TSH350_07 Datasheet, PDF (16/22 Pages) STMicroelectronics – 550MHz low noise current feedback amplifier

Intermodulation distortion product
6
Intermodulation distortion product
TSH350
The non-ideal output of the amplifier can be described by the following series:
V_out = C₀ + C₁V_in + C₂V²ⁱⁿ + …+ C_nVⁿⁱⁿ
where the input is V_in=Asinωt, C₀ is the DC component, C₁(V_in) is the fundamental and C_n is
the amplitude of the harmonics of the output signal V_out.
A one-frequency (one-tone) input signal contributes to harmonic distortion. A two-tone input
signal contributes to harmonic distortion and to the intermodulation product.
The study of the intermodulation and distortion for a two-tone input signal is the first step in
characterizing the driving capability of multi-tone input signals.
In this case:
V_in = A sin ω₁t + A sin ω₂t
then:
V_out = C₀ + C₁(A sin ω₁t + A sin ω₂t) + C₂(A sinω₁t + A sin ω₂t)²â€¦+ C_n(A sin ω₁t + A sin ω₂t)ⁿ
From this expression, we can extract the distortion terms, and the intermodulation terms
from a single sine wave:
● second order intermodulation terms IM2 by the frequencies (ω₁-ω₂) and (ω₁+ω₂) with an
amplitude of C2A²
● third order intermodulation terms IM3 by the frequencies (2ω₁-ω₂), (2ω₁+ω₂), (−ω₁+2ω₂)
and (ω₁+2ω₂) with an amplitude of (3/4)C3A³
The intermodulation product of the driver is measured by using the driver as a mixer in a
summing amplifier configuration (see Figure 33). In this way, the non-linearity problem of an
external mixing device is avoided.
Figure 33. Inverting summing amplifier (using evaluation board SO8_S_MULTI)
V_in1
R1
V_in2
R2
R_fb
_
V_out
+
100
R
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