Operational Amplifiers

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Operational Amplifiers
Benchmark Companies Inc PO Box 473768 Aurora CO
80047
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What is an Op-Amp?
INTRODUCTION What is an operational
amplifier? In this chapter, we will define what
an operational amplifier is, and discuss the many
parameters that distinguish one type of device
from another.
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What is an Op-Amp?

• OBJECTIVES
• At the completion of this chapter, you will be
  able to define the following

Define the following terms

channel separationclosed-loop gaincommon-mode
rejection ratiogain-bandwidth productinput bias
Current offsetinput offset Currentinput offset
voltageInput resistance input voltage range
inverting input loop gain non-inverting input
open-loop gain operational amplifier output
resistance output voltage swing slew rate
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What is an Op-Amp?

• Objective cont.
• Interpret a typical op-amp data sheet.
• Measure some of the common op-amp parameters.

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What is an Op-Amp?

• THE IDEAL OP-AMP
• Before we start looking at actual operational
  amplifier circuits, we will briefly consider the
  operational amplifier, hereafter referred to Op
  amp, by itself.

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What is an Op-Amp?

• THE IDEAL OP-AMP
• The term Op-amp was originally used to describe a
  series of high-performance dc amplifiers that
  were used as the basis for analog computers.

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What is an Op-Amp?

• THE IDEAL OP-AMP
• Todays integrated circuit op-amp is a very
  high-gain dc amplifier that uses external
  feedback networks to control its response.

Feedback
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What is an Op-Amp?

• Open Loop Mode
• The op-amp without any external feedback is
  described as being used in an open-loop mode.

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What is an Op-Amp?

• Open Loop Mode
• It is in this mode that we can describe the
  characteristics of the ideal op-amp
• The open-loop gain is infinite.

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What is an Op-Amp?

• Open Loop Mode
• It is in this mode that we can describe the
  characteristics of the ideal op-amp
• The input resistance is infinite.
• The output resistance is zero.

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What is an Op-Amp?

• Open Loop Mode
• It is in this mode that we can describe the
  characteristics of the ideal op-amp
• The bandwidth is infinite.
• The output voltage is zero when the input voltage
  is zero (i.e., zero offset).

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What is an Op-Amp?

• Open Loop Mode
• In practice, however, no op-amp can meet these
  five ideal open-loop characteristics. However, as
  we shall see in the next few chapters, the world
  doesnt come to an end because there is no such
  thing as the ideal op-amp.

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What is an Op-Amp?

• THE OP-AMP SCHEMATIC SYMBOL
• the op-amp has two inputs one inverting, or -
  input, and one non-inverting, or input.
• a single output

Inverting
Output
Non-Inverting
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What is an Op-Amp?

• THE OP-AMP SCHEMATIC SYMBOL
• the op-amp is powered normally by a dual-polarity
  power supply, typically in the range of 5 to
  15 volts.

Neg
Pos
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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Perhaps the best way to understand the many
  characteristics of an op-amp is to examine a
  manufacturers data sheet.

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• the data sheet usually contains the following
  information
• A general description of the op-amp.
• An internal equivalent circuit schematic.
• Pin configuration of the device.
• The absolute maximum ratings.
• The electrical characteristics.
• Typical performance curves.

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• we will cover most of the important parameters,
  using the type 741 op-amp as a representative
  example.

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Important Parameters cont.
• Maximum Ratings The maximum ratings given in the
  data sheet are the maximum the op-amp can safely
  tolerate without the possibility of destruction.

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Maximum Ratings
• Supply Voltage (Va )
• This is the maximum positive and negative
  voltage that can be used to power the op-amp.
• Internal Power Dissipation (PD)
• This is the maximum power that the op-amp is
  capable of dissipating, given a specified
  ambient temperature (i.e., 500 mW _at_ lt75C).
• Differential Input Voltage (Vid )
• This is the maximum voltage that can be applied
  across the and - inputs.
• Input Voltage (Vicm )
• This is the maximum input voltage that can be
  simultaneously applied between both inputs and
  ground, also referred to as the common-mode
  voltage. In general, this maximum voltage is
  equal to the supply voltage.
• Operating Temperature (Ta)
• This is the ambient temperature range for which
  the op-amp will operate within the manufacturers
  specifications. Note that the military grade
  version (741) has a wider temperature range than
  the commercial, or hobbyist, grade version
  (741C).
• Output Short-Circuit Duration
• This is the amount of time that the op-amps
  output can be short-circuited to ground or either
  supply voltage.

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Important Parameters cont.
• Electrical Characteristics
• The Op amps electrical characteristics are
  usually specified for a supply voltage and
  ambient temperature. However, certain parameters
  may also have other conditions attached, such as
  a particular source resistance. Generally, each
  parameter will have a minimum typical, and/or
  maximum value. (See data sheet for examples)

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Electrical Characteristics cont.
• Input Parameters
• Input Offset Voltage (Voi)
• This is the voltage that must be applied to one
  of the input terminals to give a zero output
  voltage. Remember, for an ideal op-amp, the
  output voltage offset is zero!
• Input Bias Current (Ib)
• This is the average of the currents flowing into
  both inputs. Ideally, the two input bias currents
  are equal.
• Input Offset Current (los)
• This is the difference of the two input bias
  currents when the output voltage is zero.
• Input Voltage Range (Vcm )
• This is the range of the common-mode input
  voltage (i.e., the voltage common to both inputs
  and ground).
• Input Resistance (Zi)
• This is the resistance looking in at either
  input with the remaining input with the remaining
  input grounded.

