Chapter 11 Amplifiers: Specifications and External Characteristics

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Chapter 11Amplifiers Specifications
andExternal Characteristics
Basic Amplifier Concepts Cascaded
Amplifiers Power Supplies and Efficiency Additiona
l Amplifier Models Importance of Amplifier
Impedances in Various Applications Ideal
Amplifiers Frequency Response Linear Waveform
Distortion Pulse Response Transfer Characteristic
and Nonlinear Distortion Differential
Amplifiers Offset Voltage Bias Current, and
Offset Current
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Chapter 11Amplifiers Specifications
andExternal Characteristics

• Use various amplifier models to calculate
  amplifier performance for given sources and
  loads.
• 2. Compute amplifier efficiency.

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3. Understand the importance of input and output
impedances of amplifiers. 4. Determine the best
type of ideal amplifier for various
applications. 5. Specify the frequency-response
requirements for various amplifier
applications. 6. Understand linear and nonlinear
distortion in amplifiers.
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7. Specify the pulse-response parameters of
amplifiers. 8. Work with differential amplifiers
and specify common-mode rejection
requirements. 9. Understand the various sources
of dc offsets and design balancing circuits.
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BASIC AMPLIFIER CONCEPTS
Ideally, an amplifier produces an output signal
with identical waveshape as the input signal, but
with a larger amplitude.
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Inverting versus Noninverting Amplifiers
Inverting amplifiers have negative voltage gain,
and the output waveform is an inverted version of
the input waveform. Noninverting amplifiers have
positive voltage gain.
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Voltage-Amplifier Model
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Current Gain
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Power Gain
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CASCADED AMPLIFIERS
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Simplified Models for Cascaded Amplifier Stages
First, determine the voltage gain of the first
stage accounting for loading by the second
stage. The overall voltage gain is the product
of the gains of the separate stages. The input
impedance is that of the first stage, and the
output impedance is that of the last stage.
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POWER SUPPLIES AND EFFICIENCY
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Current-Amplifier Model
Aisc is the current gain of the amplifier with
the output short circuited.
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Transconductance-Amplifier Model
Connect a short circuit across the output
terminals and analyze the circuit to determine
Gmsc.
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Transresistance-Amplifier Model
Open circuit the output terminals and analyze the
circuit to determine Rmoc.
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IMPORTANCE OF AMPLIFIER IMPEDANCES IN VARIOUS
APPLICATIONS
Some applications call for amplifiers with high
input (or output) impedance while others call for
low input (or output) impedance.
Other applications call for amplifiers that have
specific input and/or output impedances.
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The proper classification of a given amplifier
depends on the ranges of source and load
impedances with which the amplifier is used.
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FREQUENCY RESPONSE
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Determining Complex Gain
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LINEAR WAVEFORM DISTORTION
If the gain of an amplifier has a different
magnitude for the various frequency components of
the input signal, a form of distortion known as
amplitude distortion occurs.
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Phase Distortion
If the phase shift of an amplifier is not
proportional to frequency, phase distortion
occurs.
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Requirements for Distortionless Amplification
To avoid linear waveform distortion, an amplifier
should have constant gain magnitude and a
phase response that is linear versus frequency
for the range of frequencies contained in the
input signal.
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PULSE RESPONSE
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Rise Time
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Tilt
For small amounts of tilt,
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TRANSFER CHARACTERISTIC AND NONLINEAR DISTORTION
The transfer characteristic is a plot of
instantaneous output amplitude versus
instantaneous input amplitude. Curvature of the
transfer characteristic results in nonlinear
distortion.
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Harmonic Distortion
For a sinewave input, nonlinear distortion
produces output components having frequencies
that are integer multiples of the input frequency.
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Total Harmonic Distortion (THD)
Total harmonic distortion is a specification that
indicates the degree of nonlinear distortion
produced by an amplifier.
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DIFFERENTIAL AMPLIFIERS
A differential amplifier has two input terminals
an inverting input and a noninverting
input. Ideally, a differential amplifier
produces an output that is proportional to the
difference between two input signals.
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Common-mode Signal
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Common-Mode Rejection Ratio
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OFFSET VOLTAGE, BIAS CURRENT, AND OFFSET CURRENT
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Real differential amplifiers suffer from
imperfections that can be modeled by several dc
sources two bias-current sources, an offset
current source, and an offset voltage source. The
effect of these sources is to add a (usually
undesirable) dc term to the ideal output.
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