THS4502 Datasheet, PDF (26/40 Pages) Texas Instruments – WIDEBAND, LOW-DISTORTION FULLY DIFFERENTIAL AMPLIFIERS

THS4502
THS4503
SLOS352D − APRIL 2002 − REVISED JANUARY 2004
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 103 and Figure 104.
P_O P_O
∆f_c = f_c − f1
∆f_c = f2 − f_c
IMD₃ = P_S − P_O
P_S
P_S
f_c − 3∆f f1 f_c f2 f_c + 3∆f
f − Frequency − MHz
Figure 103
26
P_OUT
(dBm)
OIP₃
P_O
www.ti.com
1X
IMD₃
IIP₃
3X
P_IN
(dBm)
P_S
Figure 104
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.
ǒ Ǔ OIP₃ + P_O )
Ť_IMD
Ť
3
2
where
(10)
ǒ Ǔ ^V2
P_O + 10 log
2R_L
Pdiff
0.001
(11)
NOTE^: P_o is the output power of a single tone, R_L is the differential load
resistance, and V_P(diff) is the differential peak voltage for a
single tone.