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Electrical Characteristics cont.
• Output Parameters
• Output Resistance (Zoi)
• This is the resistance seen looking into the
  op-amps output.
• Output Short-Circuit Current (Iosc )
• This is the maximum output current that the
  op-amp can deliver to a load.
• Output Voltage Swing (Vo max)
• Depending on the load resistance, this is the
  maximum peak output voltage that the op-amp can
  supply without saturation or clipping.

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Electrical Characteristics cont.
• Dynamic Parameters
• Open-Loop Voltage Gain (AOL)
• This is the ratio of the output to input voltage
  of the op-amp without external feedback.
• Large-Signal Voltage Gain
• This is the ratio of the maximum voltage swing
  to the change in the input voltage required to
  drive the output from zero to a specified voltage
  (e.g., 10 volts).
• Slew Rate (SR)
• This is the time rate of change of the output
  voltage with the op-amp circuit having a voltage
  gain of unity (1.0).

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What is an Op-Amp?

• THE OP-AMP DATA SHEET
• Electrical Characteristics cont.
• Other Parameters
• Supply Current
• This is the current that the op-amp will draw
  from the power supply
• Common-Mode Rejection Ratio (CMRR)
• This is a measure of the ability of the op-amp
  to reject signals that are simultaneously present
  at both inputs. It is the ratio of the
  common-mode input voltage to the generated output
  voltage, usually expressed in decibels (dB).
• Channel Separation
• Whenever there is more than one op-amp in a
  single package, such as a type 747 op-amp, a
  certain amount of crosstalk will be present.
  That is, a signal applied to the input of one
  section of a dual op-amp will produce a finite
  output signal in the remaining section, even
  though there is no input signal applied to the
  unused section.

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What is an Op-Amp?

• GAIN AND FREQUENCY RESPONSE
• Unlike the ideal op-amp, the Op-amp that is used
  in various circuits does not have infinite gain
  and bandwidth. As shown in Fig.1, the open-loop
  gain AOL for a type 741 op-amp is graphed as a
  function of frequency.

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What is an Op-Amp?

• GAIN AND FREQUENCY RESPONSE
• At very low frequencies, the open-loop gain of
  op-amp is constant, but begins to roll off at
  approximately 6 Hz at a rate of -6 dB/octave or
  -20 dB/decade An octave is a doubling in
  frequency and a decade is a ten-fold increase in
  frequency.

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What is an Op-Amp?

• GAIN AND FREQUENCY RESPONSE
• This decrease continues until the gain is unity,
  or 0 dB. The frequency at which the gain is unity
  is called the unity gain frequency, fT.

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What is an Op-Amp?

• Open Loop and Closed Loop Gain
• When some of the output signal is fed back to
  the op-amps input, the ratio of the output to
  input voltage is termed the closed-loop gain,
  ACL, and is always less than the open-loop gain.

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What is an Op-Amp?

• Open Loop and Closed Loop Gain
• The difference in decibels between the open-loop
  and closed-loop gains is the loop gain, AL. When
  AOL and ACL are expressed as simple
  output-to-input ratios, the loop gain is
  expressed mathematically as
• AL AOL/ACL

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What is an Op-Amp?

• Gain Bandwidth
• Perhaps the first factor in the consideration of
  a particular op-amp for a given application is
  its gain-bandwidth product, or GBP.

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What is an Op-Amp?

• Gain Bandwidth
• For the response curve below, the product of the
  open-loop gain and frequency is a constant at any
  point on the curve, so that
• GBP AOLBW

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What is an Op-Amp?

• Gain Bandwidth
• Graphically, the bandwidth is the point at which
  the closed-loop gain curve intersects the
  open-loop gain curve, as shown in the figure
  below for a family of closed-loop gains.

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What is an Op-Amp?

• Gain Bandwidth
• Therefore, one obtains the bandwidth for any
  desired closed-loop gain by simply drawing a
  horizontal line from the desired value of gain to
  intersect the roll-off of the open-loop gain
  curve.

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What is an Op-Amp?

• Gain Bandwidth
• For a practical design situation, the actual
  design gain of an opamp circuit should be about
  a factor of 1/10 to 1/20 of the open-loop gain at
  a given frequency.

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What is an Op-Amp?

• Gain Bandwidth
• This ensures that the op-amp will function
  properly without distortion. As an example, using
  the response of Fig. 1-3, the closed-loop gain at
  10 kHz should be about 5 to 10, since the
  open-loop gain is 100 (40 dB).

ACL
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What is an Op-Amp?

• Transient Response, (Rise Time)
• The time that it takes for the output signal to
  go from 10 to 90 of its final value when a
  step.

Input Signal
Output Signal
time
Timing Diagram
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What is an Op-Amp?

• Transient Response, (Rise Time)
• A function pulse is used as an input signal, and
  is specified under closed-loop conditions. From
  electronic circuit theory, the rise time is
  related to the bandwidth of the op-amp by the
  relation
• BW 0.35/rise time

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What is an Op-Amp?

• Summary
• Op-amps are designed to be powered from a dual,
  or bipolar, voltage supply which is typically in
  the range of 5 to 15 volts.

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What is an Op-Amp?

• Summary
• That is, one supply is 5 to 15 volts with
  respect to ground, and another supply voltage of
  -5 to -15 volts with respect to ground.

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What is an Op-Amp?

• Summary
• However, in certain cases, an op-amp may be
  operated from a single supply voltage, which is
  explained in Chapters 8 and 9.

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• End of Lesson