ITT Technical Institute
ET245
Devices II
Unit 5
Chapter 7.1 – 7.3
Unit 5 Agenda
Lecture:
• Chapter 7, Sections 7.1 – 7.3
Lab 3, Linear Op amp Circuits
continued from last week
Assignment:
• Complete Problems (pg 413):
#14, 8, 10, & 12
(Question 12: assume Aol = 89dB = 28,184)
• Read Chapter 8, Sections 8.1 – 8.3
• Prepare for Quiz #5 on Unit 5
Unit 5 Objective
Class Objectives:
• Learn Op-Amp Basic Concepts of Gain
• Learn OP-Amp Basic Concepts of Bandwidth
Chapter 7 – Section 7.1
Op-Amp Responses - Basic Concepts
Open-Loop Gain
The open-loop gain (Aol) of an op-amp is the internal
voltage gain of the device and represents the ratio of
output voltage to input voltage.
Notice no external connections.
Data sheets often refer to the
open-loop gain as
large-signal voltage gain.
Op-Amp Responses - Basic Concepts
Closed-Loop Gain
The closed-loop gain (Acl) is the voltage gain of an op-amp
with external feedback.
The closed-loop gain is determined by the external
components.
Closed-loop gain can be precisely
controlled by external component
values.
Op-Amp Responses - Basic Concepts
Gain is Frequency Dependant
In chapter 6, all of the gain expressions applied to the
midrange gain and were considered independent of the
frequency.
The midrange open-loop gain of an op-amp extends from
zero frequency (dc) up to a critical frequency at which the
gain is 3 dB less than the midrange value.
Op-Amp Responses - Basic Concepts
Gain is Frequency Dependant (continued)
An open-loop response curve (Bode plot) for a certain
op-amp is shown.
Most of the op-amp
data sheets show this
type of curve or specify
the midrange open-loop
gain.
The critical (cutoff)
frequency is 10Hz.
Op-Amp Responses - Basic Concepts
3 dB Open-Loop Bandwidth
The bandwidth of an ac amplifier is the frequency range
between the points where the gain is 3 dB less than the
midrange gain. Therefore, bandwidth is equal to the upper
critical frequency minus the lower critical frequency.
BW = fcu – fcl
Since the lower critical frequency is zero (dc), the
bandwidth is simply the upper critical frequency.
BW = fcu ( referred to as fc )
Op-Amp Responses - Basic Concepts
Unity-Gain Bandwidth
Note that the gain steadily decreases to a point where it is
equal to one (0 dB).
The value of frequency
at which this unity gain
occurs is the
unity-gain bandwidth.
Op-Amp Responses - Basic Concepts
Gain-Versus-Frequency Analysis
The RC lag (low-pass) networks within an op-amp are
responsible for the roll-off in gain as frequency increases.
Attenuation is expressed as:
Vout
Vin
=
XC
R2 + XC2
1
=
1 + R2 / XC
1
=
FIGURE 7-3
RC lag network.
1 + f 2 / f c2
Op-Amp Responses - Basic Concepts
Gain-Versus-Frequency Analysis (continued)
If an op-amp is represented by a voltage gain element with
a gain of Aol(mid) and a single RC lag network, then the total
open-loop gain of the op-amp is the product of the
midrange open-loop gain Aol(mid) and the attenuation of the
RC network.
Aol =
Aol(mid)
1 + f 2 / fc 2
FIGURE 7-4
Op-amp represented by gain element and internal RC network.
Op-Amp Impedances
EXAMPLE 7.1:
Determine Aol for the following values of f.
Assume fc(ol) = 100 Hz, and Aol(mid) = 100,000.
a) f = 0 Hz b) f = 10 Hz c) f = 100 Hz d) f = 1000 Hz
Solution: pg 385
Op-Amp Responses - Basic Concepts
Phase Shift
As you know, an RC network causes a propagation delay
from the input to output, thus creating a phase shift
between the input signal and the output signal.
