NORTH EET 160 - Chapter 11: Op Amps and Op Amp Circuits (17 pages)

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Chapter 11: Op Amps and Op Amp Circuits



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Chapter 11: Op Amps and Op Amp Circuits

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Pages:
17
School:
North Seattle College
Course:
Eet 160 - Introduction to Electricity and Electronics
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S l i d e Chapter 11 Op Amps and Op Amp Circuits 1 S 11 1a Introduction l 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 i d e 2 2 S l i d e 3 Op 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 3 S Op Amp Characteristics l 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 voltage and can be either polarity i d e In some designs the compensation pins are used to null out the offset voltage 4 4 S l i d e Schematic 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 output 7 1 5 U3 3 6 2 741 4 U2 5 S OPAMP 3T VIRTUAL 5 Schematic Representation l i d e 6 11 1a End 6 S l i d e 7 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 theOffset basesadjustments of Used to fine tune transistors S 7 inputs to diff amp V1 and V2 are inputs Diff amp Q1 and Q2 l Q13 and Q14 Current Mirror or tail resistor Q4 is an active load RC i d e 8 11 1b Internal Circuitry Actual Circuit http en wikipedia org wiki Image Opa mptransistorlevelcoloredlabeled png Q5 emitter follower No gain but steps up impedance Q6 additional gain Q11 Active load Generates huge gain Q9 and Q10 are class B push pull emitter follower No gain but low output impedance and 0 volts out with no input 8 S Common Chip Connections l i d e 9 9 S l i d e 1 Input Bias Current and Input Resistance l i d e 1 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 B2 2 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 0 S 10 11 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 11 1 S l i d e 1 Output 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 The 741 op amp has 25 ohms of resistance with AOL and nearly zero with Negative Feedback ACL It is low enough that for practical purposes we will consider it to be zero 12 2 S l i d e 1 Output Voltage Swing The output of an op amp can normally have output voltage swings that are within 2 3 V of the supply voltages If the output tries to exceed these limits it will be distorted Clipping Output saturation voltage Where clipping occurs is commonly referred to as rails 13 3 S l i d e Open versus Closed loop Gain An ideal op amp has infinite voltage gain at all frequencies Practical op amps have open loop gains ranging from 100 000 to over 1 000 000 at low frequencies We will look at a graph in a later presentation The internal voltage gain of an op amp decreases at higher frequencies 1 11 1c End 14 4 S l i d e 11 1d Common mode versus Differential mode Gain Differential mode voltage gain is the gain given to a voltage that appears between the two input terminals Two different voltages on the inputs By contrast common mode voltage gain is the gain given to a voltage that appears on both input terminals with respect to ground Ideally an op amp will reject voltages that appear on both input terminals resulting in a commonmode voltage gain of zero 15 5 Same input on both terminals 1 S l i d e Frequency Response An ideal op amp has infinite voltage gain over an infinite range of frequencies An op amp will also respond to very low frequencies down to dc In a practical op amp voltage gain decreases as frequency increases By adding external circuitry the closed loop voltage gain can be decreased to a practical value that is constant over a wide frequency range 1 16 6 S l i d e 1 Open Loop Voltage Gain Vs Frequency Anti log of 100 20 100 000 Note gain is only 100K up to about 10 Hz Dramatically decrease at rate of 20 db per decade of frequency At 0 db gain 1 and frequency 1 Meg Hz for 741 Op Amp Called F Unity With closed loop gain can be stabilized over a wide range of frequencies 17 7 S l i d e 1 8 Output Slew Rate The maximum output voltage swing of an op amp is limited to the maximum values of the source voltage and the internal design of the op amp The output voltage swing is also limited regarding how fast it can change The maximum rate of change of the output voltage is called the slew rate of the op amp Slew rate is specified in volts per second or volts per microsecond V S Op amp 741 has 5V us Others are as high as 5500V us This limitation is designed into the op amp to prevent unwanted high frequency oscillation 11 1d End 18 S l i d 11 2a Basic Amplifier Configurations There are two basic op amp configurations in wide use Inverting amplifiers Noninverting amplifiers These configurations form the basis for many other related applications e 1 19 9 …


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