AWR Software for the Design of a High-Efficiency Broadband GaN HEMT Doherty Amplifier for Cellular Transmitters
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The impedance conditions at different points of the load network of the peaking amplifier when it is turned off are shown in
Figure 6, where Z
match
shown in Figure 6a indicates low reactance at the output of the load network over the required frequency
range from 1.8-2.7GHz, having near-zero reactance at the mid-band frequency with some inductive and capacitive reactances
when the operating frequency approaches the bandwidth edges. At the same time, by using a series transmission line one
quarter-wavelength long at high-band frequency, an open-circuit condition is provided at higher band frequencies with suffi-
ciently high inductive and capacitive reactances across the frequency bandwidth, indicated by Z
peaking
shown in Figure 6b.
Hence, the broadband performance of such an inverted Doherty structure can potentially be achieved in a practical realization.
Figure 6: Impedances for peaking amplifier
Figure 7a shows the load-network equivalent circuit for a carrier amplifier with a frequency behavior of the impedance Z
carrier
seen by the carrier device, whose real component slightly varies around 10ohms, shown in Figure 7b. This means that, taking
into account the device output shunt capacitance C
out
of about 5pF and series output inductance L
out
provided by the overall
bond wire and package leadframe inductances, the impedance seen by the device multi-harmonic current source at the
fundamental frequency across the entire frequency bandwidth of 1.8-2.7GHz has been increased by two times from the initial
5ohms at the input of the broadband output impedance transformer. This is a high enough impedance to achieve high
efficiency at backoff output power levels.
Figure 7: Matching network and load impedance for carrier amplifier
a) b)
a) b)
