IEEE Circuits and Systems Magazine - Q4 2020 - 49

The new scenario sparked a renaissance in the use of
Doherty PAs in the 1990s, where base stations began to
demand for linear radio frequency amplifiers. These amplifiers were required to maintain high efficiency even
with modulated signals that had large PAPRs. However,
at that time the bandwidth of Doherty PAs were narrow,
which was sufficient for radio broadcasting. With the
proliferation of wireless data communication, there was
a push for significantly increased bandwidth. With MIMO
and carrier aggregation techniques being proposed and
widely adopted in the modern wireless communication
systems, this urgency had been given a further impetus.
This impetus has in recent years resulted in innovative
circuits for broadband Doherty PAs. An overview of the
conventional (narrowband) and broadband Doherty PA
are presented in this section.
The conventional Doherty PA topology is shown in
Fig. 1. It is composed of two active devices in phase
quadrature (carrier device: 0Â°, peaking device: 90Â°), two
fundamental impedance inverters (FIIs) and two quarter-wavelength transmission lines. Rcarrier and Rpeaking are
the load impedances (purely resistive) of the carrier
and peaking amplifier, respectively. RC.N. is the combining impedance. ZT is the transmission line characteristic
impedance. IC1 is the output current of carrier amplifier.
The Doherty PAs are biased on a Class AB PA (carrier PA) and a Class C PA (peaking PA). Combining two
active devices in quadrature results in a high efficiency
over a large dynamic range. The mechanism behind this
high efficiency in Doherty PAs can be explained using
the load modulation mechanism. The load modulation
concept is a voltage-controlled voltage source (VCVS) in
parallel with a voltage-controlled current source
(VCCS) and a load resistor (R0). For the carrier and the

peaking PA, both of them can be considered as a VCCS
at the output (location of Ic and Ip). With the transformation of a quarter-wave transmission line, the VCVS can
be obtained at the output of the carrier branch (location
of Ic1). A peaking VCCS is then used to modulate the load
impedance of the carrier branch.
To aid in analysis, the ABCD matrix of a m /4 transmission line with its equivalent parameters, can be
written as:
Vc
cos i
jZ T sin i VC.N.
E;
E (1)
; E=;
Ic
j ^1/Z T h sin i cos i
I c1

	

The relationship between VC.N. and Ic at (f0, e.g. i = 90c)
can be simplified as
VC.N. = - jZ T I c 

	

According to Fig. 1, VC.N. can be calculated as
VC.N. = R C.N. I L = R C.N. ^ I c1 + I p h (3)

	

Resulting in (1) can be written as
Vc
cos i
jZ T sin i R C.N. (I c1 + I p)
E;
E (4)
; E=;
Ic
j ^1/Z T h sin i cos i
I c1

	

Ic1 can be found to be
I c1 =

	

I c - jI p ^ R C.N. /Z T h sin i
(5)
j ^ R C.N. /Z T h sin i + cos i

Similarly, Vc can be found to be
Vc = I p R C.N. cos i + I c1 ^ R C.N. cos i + jZ T sin i h (6)

	

Moreover, Rcarrier and Rpeaking are calculated as

VCCS

Rpeaking

Rcarrier
Ic1
ZT, Î» /4 Line

FII

(2)

Ip

VC.N.

FII
RC.N.

Ic

IL
Î» /4 Line

Vc

Vp

0Â°

90Â°
Class-AB
VCCS

VCVS

R0
(50 â¦)

Class-C

Figure 1. Structure of the narrowband Doherty PA [13].

FOURTH QUARTER 2020 		

IEEE CIRCUITS AND SYSTEMS MAGAZINE	

49
IEEE Circuits and Systems Magazine - Q4 2020

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