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Basic Op-Amp Circuits

Objectives. Explain the basic operation of a comparator circuitAnalyze summing amplifiers, averaging amplifiers, and scaling amplifiersExplain the operation of op-amp integrators and differentiatorsDiscuss the operation of several types of op-amp oscillatorsRecognize and evaluate basic op-amp fi

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Basic Op-Amp Circuits

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    1. Chapter 19 Basic Op-Amp Circuits

    2. Objectives Explain the basic operation of a comparator circuit Analyze summing amplifiers, averaging amplifiers, and scaling amplifiers Explain the operation of op-amp integrators and differentiators Discuss the operation of several types of op-amp oscillators Recognize and evaluate basic op-amp filters Describe the operation of basic series and shunt voltage regulators

    3. Comparators One application of the op-amp used as a comparator is to determine when an input voltage exceeds a certain level The inverting input is tied to a reference voltage (the reference voltage may be ground, or a voltage level), and the signal is applied to the noninverting input Because of the high open-loop gain, a very small difference voltage between the two inputs drives the amplifier into saturation, causing the output voltage to go to its limit

    4. Comparators

    5. Summing Amplifiers The summing amplifier has two or more inputs, and its output voltage is proportional to the negative of the algebraic sum of its input voltages VOUT = - (VIN1 + VIN2 + VIN3 + + VINn)

    6. Summing Amplifiers Summing Amplifier with Gain Greater than Unity When Rf is larger than the input resistors, the amplifier has a gain of Rf/R, where R is the value of each input resistor: VOUT = - (VIN1 + VIN2 + VIN3 + + VINn) Rf/R Averaging Amplifier By setting the ratio Rf/R equal to the reciprocal of the number of inputs, the result is the mathematical average of the input voltages

    7. Summing Amplifiers Scaling Adder A different weight can be assigned to each input of a summing amplifier, by adjusting the values of the individual input resistors (the smaller the value of the input resistance R, the greater the weight, and vice versa) VOUT = - ((Rf/R1)VIN1 + (Rf/R2)VIN2 + (Rf/R3)VIN3 + + (Rf/Rn)VINn)

    8. Integrators and Differentiators An op-amp integrator simulates mathematical integration, which is basically a summing process that determines the area under the curve of a function An op-amp differentiator simulates mathematical differentiation, which is a process of determining the instantaneous rate of change of a function

    9. Integrators and Differentiators An ideal integrator is shown The feedback element is a capacitor that forms an RC circuit with the input resistor

    10. Integrators and Differentiators The capacitor voltage in a simple RC circuit is not linear but is exponential When using an op-amp with an RC circuit to form an integrator, the capacitors charging current is made constant, thus producing a linear voltage rather than an exponential voltage If Vin is a constant voltage, then Iin is also a constant because the inverting input always remains at 0 V Since Iin is constant, so is IC The constant IC charges the capacitor linearly and produces a linear voltage across C

    11. Integrators and Differentiators An ideal differentiator is shown The capacitor is now the input element A differentiator produces and output that is proportional to the rate of change of the input voltage

    12. Integrators and Differentiators On a differentiator, IC = Iin and the voltage across the capacitor is equal to Vin at all times because of the virtual ground on the inverting input Since the current at the inverting input is negligible, IR=IC, both currents are constant because the slope of the capacitor voltage (VC/t) is constant The output voltage is also constant and equal to the voltage across Rf because one side of the feedback resistor is always at virtual ground Vout = -(VC/t)RfC

    13. Oscillators One type of sinusoidal oscillator is the Wien-bridge oscillator A fundamental part of the Wien-bridge oscillator is a lead-lag circuit shown below

    14. Oscillators R1 and C1 together form the lag portion of the circuit; R2 and C2 form the lead portion At lower frequencies, the lead circuit dominates due to the high reactance of C2 As the frequency increases, XC2 decreases, allowing the output voltage to increase At some specified frequency, the response of the lag circuit takes over, and the decreasing value of XC1 causes the output voltage to decrease

    15. Oscillators The output voltage peaks at the resonant frequency fr At this point, the attenuation (Vout/Vin) of the circuit is 1/3 The formula for the resonant frequency is: fr = 1 / (2?RC) The lead-lag circuit has a resonant frequency, fr, at which the phase shift through the circuit is 0 and the attenuation is 1/3 Below fr, the lead circuit dominates (output leads input) Above fr, the lag circuit dominates (output lags input)

    16. Oscillators

    17. Oscillators One practical implementation of a triangular-wave oscillator is shown below

    18. Oscillators The basic square-wave oscillator shown is a type of relaxation oscillator because its operation is based on the charging and discharging of a capacitor

    19. Active Filters The term active filter means that a gain element is used; in this case, an op-amp The circuit below is a voltage-follower and an RC filter between the input signal and the non-inverting input, to produce a low pass filter

    20. Active Filters A filter with one RC circuit that produces a -20 dB/decade roll-off beginning at fc is said to be a single-pole or first-order filter The term -20dB/decade means that the voltage gain decreases by ten times (-20 dB) when the frequency increases by ten times (decade) A two-pole (second order) low-pass filter uses two RC circuits to produce a roll-off rate of -40 dB/decade

    21. Active Filters A single-pole high-pass active filter with a -20 dB/decade roll-off is shown below Ideally all frequencies above fc pass without limit

    22. Active Filters All op-amps inherently have internal RC circuits that limit the amplifiers response at high frequencies Such is the case with the active high-pass filter There is an upper frequency limit to its response, which makes this type of filter a band-pass filter with a very wide bandwidth rather than a true high-pass filter In many applications, the internal high-frequency cutoff is so much greater than the filters critical frequency that the internal high-frequency cutoff can be neglected

    23. Active Filters One way to implement a band-pass filter is to use a cascaded arrangement of a high-pass filter followed by a low-pass filter The critical frequency of each filter is chosen so that the response curves overlap

    24. Voltage Regulators In a series regulator, the control element is in series with the load between input and output The output sample circuit senses a change in the output voltage The error detector compares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage

    25. Voltage Regulators

    26. Voltage Regulators In the basic shunt regulator, the control element is a transistor (Q1) in parallel with the load A series resistor (R1) is in series with the load The operation of the circuit is similar to that of the series regulator, except that the regulation is achieved by controlling the current through the parallel transistor Q1 As output voltage varies, Q1 is driven to compensate for the change in voltage (Q1 acts as a voltage divider with R1)

    27. Voltage Regulators

    28. Summary In an op-amp comparator, when the input voltage exceeds a specified reference voltage, the output changes state The output voltage of a summing amplifier is proportional to the sum of the input voltages An averaging amplifier is a summing amplifier with a closed-loop gain equal to the reciprocal of the number of inputs

    29. Summary In a scaling adder, a different weight can be assigned to each input, thus making the input contribute more or contribute less to the output The integral of a step is a ramp The derivative of a ramp is a step In a Wien-bridge oscillator, the closed-loop gain must be equal to 3 in order to have unity gain around the positive feedback loop In filter terminology, a single RC circuit is called a pole

    30. Summary Each pole in a filter causes the output to roll off (decrease) at a rate of -20 dB/decade Two-pole filters roll off at a maximum rate of -40 dB/decade In a series voltage regulator, the control element is a transistor in series with the load In a shunt voltage regulator, the control element is a transistor in parallel with a load The terminals on a three-terminal regulator are input voltage, output voltage, and ground

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