SAVING POWER WITH POWER-DOWN

LINEARITY: DEFINITIONS, TERMINOLOGY,

IMD3 = PS − PO

PS

PO

PO

∆fc = fc − f1

∆fc = f2 − fc

PS

fc − 3∆f

f1 fc f2

fc + 3∆f

f − Frequency − MHz

THS4504

THS4505

www.ti.com ......................................................................................................................................................... SLOS363D – AUGUST 2002 – REVISED MAY 2008

Amplifiers are generally thought of as linear devices.

FUNCTIONALITY

In other words, the output of an amplifier is a linearly

scaled version of the input signal applied to it. In

The THS4500 family of fully differential amplifiers

reality, however, amplifier transfer functions are

contains devices that come with and without the

nonlinear.

Minimizing

amplifier

nonlinearity

is

a

power-down option. Even-numbered devices have

primary design goal in many applications.

power-down capability, which is described in detail

here.

Intercept points are specifications that have long

been

used

as

key

design

criteria

in

the

RF

The power-down pin of the amplifiers defaults to the

communications

world

as

a

metric

for

the

positive supply voltage in the absence of an applied

intermodulation distortion performance of a device in

voltage (i.e. an internal pullup resistor is present),

the signal chain (for example, amplifiers, mixers,

putting the amplifier in the power-on mode of

etc.). Use of the intercept point, rather than strictly the

operation. To turn off the amplifier in an effort to

intermodulation

distortion,

allows

for

simpler

conserve power, the power-down pin can be driven

system-level calculations. Intercept points, like noise

towards the negative rail. The threshold voltages for

figures, can be easily cascaded back and forth

power-on and power-down are relative to the supply

through a signal chain to determine the overall

rails and given in the specification tables. Above the

receiver chain intermodulation distortion performance.

enable threshold voltage, the device is on. Below the

The relationship between intermodulation distortion

disable threshold voltage, the device is off. Behavior

and intercept point is depicted in Figure 85 and

in between these threshold voltages is not specified.

Figure 86.

Note that this power-down functionality is just that;

the amplifier consumes less power in power-down

mode. The power-down mode is not intended to

provide a high-impedance output. In other words, the

power-down functionality is not intended to allow use

as a 3-state bus driver. When in power-down mode,

the impedance looking back into the output of the

amplifier is dominated by the feedback and gain

setting resistors.

The time delays associated with turning the device on

and off are specified as the time it takes for the

amplifier to reach 50% of the nominal quiescent

current. The time delays are on the order of

microseconds because the amplifier moves in and out

of the linear mode of operation in these transitions.

CIRCUIT TECHNIQUES, AND DESIGN

TRADEOFFS

Figure 85. 2-Tone and 3rd-Order Intermodulation

The

THS4500

family

of

devices

features

Products

unprecedented distortion performance for monolithic

fully differential amplifiers. This section focuses on

Due to the intercept point's ease of use in system

the fundamentals of distortion, circuit techniques for

level calculations for receiver chains, it has become

reducing nonlinearity, and methods for equating

the

specification

of

choice

for

guiding

distortion of fully differential amplifiers to desired

distortion-related

design

decisions.

Traditionally,

linearity specifications in RF receiver chains.

these systems use primarily class-A, single-ended RF

amplifiers as gain blocks. These RF amplifiers are

typically designed to operate in a 50-

Ω environment,

just like the rest of the receiver chain. Since intercept

points are given in dBm, this implies an associated

impedance (50

Ω).

Copyright © 2002–2008, Texas Instruments Incorporated

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