- 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:
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)
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.
741 Op-Amp Schematic 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
- Rout is zero
Ideal Op-Amp Analysis - To analyze an op-amp feedback circuit:
Inverting Amplifier Analysis Non-Inverting Amplifier Analysis Op-Amp Buffer - Vout = Vin
- Isolates loading effects
Op-Amp Differentiator Op-Amp Integrator Op-Amp Summing Amplifier Applications of Op-Amps - Filters
- Types:
- Low pass filter
- High pass filter
- Band pass filter
- Cascading (2 or more filters connected together)
- 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 - Using KCL at the inverting and non-inverting terminals of the op amp we find that
- 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
- 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
Applications PID Controller – PID Controller Circuit Diagram 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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