IEEE Circuits and Systems Magazine - Q4 2020 - 51

RL Is the Load Impedances of the Carrier
and Peaking Devices
Rcarrier

Zcarrier

Rpeaking

Zpeaking

C.N.

FII
90°

Ic

Offset Line
90°

RC.N.

FII
90°

Ip
IL

Vc

PMN

Vp

90°

0°
Class-AB

Class-C

R0
(50 Ω)

Figure 3. Structure of the post-matching Doherty PA [23].

the impedance RC.N. equals to Rcarrier which is RL . While
at saturation, full conduction can be obtained in both
carrier and peaking PAs and in parallel. Under this
other condition Rcarrier and Rpeaking operates into 2RL to
ensure that the impedance of RC.N. remains at RL . Fundamental frequency load conditions for the post-matching
Doherty PA can be expressed by
	

R carrier = *

0 # v in # 0.5
Ip
(15)
R L c 1 + m, 0.5 # v in # 1
Ic

R peaking = *

3,

	

Carrier Branch, 90°

R L,

0 # v in # 0.5
0.5 # v in # 1

(16)

Unlike the conventional Doherty PA, bandwidth of the
post-matching Doherty PA only depends on the relationship between RL and Zcarrier which is mainly determined
by the equivalent circuit of FII. With these load conditions, the impedance transformation ratio of carrier FII
is smaller than the one for conventional Doherty PAs
at OBO point, which results in a wider carrier FII bandwidth. Consequently, the bandwidth of the Doherty PA
can be effectively extended.
Last but not least, the post-matching Doherty PA topology (shown in Fig. 4(a)) is a naturally broadband amplifier,
but only in the small signal region. A pass band frequency
response can be obtained under small signal conditions
because the 90° carrier FII is in parallel with the 180° peaking branch circuit. The simulated frequency response in a
50 ohm system for this LMN is given in Fig. 4(b).
FOURTH QUARTER 2020 		

(a)
0

S-Parameter (dB)

Ip
R L c 1 + m,
Ic

Peaking Branch, 180°

-10
-20
-30
S11
S21

-40
-50
0.2

0.4

0.6 0.8 1 1.2 1.4 1.6
Normalized Frequency ω ′

1.8

(b)
Figure 4. Geometry of LMN and its frequency response.

IEEE CIRCUITS AND SYSTEMS MAGAZINE	

51

IEEE Circuits and Systems Magazine - Q4 2020

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