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Chapter 15

Chapter 15. Operational Amplifiers. Components. Discrete – Components housed in individual packages; that is, one package – one component. The traditional 2N3904 (TO92 package) is an example.

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Chapter 15

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  1. Chapter 15 Operational Amplifiers

  2. Components • Discrete – Components housed in individual packages; that is, one package – one component. • The traditional 2N3904 (TO92 package) is an example. • Integrated circuit (IC) – A single package that contains any number of active and/or passive components, all constructed on a single piece of semiconductor material. • The KA741 operational amplifier is an example.

  3. Operational Amplifier (Op-Amp) • Op-amp – A high-gain dc amplifier that has high input impedance and low output impedance. • The inverting (–) input and noninverting (+) input are signal inputs. • The +V and –V inputs are supply voltage inputs.

  4. Op-Amp Supply Connections • Op-amp supply pins are connected • To equal positive and negative supply voltages, or • To one supply voltage and ground.

  5. Differential Amplifier • Differential amplifier – A circuit that amplifies the difference between two input voltages. • The op-amp signal inputs are connected to a differential amplifier. • The circuit amplifies the difference between the voltages at its inverting (-) and noninverting (+) inputs.

  6. Op-Amp Voltage Gain • Open-loop voltage gain (AOL) – The maximum possible voltage gain of a given op-amp. • The term open-loop indicates that AOL is the voltage gain of an op-amp with no feedback path. • Adding a feedback path to an op-amp (below) reduces the overall voltage gain to some value less than AOL.

  7. Input/Output Polarity • When the (+) input is the more positive of the two, the output is positive. • When the (-) input is the more positive of the two, the output is negative.

  8. Inverting and Noninverting Circuits • Inverting input – The op-amp input that produces a 180° voltage phase shift (from input to output). • Noninverting input – The op-amp input that does not produce a voltage phase shift (from input to output).

  9. Output Voltage Curves • The limits on output voltage are determined primarily by load resistance and the supply voltages.

  10. Basic Differential Amplifier

  11. Differential Amplifier Input/Output Relationships

  12. Modes of Operation • Differential amplifiers have three operating modes. • Single-ended mode – An active input is applied to one input only. • Differential mode – Two active inputs are applied. The output reflects the relationship between the input signals. • Common mode – Occurs when two identical signals are applied to the inputs simultaneously. The ideal differential has a 0 V output when common mode signals are applied. • Common-mode signals are usually undesired signals.

  13. Output Offset Voltage • Output offset voltage – A voltage that may appear at the output for an op-amp, caused by an imbalance in the differential amplifier.

  14. Input Offset Voltage • Input offset voltage – The difference between the base-emitter voltages in a differential amplifier that produces an output offset voltage when the signal inputs are grounded. • Input offset voltage is commonly represented as a battery connected to the op-amp inputs.

  15. Eliminating Output Offset Voltage • Input offset current – A slight difference in op-amp input currents, caused by differences in the transistor beta ratings. • Compensating resistor – A resistor connected to the noninverting input to an op-amp to compensate for any difference in the input currents.

  16. Input Bias Current • Input bias current – The average value of quiescent dc biasing current drawn by the signal inputs of an op-amp. • A current path is required for both signal inputs to an op-amp for the component to operate.

  17. Common-Mode Rejection Ratio (CMRR) • Common-mode rejection ratio (CMRR) – The ratio of differential gain to common-mode gain.

  18. Other Op-Amp Ratings • Power supply rejection ratio – The ratio of a change in op-amp output voltage to a change in supply voltage. • The lower the power supply rejection ratio of an op-amp, the better. • Output short-circuit current – The maximum output current for an op-amp, measured with the load shorted.

  19. Op-Amp Slew Rate • Slew rate – The maximum rate at which op-amp output voltage can change. • Slew rate is typically measured in volts/microsecond. • Slew rate can be used to determine the maximum operating frequency for an op-amp.

  20. Slew Rate Distortion

  21. Other Op-Amp Specifications • Input voltage range – The maximum differential input that an op-amp can accept without risking damage to its input differential amplifier. • Large-signal voltage gain – The open-loop voltage gain of the op-amp. • Supply current – The value of quiescent (inactive) current that the op-amp draws from its power supply. • Power consumption – The amount of power dissipated by an op-amp when operating in its quiescent (inactive) state.

  22. Inverting Amplifiers • Inverting amplifier – A basic op-amp circuit that produces a 180° signal phase shift (from input to output). • The op-amp counterpart of the common-emitter and common-source amplifiers.

  23. Inverting Amplifier Operation

  24. Closed-Loop Voltage Gain (ACL) • Closed-loop voltage gain (ACL) – The voltage gain of an op-amp with a feedback path. • The value of ACL is always lower than the value of open-loop voltage gain (AOL). • For the inverting amplifier:

  25. Inverting Amplifier Analysis • The amplifier output impedance (Zout) is lower than the output impedance of the op-amp.

  26. Noninverting Amplifiers • Noninverting amplifier – An op-amp circuit with no signal phase shift (from input to output). • The closed-loop voltage gain of a noninverting amplifier is found using:

  27. Noninverting Amplifier Analysis • The amplifier input impedance (Zin) is greater than (or equal to) the input impedance of the op-amp. • The amplifier output impedance (Zout) is lower than (or equal to) the output impedance of the op-amp.

  28. The Voltage Follower • Voltage follower – The op-amp counterpart of the emitter follower and the source follower. The circuit has: • High Zin and low Zout. • ACL = 1 • Input and output signals that are in phase.

  29. Op-Amp Frequency Response • DC amplifier – Any amplifier that exhibits midband gain when operated at 0 Hz. • When operated above its cutoff frequency, op-amp voltage gain rolls off at 20 dB per decade.

  30. Gain-Bandwidth Product • Unity gain frequency – The maximum possible operating frequency for an op-amp, measured at AOL = 0 dB.

  31. Negative Feedback • Negative feedback – A type of feedback in which the feedback signal is 180° out of phase with the input signal.

  32. Positive Feedback • Positive feedback – A type of feedback in which the feedback signal is in phase with the input signal.

  33. Inverting Amplifier Operation • The negative feedback path in the inverting amplifier: • Reduces voltage gain (from its open-loop value). • Increases bandwidth (relative to its open-loop value).

  34. Noninverting Amplifier Operation • The negative feedback path in the noninverting amplifier: • Reduces voltage gain (from its open-loop value). • Increases bandwidth (relative to its open-loop value).

  35. Attenuation • Attenuation – Any reduction in the amplitude of a signal. • Attenuation factor (av) – The ratio of feedback voltage to output voltage. • The value of av is always less than one. • Feedback factor (1+avAOL) – A value used in the gain and impedance calculations for a given feedback amplifier.

  36. Feedback Factor and Gain

  37. Feedback Factor and Circuit Impedance Values

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