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Operational Amplifiers
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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.
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 separation
closed-loop gain
common-mode rejection ratio
gain-bandwidth product
input bias Current offset
input offset Current
input offset voltage
Input resistance
input voltage range
inverting input
loop gain
non-inverting input
open-loop gain
operational amplifier
output resistance
output voltage swing
slew rate
What is an Op-Amp?
Objective cont.


Interpret a typical op-amp data sheet.
Measure some of the common op-amp parameters.
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.
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.
What is an Op-Amp?

THE IDEAL OP-AMP
 Today’s integrated circuit op-amp is a very high-gain
dc amplifier that uses external feedback networks to
control its response.
Feedback
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.
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.
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.
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).
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 doesn’t come
to an end because there is no such thing as the ideal
op-amp.
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
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
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
manufacturer’s data sheet.
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.
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.
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.
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 @ <75C).
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 manufacturer’s 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-amp’s output can be short-circuited to
ground or either supply voltage.
What is an Op-Amp?

THE OP-AMP DATA SHEET
 Important Parameters cont.
 Electrical Characteristics:

The Op amp’s 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)
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.
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-amp’s 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.
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 opamp circuit having a voltage gain of unity (1.0).
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.
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.
What is an Op-Amp?

GAIN AND FREQUENCY RESPONSE
 At
very low frequencies, the open-loop gain of opamp 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.
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.
What is an Op-Amp?

Open Loop and Closed Loop Gain

When some of the output signal is fed back to the
op-amp’s 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.
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
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.
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
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.
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.
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.
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
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
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
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.
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.
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.
End of Lesson