IEEE Solid-State Circuits Magazine - Fall 2015 - 76

The complete definition of amplifier class
is an operating definition and depends on
how often the drain current flows during
any single cycle of the input frequency.
majority of those generally considered linear amplifiers. Also included
among the single-path amplifiers
are many switching "amplifiers"-
which are actually not amplifiers in
the usual sense. The special case of
switching amplifiers will be the focus
of Part 5 in this series.

Multiple-path amplifiers are designed to improve on the performance weaknesses of single-path
amplifiers. The major weakness addressed is low energy efficiency,
which, as Part 1 of this series shows,
is a direct consequence of Ohm's
law. An amplifier circuit where the
transistor is operating linearly cannot have good efficiency. This article
provides more detail as to why this
is physically required.

VDD

The Basic Amplifier Circuit

RL

The basic amplifier circuit is shown
in Figure 1. It is formed by adding a
resistor R L in series with the transistor drain and connected to a supply voltage VDD . This resistor does
two things:
1) It converts the varying current
at the transistor drain to a correspondingly varying output
voltage.

VOUT
VIN

Figure 1: Adding a load resistor to a
transistor makes a basic amplifier and provides for an output signal voltage.

Cutoff

Active

Compression

0.7

sR
S

0.5

as

0.4

Cl

Drain Current (A)

0.6

0.3

Threshold

0.2

Class A

Class C

0.1
0

Class AB1
0

0.1 0.2

0.3 0.4
Class B

0.5 0.6 0.7

0.8 0.9

1

Class AB2
Vbias (V)

Figure 2: A transistor transfer function (solid black curve) with several points marked
that correspond to bias selection options. The transistor nonlinear operations of cutoff and
compression are highlighted.

76

fa l l 2 0 15

IEEE SOLID-STATE CIRCUITS MAGAZINE

2) It limits the maximum amount
of current that can flow into the
transistor at VDD /R L .
Using the circuit shown in Figure 1,
we can measure the transfer function from the transistor input VGS to
any other parameter of interest. One
common parameter of interest is
the drain current I D . Figure 2 shows
an example transfer function and
identifies the three major regions
describing the behavior of this
function:
■ cutoff, where the transistor drain
current is zero no matter what
the input VGS value is
■ compression, where the drain current is large yet changes little, if
at all, with additional increases in
VGS
■ active, where the drain current
changes significantly with variations in VGS .
Several points have been marked
along the transfer function curve
that identify options for biasing
this amplifier.

Classes of Amplifiers
An amplifier's class is often described
in terms of where along this transfer function the setting of the bias
supply [1] sets the drain current.
However, this is only partially true.
The complete definition of amplifier
class is an operating definition and
depends on how often the drain current flows during any single cycle of
the input frequency:
■ for class-A amplifiers, the drain
current never goes to zero
■ for class-B amplifiers, the drain
current flows for only one half of
the cycle
■ for class-C amplifiers, the drain
current flows for less than one
half of the cycle.
From these fundamental definitions we reach several immediate
conclusions. First, class-AB describes
amplifiers that operate such that
some interval of the drain current
goes to zero during each input cycle
but the drain current is not zero for
an entire half cycle. Second, transistor bias for class-B amplifiers



Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Fall 2015

IEEE Solid-State Circuits Magazine - Fall 2015 - Cover1
IEEE Solid-State Circuits Magazine - Fall 2015 - Cover2
IEEE Solid-State Circuits Magazine - Fall 2015 - 1
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