IEEE Solid-State Circuits Magazine - Winter 2016 - 45

Three-Port operating Modes

IPA

Port 1

Both amplifier input ports are independent, so now we must consider
the output power POUT as a function of the two independent input
parameters, the input power PIN and
the supply voltage VPA
POUT = f (PIN, VPA) .

VPA

(1)

The function f ($) in (1) is not linear,
in general, except possibly under some
restrictions on the input parameters.
This single output being dependent
on two independent inputs provides
the overall amplifier response as a
surface. One measured example of
such an amplifier response surface is
shown in Figure 2.
Back in the late 1990s, an engineer
working on variable supply power
amplifier transmitter design developed a simple tool to illustrate the
various aspects of amplifier operation when operated as a three-port.
Now called the Booth chart [2], this
graphical tool is more than the collection of amplifier response curves
from different supply voltage values
that comprise it. This tool also provides insight into how the amplifier
behaves as a full three-port circuit.
The Booth chart in Figure 3 corresponds to the measured amplifier
surface from Figure 2. The nonlinear
nature of (1) is clearly evident.
The Booth chart shows that there
are three major regions of operation
for this (and any) amplifier. Two of

RF
Signal In

Port 3

PA
Port 2

RF
Signal Out

Figure 1: All amplifiers actually are, and
always have been, three-port circuits

them are commonly known, which
are linear operation (L-mode) and
amplifier compression (C-mode). A
third operating region is apparent
when both the supply voltage and the
input signal power are small. This
third region is called P-mode, for reasons that are discussed shortly.
The fundamental definition of
linear amplification, which here
means class-A operation, is that
the magnitude of the output signal is directly proportional to the
input signal magnitude. Also, in any

5.0

4.0

3.0

2.0

PA Supply Voltage (V)

1.0

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0.0

class-A amplifier, this output magnitude is independent of the supply
voltage value. These two characteristics are a direct consequence of the
transistor operating as a controlled
current source (CCS) and appear
on the Booth chart as an overlay
of amplifier response curves showing the same output power for multiple values of the supply voltage.
The L-mode region is highlighted in
Figure 4(a). The conventional circuit
operating model that corresponds
with this measured operation is
shown in Figure 4(b).
At higher values of the input
signal power from those within
L-mode, the amplifier compresses,
and we observe a complete reversal
of the characteristics that describe
L-mode. In this compressed region,
correspondingly called C-mode, the
output power becomes independent of the input signal power-the

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30
20
10
0
-10
-20
-30
-40
-50
-60
-70

PA Output Power (dBm)

so we refer to the supply input as
Port 3 as shown in Figure 1.

Input RF Power (dBm)

Figure 2: The three-port characterization of an amplifier provides a characteristic surface
similar to this example.

IEEE SOLID-STATE CIRCUITS MAGAZINE

W I N T E R 2 0 16

Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Winter 2016