THS4501CD datasheet(27/37 Pages) TI

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THS4500

THS4501

SLOS350D − APRIL 2002 − REVISED JANUARY 2004

www.ti.com

LINEARITY: DEFINITIONS, TERMINOLOGY,

CIRCUIT TECHNIQUES, AND DESIGN

TRADEOFFS

The THS4500 family of devices features unprecedented

distortion performance for monolithic fully differential

amplifiers. This section focuses on the fundamentals of

distortion, circuit techniques for reducing nonlinearity,

and methods for equating distortion of fully differential

amplifiers to desired linearity specifications in RF receiver

chains.

Amplifiers are generally thought of as linear devices. In

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

version of the input signal applied to it. In reality, however,

amplifier transfer functions are nonlinear. Minimizing

amplifier nonlinearity is a primary design goal in many

applications.

Intercept points are specifications that have long been

used as key design criteria in the RF communications

world as a metric for the intermodulation distortion

performance of a device in the signal chain (e.g.,

amplifiers, mixers, etc.). Use of the intercept point, rather

than strictly the intermodulation distortion, allows for

simpler system-level calculations. Intercept points, like

noise figures, can be easily cascaded back and forth

through a signal chain to determine the overall receiver

chain’s intermodulation distortion performance. The

relationship between intermodulation distortion and

intercept point is depicted in Figure 107 and Figure 108.

Figure 107

IMD3 = PS − PO

∆fc = fc − f1

∆fc = f2 − fc

fc − 3∆ff1 fc f2

fc + 3∆f

f − Frequency − MHz

Figure 108

IMD3

OIP3

IIP3

(dBm)

POUT

(dBm)

Due to the intercept point’s ease of use in system level

calculations for receiver chains, it has become the

specification of choice for guiding distortion-related design

decisions. Traditionally, 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).

However, with a fully differential amplifier, the output does

not require termination as an RF amplifier would. Because

closed-loop amplifiers deliver signals to their outputs

regardless of the impedance present, it is important to

comprehend this when evaluating the intercept point of a

fully differential amplifier. The THS4500 series of devices

yields optimum distortion performance when loaded with

200

Ω to 1 kΩ, very similar to the input impedance of an

analog-to-digital converter over its input frequency band.

As a result, terminating the input of the ADC to 50

Ω can

actually be detrimental to system performance.

This discontinuity between open-loop, class-A amplifiers

and closed-loop, class-AB amplifiers becomes apparent

when comparing the intercept points of the two types of

devices. Equation 10 gives the definition of an intercept

point, relative to the intermodulation distortion.

OIP

3 + PO )

IMD

where

O + 10 log

Pdiff

0.001

(10)

(11)

NOTE: Po is the output power of a single tone, RL is the differential load

resistance, and VP(diff) is the differential peak voltage for a

single tone.

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