What is an Op-Amp? – The Surface



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What is an Op-Amp? – The Surface

  • An Operational Amplifier (Op-Amp) is an integrated circuit that uses external voltage to amplify the input through a very high gain.
  • We recognize an Op-Amp as a mass-produced component found in countless electronics.
  • What an Op-Amp looks like to a lay-person
  • What an Op-Amp looks like to an engineer

What is an Op-Amp? – The Layout

  • There are 8 pins in a common Op-Amp, like the 741 which is used in many instructional courses.

What is an Op-Amp? – The Inside

  • The actual count varies, but an Op-Amp contains several Transistors, Resistors, and a few Capacitors and Diodes.
  • For simplicity, an Op-Amp is often depicted as this:
  • Non-Inverting Input
  • Inverting Input
  • Negative Power Supply
  • Output
  • -
  • +

History of the Op-Amp – The Dawn

  • Before the Op-Amp: Harold S. Black develops the feedback amplifier for the Western Electric Company (1920-1930)
  • A
  • β
  • Input
  • Output
  • Forward Gain
  • Feedback

History of the Op-Amp – The Dawn

  • The Vacuum Tube Age
  • The First Op-Amp: (1930 – 1940) Designed by Karl Swartzel for the Bell Labs M9 gun director
  • Uses 3 vacuum tubes, only one input, and ± 350 V to attain a gain of 90 dB
  • Loebe Julie then develops an Op-Amp with two inputs: Inverting and Non-inverting

History of the Op-Amp – The Shift

  • The end of Vacuum Tubes was built up during the 1950’s-1960’s to the advent of solid-state electronics
  • The Transistor
  • The Integrated Circuit
  • The Planar Process

History of the Op-Amp – The Shift

  • 1960s: beginning of the Solid State Op-Amp
  • Example: GAP/R P45 (1961 – 1971)
    • Runs on ± 15 V, but costs $118 for 1 – 4
  • The GAP/R PP65 (1962) makes the Op-Amp into a circuit component as a potted module

History of the Op-Amp – The Evolution

  • The solid-state decade saw a proliferation of Op-Amps
    • Model 121, High Speed FET family, etc.
  • Robert J. Widlar develops the μA702 Monolithic IC Op-Amp (1963) and shortly after the μA709
  • Fairchild Semiconductor vs. National Semiconductor
    • National: The LM101 (1967) and then the LM101A (1968) (both by Widlar)
    • Fairchild: The “famous” μA741 (by Dave Fullager 1968) and then the μA748 (1969)

Mathematics of the Op-Amp

  • The gain of the Op-Amp itself is calculated as:
  • G = Vout/(V+ – V-)
  • The maximum output is the power supply voltage
  • When used in a circuit, the gain of the circuit (as opposed to the op-amp component) is:
  • Av = Vout/Vin

Op-Amp Saturation

  • As mentioned earlier, the maximum output value is the supply voltage, positive and negative.
  • The gain (G) is the slope between saturation points.
  • Vout
  • Vin
  • Vs-
  • Vs+

741 Op-Amp Schematic

  • differential amplifier
  • high-gain amplifier
  • output stage
  • current mirror
  • current mirror
  • current mirror

Op-Amp Characteristics

  • Open-loop gain G is typically over 9000
    • But closed-loop gain is much smaller
  • Rin is very large (MΩ or larger)
  • Rout is small (75Ω or smaller)
    • Effective output impedance in closed loop is very small

Ideal Op-Amp Characteristics

  • Open-loop gain G is infinite
  • Rin is infinite
    • Zero input current
  • Rout is zero

Ideal Op-Amp Analysis

  • To analyze an op-amp feedback circuit:

Inverting Amplifier Analysis

  • virtual ground

Non-Inverting Amplifier Analysis

Op-Amp Buffer

  • Vout = Vin
  • Isolates loading effects
  • A
  • High output impedance
  • B
  • Low input impedance

Op-Amp Differentiator

Op-Amp Integrator

Op-Amp Summing Amplifier

Op-Amp Differential Amplifier

  • If R1 = R2 and Rf = Rg:

Applications of Op-Amps

  • Filters
  • Types:
  • Low pass filter
  • High pass filter
  • Band pass filter
  • Cascading (2 or more filters connected together)
  • R2
  • +
  • -
  • +
  • V0
  • __
  • + Vcc
  • - Vcc
  • -
  • +
  • R1
  • C
  • Low pass filter
  • Low pass filter Cutoff frequency 
  • Low pass filter transfer function

Applications of Op-Amps

  • Electrocardiogram (EKG) Amplification
    • Need to measure difference in voltage from lead 1 and lead 2
    • 60 Hz interference from electrical equipment

Applications of Op-Amps

  • Simple EKG circuit
    • Uses differential amplifier to cancel common mode signal and amplify differential mode signal
  • Realistic EKG circuit
    • Uses two non-inverting amplifiers to first amplify voltage from each lead, followed by differential amplifier
    • Forms an “instrumentation amplifier”

Strain Gauge

  • 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

Strain Gauge

  • Half-Bridge Arrangement
  • Using KCL at the inverting and non-inverting terminals of the op amp we find that 
  • ε ~ Vo = 2ΔR(Rf /R2)
  • R + ΔR
  • Rf
  • +
  • -
  • +
  • V0
  • __
  • + Vcc
  • - Vcc
  • -
  • +
  • Rf
  • Vref
  • R
  • R - ΔR
  • R
  • Op amp used to amplify output from strain gauge

Applications of Op-Amps

  • Piezoelectric Transducer
    • Used to measure force, pressure, acceleration
    • Piezoelectric crystal generates an electric charge in response to deformation
  • Use Charge Amplifier
    • Just an integrator op-amp circuit
  • 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
  • P
  • I
  • D
  • Output Process
  • Sensor
  • VERROR
  • VSET
  • VOUT
  • VSENSOR
  • PID Controller – System Block Diagram

Applications PID Controller – System Circuit Diagram

  • Source: http://www.ecircuitcenter.com/Circuits/op_pid/op_pid.htm
  • Calculates VERROR = -(VSET + VSENSOR)
  • Signal conditioning allows you to introduce a time delay which could account for things like inertia
  • -VSENSOR

Applications PID Controller – PID Controller Circuit Diagram

  • VERR
  • Adjust
  • Change
  • Kp
  • RP1, RP2
  • Ki
  • RI, CI
  • Kd
  • RD, CD
  • VERR PID

Applications of Op-Amps

  • Example of PI Control: Temperature Control
  • Thermal System we wish to automatically control the temperature of:
  • Block Diagram of Control System:

Applications of Op-Amps

  • Voltage Error Circuit:
  • Proportional-Integral Control Circuit:
  • Example of PI Control: Temperature Control

References

  • Cetinkunt, Sabri. Mechatronics. Hoboken, NJ: John Wiley & Sons Inc., 2007.
  • Jung, Walter G. Op Amp Applications Handbook. Analog Devices, Inc., 2005.
  • “Operational Amplifier.” http://en.wikipedia.org/wiki/Operational_amplifier.
  • “Operational Amplifier Applications.” http://en.wikipedia.org/wiki/Operational_amplifier_applications.

References

  • Rizzoni, G. Principles and Applications of Electrical Engineering, McGraw Hill, 2007.
  • http://web.njit.edu/~joelsd/electronics/Labs/ecglab.pdf


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