Chapter 15 Operational Amplifiers Components n Discrete Components

Chapter 15 Operational Amplifiers

Components n Discrete – Components housed in individual packages; that is, one package – one component. n n The traditional 2 N 3904 (TO 92 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. n The KA 741 operational amplifier is an example.

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

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

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

Op-Amp Voltage Gain n Open-loop voltage gain (AOL) – The maximum possible voltage gain of a given op-amp. n n 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.

Input/Output Polarity n n 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.

Inverting and Noninverting Circuits n n 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).

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

Basic Differential Amplifier

Differential Amplifier Input/Output Relationships

Modes of Operation n Differential amplifiers have three operating modes. n n n 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. n Common-mode signals are usually undesired signals.

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

Input Offset Voltage n 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. n Input offset voltage is commonly represented as a battery connected to the op-amp inputs.

Eliminating Output Offset Voltage n n 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.

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

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

Other Op-Amp Ratings n Power supply rejection ratio – The ratio of a change in op-amp output voltage to a change in supply voltage. n n 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.

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

Slew Rate Distortion

Other Op-Amp Specifications n n 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 opamp when operating in its quiescent (inactive) state.

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

Inverting Amplifier Operation

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

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

Noninverting Amplifiers n n 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:

Noninverting Amplifier Analysis n n 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.

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

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

Gain-Bandwidth Product n Unity gain frequency – The maximum possible operating frequency for an op-amp, measured at AOL = 0 d. B.

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

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

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

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

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

Feedback Factor and Gain

Feedback Factor and Circuit Impedance Values