IEEE Solid-State Circuits Magazine - Winter 2016 - 46

30
Output RF Power (dBm)

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Output RF Power (dBm)

Figure 3: Representing an amplifier three-port characteristic with a family of power transfer function curves, called a Booth chart. Each constituent curve is taken at a different value
of the supply voltage input.

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L-Mode
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RL
IL = IDDQ + ISIGNAL

Control
Input
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Active
Device

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(b)

Output RF Power (dBm)

Figure 4: L-mode, which is conventional linear amplifier operation, is (a) noted on the
Booth chart where the output power depends only on the input power, and not on variations
of the supply voltage, and (b) depicted as a functional model for L-mode operation where
the transistor operates as a CCS.

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C-Mode
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RSW

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(b)

Figure 5: C-mode is the region of amplifier compression: (a) noted on the Booth chart
where the output power depends only on the supply voltage, and not on variations of the
input power (amplifier self-limiting), and (b) shown as a functional model for C-mode operation where the transistor operates as a switch.

W I N T E R 2 0 16

IEEE SOLID-STATE CIRCUITS MAGAZINE

amplifier achieves power saturation and acts as a self-limiter. Now,
though, the output power does vary
with changes in the supply voltage,
something not seen in L-mode. The
C-mode region of this Booth chart is
shown in Figure 5(a).
With this reversal of amplifier output sensitivities, it is expected that
the circuit operating model must also
be different. This is indeed true, and
the C-mode operating model is shown
in Figure 5(b). Here the transistor no
longer acts as a CCS and changes to
acting as a switch. The transistor no
longer regulates the current flowing through the load. Instead, it only
selects when current will flow, or not,
through the load. Limitations on the
load current are set by the amplifier
external circuitry and the load resistance value itself [3].
The recognition of P-mode is
readily done on the Booth chart, as
shown in Figure 6(a). Here the wide
separation of the individual curves
signifies a very high sensitivity to
the supply voltage value, something
that is very different from both
L-mode and C-mode, and proves
that the transistor is not operating as a CCS. Rather, the transistor
operates as a controlled-value resistance, as shown in the functional
model in Figure 6(b).
With the transistor operating as
a variable resistance, the current
through the load is governed by the
product of the input signal voltage
with the supply voltage, hence the
name of P-mode. In the transistor's
region of resistive operation, its
transconductance is much lower than
in the CCS region. This is the reason
for lower gain in P-mode. The Booth
chart also shows that, in P-mode, the
output power changes with the same
slope as with L-mode, but all at lower
power levels than L-mode, with the
level dependent on the supply voltage value. This is the definition of a
controlled variable gain circuit.
It is also common in the power amplifier literature to evaluate designs
using a set of gain plots similar to
that in Figure 7. This particular set of

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