IEEE Circuits and Systems Magazine - Q4 2020 - 52

There have been many excellent works on broadband
Doherty PAs in recent years. The following chapter will
discuss their operating principles in more detail.
A. Broadband Doherty Power Amplifier Focusing
on the Fundamental Terminations
For the Doherty PA works of [22]-[25] and [50] mainly
focus on the fundamental termination. Some creative
ideas about the broadband configuration were pro posed. The details are as follow.
In [22], a " transformer-less load-modulated " (TLLM)
architecture was proposed. Similar to the conventional
Doherty structure, the TLLM architecture has two amplifiers-the carrier PA and a peaking PA. A different output
matching technique was employed without the use of
impedance transformers. Doherty behaviors could there-

fore be realized over a wide bandwidth. A theoretical design procedure for this architecture was presented, which
allows the S-parameter of the output matching networks
of carrier and peaking amplifiers to be obtained.
For the transistors in Doherty PAs, a drain impedance
transformation keeps the real part nearly unchanged
while making the necessary decrease in the imaginary
part. In order to meet such requirements, low-order L-type
impedance inverters have been used for the post-matching Doherty PA FII [23]. The L-type can be readily modelled
as an LC low pass network, which is shown in Fig. 5.
Zcarrier can be calculated as

C

Rcarrier /2 = RL

Figure 5. Equivalent circuit of the L-type FII [23].

Real Part of Zcarrier

2
RL
2RL

1.2

	

0.8
0.4

40%

Imaginary Part of Zcarrier

2

1.6
1.2

	

	

0.8
RL
2RL

40%
0.4
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Normalized Frequency Ï â²
(b)

2

Figure 6. Normalized L-type impedance transformer in [23].
(a) Real part of impedance in different load conditions. (b)
Imaginary part of impedance in different load conditions.

52â	

RL
2R L
(19)
=
1 + R 2L ~ 20 C 2
1 + ^2R L h2 ~ 20 C 2

~0 =

1 (20)
2C 2 R 2L

Therefore, the real part of Zcarrier,sat at ~ 0 has a relationship with RL and a , given by

2

0

RL
~C
+ j ~L (18)
1
f
1 + R 2L ~ 2 C 2
~2 C2 + 2 p
RL

Rearranged in terms of ~ 0,

0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Normalized Frequency Ï â²
(a)

0.4

Z carrier =

The load modulation results in Rcarrier changing from RL
(back-off) to 2RL (saturation) at the intrinsic plane. However, in practice, due to transistor parasitics the optimal
Zcarrier at the package plane will have an imaginary part,
which is dependent on the output power. With increase
in output power (from back-off to saturation), the real
part of optimal Zcarrier remains unchanged while the
imaginary part moves towards the real axis. This phenomenon is also verified in simulated load-pull results
for the GaN transistor [23],

L

1.6

1
(17)
j~C + 1/R L

This equation can be expanded into its real and imaginary parts as
	

Zcarrier

Z carrier = j~L +

	

Re " Z carrier, sat , = 2R L (21)
3

Fig. 6 shows the L-type network variation of Zcarrier
impedance values when RL becomes 2RL . The abscissa
~l = ~/~ 0 and we normalize ~ 0 and RL to 1. It can be
found that the variation of the real part of Zcarrier is less
than 16% within the 40% bandwidth, while the imaginary
part decreased to a smaller value. These characteristics
match ideally with the impedance conditions required
for wideband Doherty operation.
It should be noted that although the bandwidth of
Doherty PAs can be extended by the lower Rcarrier, its

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IEEE Circuits and Systems Magazine - Q4 2020

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