BC447 Datasheet, PDF (23/36 Pages) ON Semiconductor – High Voltage Transistors

www.ti.com
directly. The total load includes the feedback network; in
the noninverting configuration (Figure 75) this is the sum
of R_f and R_g, while in the inverting configuration
(Figure 76), only R_f needs to be included in parallel with
the actual load.
POUT
(dBm)
OIP3
THS4271
THS4275
SLOS397E − JULY 2002 − REVISED JANUARY 2004
1X
LINEARITY: DEFINITIONS, TERMINOLOGY,
CIRCUIT TECHNIQUES, AND DESIGN
TRADEOFFS
The THS4271 features excellent distortion performance
for monolithic operational amplifiers. This section focuses
on the fundamentals of distortion, circuit techniques for
reducing nonlinearity, and methods for equating distortion
of operational amplifiers to desired linearity specifications
in RF receiver chains.
Amplifiers are generally thought of as linear devices. The
output of an amplifier is a linearly scaled version of the
input signal applied to it. However, amplifier transfer
functions are nonlinear. Minimizing amplifier nonlinearity
is a primary design goal in many applications.
Intercept points are specifications long 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 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 85 and
Figure 86.
PO PO
∆fc = fc − f1
∆fc = f2 − fc
IMD3 = PS − PO
PS
PS
fc − 3∆f f1 fc f2 fc + 3∆f
f − Frequency − MHz
Figure 85
PO
IMD3
IIP3
3X
PIN
(dBm)
PS
Figure 86
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. Giving intercept points in dBm, implies an
associated impedance (50 Ω).
However, with an operational 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 an
operational amplifier. The THS4271 yields optimum
distortion performance when loaded with 150 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 systems performance.
The 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. Equations 1 and 2 gives the definition of an
intercept point, relative to the intermodulation distortion.
ǒ Ǔ OIP₃ + P_O )
Ť_IMD
Ť
3
2
where
(2)
ǒ Ǔ P_O + 10 log
2R_L
V^2P
0.001
(3)
NOTE: PO is the output power of a single tone, RL is the load
resistance, and VP is the peak voltage for a single tone.
23