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Operational Amplifiers

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Past fc the gain of the non-inverting amplifier follows the ADM (open-loop gain) plot. ... Beta Circuit. Will not give you 1/ , but ... – PowerPoint PPT presentation

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Title: Operational Amplifiers


1
Operational Amplifiers
  • Team 05
  • Emily Baker
  • Rollin Garcia
  • Andrew Herman
  • Bryan Reemmer
  • Tania Yusaf

2
Topics
  • History
  • Ideal Op-Amps
  • Limitations
  • Fixes
  • Op-Amp Applications

3
Op-Amp
  • High Gain Voltage amplifier
  • Amplifies difference between 2 inputs
  • Positive and negative effect

4
History
  • Vacuum Tube Era, 1950s
  • 1st used in Analog Computers
  • Addition
  • Subtraction
  • Integration
  • Differentiation
  • Heavy
  • Prone to failure

Analog Computer
K2-W tubes general purpose Op-Amp. 1952
5
History (cont.)
  • Now in Integrated Circuits
  • 1965- Bob Widlar
  • 1st integrated Op Amp bipolar Fairchild µA709
  • Lacked short circuit protection
  • 2nd 741
  • Better performance
  • More stable
  • Easier to use

6
History (cont.)
  • Major Differences
  • Internal compensation capacitor
  • Extra transistorsshort circuit protection
  • New Designs based on
  • FET (1970s)
  • MOSFET (1980s)

7
Implementation of Op-Amps
  • Non-Inverting
  • Inverting

8
Implementation of Op-Amps
  • Integrator
  • Differentiator

9
Implementation of Op-Amps
  • Summing

10
Op-Amp Data Sheets
  • V Max rail voltage
  • V- Min rail voltage
  • OUTPUT Vin (resistor ratio 1)

11
The Ideal Op-Amp
12
Voltage Clipping
  • Output is AT MOST the supply voltage.
  • Output swing further limits the range.
  • Parasitic values lead to less than 100
    efficiency.
  • With a 15V supply, one gets a 28V output swing.

13
Slew Rate
  • Maximum rate of change of the output voltage for
    all possible input signals.
  • More than 3 distortion if you exceed the slew
    rate.

14
Output Swing vs. Frequency
  • This graph reflects a combination of both the DC
    output swing and the slew rate.

15
Gain-Bandwidth Product
  • The GBW for an amplifier is the product of the
    open loop gain and its 3 dB bandwidth.
  • This is about 1 MHz for the LM741.

16
GBP Closed-Loop
  • It can be shown that
  • Where
  • K ideal gain
  • ADM open-loop gain

17
GBP Closed-Loop
  • It can be shown that
  • Where
  • K ideal gain
  • ADM open-loop gain

Error Multiplier
18
What Does This Mean?
  • With a slight approximation of ADM, the error
    term reduces to the equation for a pole
  • Where

19
High-Frequency Response
  • Constant gain.
  • What happens at higher frequencies?

20
High Frequency Response
  • At a frequency fc the Bode plot of the Op-Amp and
    the gain of the amplifier meet.
  • Past fc the gain of the non-inverting amplifier
    follows the ADM (open-loop gain) plot.

21
Stability
  • The distance from ADM to 1/? is ADM?.
  • Intersection point is ADM?1.
  • IF ADM?gtgt1 then the gain is K.
  • IF ADM?ltlt1then the gain is ADM?K.
  • IF ADM?1 then the gain is

22
Stability
  • The meaning of this equation is at fc measure the
    angle of ADM and place that angle into ADM?.
  • This equation can cause the instability of the
    circuit.
  • If ADM?180 then the gain is infinity.

23
Stability
  • A circuit will be stable if at the frequency
    where
  • ANG(ADM?) 180 MAG(ADM?)lt1
  • PHASE MARGIN(PM) 180-ANG(ADM?)
  • ROC (SLOPE(1/?)-SLOPE(ADM)) at fc

24
Stability
  • For a circuit to be stable
  • PMgt45
  • 20Db/decltROClt40Db/dec
  • For a circuit to be marginally stable
  • 0ltPMlt45
  • ROC40Db/dec
  • For a circuit to be unstable
  • PMlt0
  • ROCgt40Db/dec

25
Beta Circuit
  • Unstable.
  • Due to the feedback loop.
  • Also due to the 1/? term.

26
Beta Circuit
27
Beta Circuit
  • Let C10.1µF and R210K?
  • Gain(-R2C2)S where Sj?
  • 1/?(1C1 R2S)
  • f11/(2?C1R2)159.15Hz
  • fc12.616kHz

28
Beta Circuit
  • Will not give you 1/?, but ?.
  • Take off the op-amp and apply a test voltage at
    the output.
  • Short the input.

29
Beta Circuit
  • Gives ?
  • Is a flip over the ? axis
  • Need to add a zero instead of a pole
  • V?? (Voltage divider)
  • (1C1 R2S)-1
  • Add R1 in series with C1

30
Beta Circuit
  • Now there is a zero.

31
Beta Circuit
  • Need the zero before fc
  • fz1/(2?C1R1)
  • Choose fz to be at 9kHz.
  • R1177?
  • Since R2gtgtR1, f1 stays approximately where it is.

32
Beta Circuit
33
Beta Circuit
34
Beta Circuit
  • Check the phase margin.
  • Check the ROC.
  • The data sheet gives the open loop gain.

35
Op-Amp Applications
  • Op-Amps are used in many different situations
    such as
  • Digital-to-Analog Converters (DAC)
  • Active Filters
  • Precision Rectifiers
  • Pre-Amplifiers
  • Voltage Clamps

36
Op-Amp Applications (cont.) Digital-to-Analog
Converter (DAC)
  • DACs commonly used in MP3 and CD players.
  • Op-Amp significance
  • Serves as a summer of digital inputs.
  • Outputs an analog signal based on gain.

37
Op-Amp Applications (cont.) Active Filters
  • Op-Amps frequently used in designs.
  • Active Components
  • Similar to transistors or vacuum tube
    applications.
  • Op-Amp significance
  • Used to shape filters response.
  • Used to buffer the filter from other components.

38
Op-Amp Applications (cont.) Precision Rectifiers
  • a.k.a Super Diode
  • Used in high precision signal processing.
  • Converts AC to DC.
  • Op-Amp significance
  • Aids in controlling the diode
  • operation.
  • Input positive Diode on
  • Input negative Diode off

39
Op-Amp Applications (cont.) Pre-Amplifiers
  • Precedes a larger amplifier.
  • Used to prepare a signal for greater
    amplification.
  • Used in many audio applications.
  • Op-Amp significance
  • Input Low-level signal (Microphone)
  • Output Line-level signal (MP3, TV)

40
Op-Amp Applications (cont.) Voltage Clamps
  • Used to measure ion currents.
  • Voltage held at set level.
  • Consists of generator with two electrodes.
  • Op-Amp significance
  • Holds voltage at designated set level.
  • Sends error signal.

41
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