Chapter 31
Comparators
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Op-amp as a Comparator |
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No negative feedback |
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Output saturates with very small + or
input |
Comparators
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Comparator |
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Non-linear device |
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vout has two discrete
values, ±VSAT |
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vout = +VSAT if +
input is greater than input |
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vout = VSAT if
input is greater than + input |
Comparators
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A comparator circuit: Sine wave in,
square wave out |
Comparators
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Input sine wave |
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Output square wave Vout = ±VSAT |
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+VSAT (determined by VCC)
when sinusoid is + |
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VSAT (determined by VEE)
when sinusoid is |
Comparators
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Compare input waveform to reference |
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Reference can be ground or dc source |
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Can compare two waveforms |
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Specialized comparator ICs also
available |
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Detects when waveform reaches given
level |
Comparators
Voltage Summing Amplifier
Voltage Summing Amplifier
Voltage Summing Amplifier
Integrators and
Differentiators
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In general |
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Using resistors and capacitors |
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Integrators |
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Differentiators |
Integrators and
Differentiators
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Voltage across capacitor |
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Current through capacitor |
Integrators and
Differentiators
Integrators and
Differentiators
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Op-amp differentiator |
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Circuit inherently unstable |
Integrators and
Differentiators
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Stable op-amp differentiator |
Instrumentation
Amplifiers
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Op-amp in differential amplifier
configuration |
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Noise suppression |
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High CMRR |
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Reasonable gain |
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IC instrumentation amps |
Instrumentation
Amplifiers
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An op-amp instrumentation amp circuit |
Instrumentation
Amplifiers
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Measurement of very small voltages |
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Transducer |
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Converts a physical change into an
electrical change |
Instrumentation
Amplifiers
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Strain gage |
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Converts force into ∆R |
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∆R is milliohms |
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Use bridge circuit |
Instrumentation
Amplifiers
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Strain gage example |
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Thin metal foil (resistor) on plastic
backing |
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Glued to metal bar |
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Bar subjected to tension and
compression |
Instrumentation
Amplifiers
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Strain gage example |
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Tension |
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Resistance of strain gage is R + ∆R |
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Compression |
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Resistance of strain gage is R ∆R |
Active Filters
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Basic filter types |
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Passive elements, gain < 1 |
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Low-pass |
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High-pass |
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Bandpass |
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Band reject |
Active Filters
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With op-amps/active filters |
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Gain can be ≥ 1 |
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Filter response closer to ideal |
Active Filters
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Low-pass (RF = R1) |
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Add resistor for gain > 1 |
Active Filters
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High-pass (RF = R1) |
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Add resistor for gain > 1 |
Active Filters
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dc gain |
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Easily achieved |
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Not used much due to gain-bandwidth
product |
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Example |
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GBWP = 106, Gain = 10 |
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Cutoff for filter (HP or LP) only 105 |
Active Filters
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Bandpass |
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Wideband |
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Cascade HP and LP active filters |
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LP must have higher cutoff frequency |
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HP and LP cutoff frequencies far apart |
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Narrowband |
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Can use single op-amp |
Active Filters
Active Filters
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Active notch filter |
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Cascade narrowband BP filter |
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Adder circuit |
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Result is 1 (frequency response of BP
filter) |
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Frequency at resonant frequency of BP
filter will be eliminated |
Voltage Regulation
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Voltage regulator |
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Constant voltage to load |
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Specified current range |
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Specified input voltage range |
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Zener diode regulator |
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Inefficient |
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Dissipates power |
Voltage Regulation
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Types of regulators |
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Fixed voltage regulator |
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Variable voltage regulator |
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Switching regulator |
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Specialized IC regulators |
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For different voltages, e.g. +5 V, 5
V, +12 V, 12 V, +15 V, 15 V, etc. |
Voltage Regulation
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Line Regulation |
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Small output change with change in
input |
Voltage Regulation
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Load regulation |
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Small output voltage change with
smaller RL |
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VNL = no-load voltage
(open-circuit load) |
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VFL = full-load voltage
(specified by manufacturer) |
Voltage Regulation
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Circuit to increase efficiency of Zener
regulator with an op-amp |
Voltage Regulation
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Three-terminal IC regulators |
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7800 series, positive voltage |
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7900 series, negative voltage |
Voltage Regulation
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5 V output, 7805 |
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12 V output, 7812 |
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5 V output, 7905 |
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12 V output, 7912 |
Voltage Regulation
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Ripple |
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Greatly reduced by IC regulator |
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Vr(in) = input ripple
voltage |
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Vr(out) = output ripple
voltage |