Slide 1 Chapter 11:Op Amps and Op Amp Circuits ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 2 211.1a Introduction• An operational amplifier (op amp) refers to an integrated circuit having well-defined operating characteristics.• A single IC may contain a single op amp, or multiple op amps may be contained in a single IC.• Op amps are widely used in computer circuits and in industrial computer applications. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 3 3Op Amp Characteristics• Op amps in electronic circuits can be assumed to be ideal devices for design and troubleshooting purposes.– This addresses things like infinite input impedance and zero output impedance.• The symbol for an op amp includes two input terminals, an output terminal, two power supply connections, and one or more compensation terminal.• An op amp is essentially a differential amplifier with a single-ended output.• Most op amps use dual power supplies. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________Slide 4 4Op Amp Characteristics• Some op amps have more than one compensation pin. These pins are used to fine tune the behavior of the op amp.• With 0 V on the input pins, some output voltage may result. This is called the offset voltageand can be either polarity.– In some designs the compensation pins are used to null out the offset voltage ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 5 5Schematic Representation• 741 is a common op amp– 2,3 inputs– 4 (-),7 (+) power supply connections– 1,5 offsets• Used to fine tune op amp outputs if necessary. Usually unused.–6 output• Basic Symbol below showing inputs and outputU2OPAMP_3T_VIRTUALU37413247651 ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 6 6Schematic Representation11.1a End ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________Slide 7 711.1b Internal Circuitry• Op amps are composed of standard components such as transistors, resistors, diodes, and so on.• The input terminals of an op amp are connected to the bases of transistors.Offset adjustments: Used to fine tune inputs to diff amp ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 8 8V1 and V2 are inputsDiff amp Q1 and Q2Q13 and Q14 =Current Mirror or tail resistorQ4 is an active load (RC)Q5 emitter follower, No gain, but steps up impedance.Q6 additional gain, Q11 Active load. Generates huge gainQ9 and Q10 are class B push-pull emitter follower: No gain but low output impedance and 0 volts out with no input.Actual Circuit:http://en.wikipedia.org/wiki/Image:Opamptransistorlevelcoloredlabeled.png ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 9 9Common Chip Connections ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________Slide 10 10Input Bias Current and Input Resistance• The input bias current in an op amp is considered to be one-half the sum of the two input currents applied.– I in(bias)I B1+ I B22• Op amps using bipolar transistors typically have bias currents in the tens to hundreds of nanoamperes.• Op amps are available with FET and MOSFET inputs and have much lower input currents.• An ideal op amp has infinite input resistance and zero input current.11.1b End ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 11 1111.1c Negative Feedback is Required• Most practical op amp circuits require negative feedback.• Negative feedback is the return of a portion of the output signal to the input signal (out-of-phase).• When an op amp has feedback, its operation is closed loop; with no feedback, it is open loop.• Diagrams show negative feedback – A. Utilizing inverting input– B. Utilizing non-inverting input• Since the op amp ideally has infinite input resistance, no current flows into input. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 12 12Output Resistance• The output of an op amp comes from a source internal to it that has an associated series resistance.• This effective resistance is called the output resistance of the op amp.• Negative feedback lowers the effective output resistance.– It is low enough that for practical purposes, we will consider it to be zero.The 741 op amp has 25 ohms of resistance with AOL and nearly zero with Negative Feedback (ACL) ___________________________________