to improve linearity: The third-order intermodulation current
of the class B or C amplifier has an opposite phase to the class
AB amplifier. Hence, the resultant IMD3 can be improved.
Second, this low bias of the peaking amplifier ensures a
correct load modulation in the low power region. Because
the peaking amplifier should be turned off in a low power region and supply the same current to the load at a high power
level, its gain expansion should be high enough. The sufficiently large gain expansion, which allows it to have a proper
ON/OFF transition according to the output power level, is
thus a very desirable characteristic for a peaking amplifier.
Therefore, an analysis for the gain expansion of the class
F and F-1 amplifiers was performed in detail. An output
current for the weakly nonlinear current source in general
common source amplifiers can be expressed using the Taylor series expansion:
id(t)=gmlvg (t)+gdl vd (t)+gm2vg(t)2+gmdvg(t)vd(t)+gd2vd (t)2
where vg(t) is an input signal. The gmx and gdx are the
xth-order nonlinear expansion coefficients for transcon-ductance and conductance, respectively. If the one-tone
signal is excited (vg(t) = Acosw0t), the output voltage at the
load Z(w) can be represented using each harmonic current
where id (t, w0) is the nth-order harmonic component of
the nonlinear output current and Z(nw0) is the impedance
at an nth-order harmonics band.
The output power including harmonic components can
be calculated by multiplication of the output current and
voltage. The power gain using the dominant terms for the
fundamental output power can be expressed:
GgRR P m in ≈ ⋅ 1 2 0();
md m + −
2( ) ;
where R(w0) is the real part of the load impedance for
the fundamental frequency band. Rin is the input resistance. Since the gm3 has a positive value for the class B bias
point, the second term in the bracket of Equation 3 contributes to the gain expansion at moderate output levels.
However, the third term pulls down the expanded gain by
the second term as the real part of the load impedance at
the second harmonics band (Z(2wo)) becomes larger.
Therefore, the class F amplifier, which has a second harmonic impedance Z(2w0) near 0, gets a larger gain expansion than that of the class F-1 amplifier, which has a very
large Z(2w0) value. The large-signal simulation results, according to various values of the second harmonic imped-