 = -tan-1 ( f / fc )
Op-Amp Impedances
EXAMPLE 7.2:
Calculate the phase shift for an RC lag circuit for each
of the following frequencies, and than plot the curve of
phase shift versus frequency. Assume fc = 100 Hz.
Solution:
pg 386 - 387
Chapter 7 – Section 7.2
Op-Amp Open-Loop Response
Frequency Response
In section 7.1, an op-amp was assumed to have a
constant roll-off of -20 dB / decade above its critical
frequency. Op-amps with this characteristic are called
compensated op-amps.
Some op-amps are more complex. The frequency
response may be determined by several internal
stages, each with its own critical frequency. An opamp with more than one critical frequency is called an
uncompensated op-amp.
Op-Amp Open-Loop Response
Frequency Response (continued)
Uncompensated op-amps require careful attention to
the feedback network to avoid oscillation.
Since the roll-off rates
are additive, the total
roll-off rate increases
by -20 dB/decade as
each critical frequency
is reached.
Op-Amp Open-Loop Response
Phase Response
In a multistage amplifier, each stage contributes to the
total phase lag.
As you have seen, an RC lag circuit can produce up to
a -90° phase shift.
A 3-stage op-amp can have up to a -270° phase shift.
The phase lag of each stage is less than -45° when
the frequency is below the critical frequency,
equal to -45° when at the critical frequency, and
greater than -45° when the frequency is above the
critical frequency.
Op-Amp Open-Loop Response
Phase Response (continued)
The phase shift lags of the stages of an op-amp are
added to produce a total phase lag, according to the
following formula for three stages:
tot = -tan-1( f / fc1 ) -tan-1( f / fc2 ) -tan-1( f / fc3 )
Op-Amp Impedances
EXAMPLE 7.3:
A certain op-amp has three internal amplifier stages
with the following gains and critical frequencies.
Stage 1: Av1 = 40 dB, fc1 = 2000 Hz
Stage 2: Av2 = 32 dB, fc2 = 40 kHz
Stage 3: Av3 = 20 dB, fc3 = 150 kHz
Determine the open-loop midrange dB gain and the
total phase lag when f = fc1.
Solution: pg 389
Chapter 7 – Section 7.3
Op-Amp Closed-Loop Response
Effect of Negative Feedback on Bandwidth
The closed-lop critical frequency of an op-amp is:
fc(cl) = fc(ol)(1 + BAol(mid))
…where B is the feedback attenuation
BWcl = BWol(1 + BAol(mid))
Op-Amp Impedances
EXAMPLE 7.4:
A certain op-amp has an open-loop midrange gain of
150,000 and an open-loop 3 dB bandwidth of 200 Hz.
The attenuation of the feedback loop is 0.002. What is
the closed-loop bandwidth?
Solution: pg 390
Op-Amp Closed-Loop Response
Effect of Negative Feedback on Bandwidth
When the open-loop gain of an op-amp is reduced by
negative feedback, the bandwidth is increased.
Op-Amp Impedances
EXAMPLE 7.5:
Determine the bandwidth of each of the amplifiers in
figure. Both op-amps have an open-loop gain of 100
dB and a unity-gain bandwidth of 3 MHz.?
Solution: pg 392
Op-Amp Closed-Loop Response
Gain-Bandwidth Product
An increase in closed-loop gain causes a decrease in
the bandwidth and vice versa, such that the product of
gain and bandwidth is a constant (as long as the roll-off
rate is a fixed -20 dB/decade).
The gain-bandwidth product is always equal to the
frequency at which the op-amp’s open-loop gain is
unity (unity-gain bandwidth).
Aclfc(cl) = unity-gain bandwidth
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Unit 5 Agenda
Lecture:
• Chapter 7, Sections 7.1 – 7.3
Lab 3, Linear Op amp Circuits
continued from last week
Assignment:
• Complete Problems (pg 413):
#14, 8, 10, & 12
(Question 12: assume Aol = 89dB = 28,184)
• Read Chapter 8, Sections 8.1 – 8.3
• Prepare for Quiz #5 on Unit 5