters: • Low Pass • Filters out frequencies above a cutoff frequency • High Pass • Filters out frequencies below a cutoff frequency • Band Pass • Passes a range of frequencies between two cutoff frequencies

  26. Active Low-Pass Filter • Cutoff frequency:

  27. Active High-Pass Filter • Switch positioning of capacitors and resistors from low-pass filter locations to create high-pass filter.

  28. Active Band-Pass Filter • Created by connecting output of a high-pass filter to the input of a low-pass filter or vice versa. • Also can create using only 1 op-amp with feedback and input capacitors

  29. No negative feedback • Vout is a non-linear function of the differential input voltage V+ - V- • V+ - V- = Vdiff • Vout = sign(Vdiff) x Vs • Binary logic and oscillator

  30. Comparator Vout ( volts ) +Vs iin = 0A + + Vs Vout Vdiff - V+ Vdiff 0V V- -Vs - Vs

  31. Comparator

  32. Questions?

  33. References • “Operational Amplifiers.” http://en.wikipedia.org/wiki/Op_amp • “Real vs. Ideal Op Amp.” http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opamp.html#c4 • “741 Op Amp Tutorial.” http://www.uoguelph.ca/~antoon/gadgets/741/741.html • “Op Amp History.” Analog Devices. http://www.analog.com/library/analogDialogue/archives/39-05/Web_ChH_final.pdf

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