FREESCALE AN3100

Freescale Semiconductor
Application Note
AN3100
Rev. 0, 3/2005
General Purpose Amplifier Biasing
by: Jeff Gengler
Freescale Semiconductor
INTRODUCTION
Freescale Semiconductor’s General Purpose Amplifier
(GPA) devices are all designed to operate from a single
positive voltage supply. The GPAs have output powers
ranging from 12 to 34 dBm. They are currently designed with
three different circuit techniques:
• Darlington Pair
• Discrete with integrated current mirror
• Field Effect Transistor (FET) operating at zero gate voltage
drain leakage current (IDSS)
and use two different device technologies:
• Indium Gallium Phosphide Heterostructure Bipolar
Transistors (InGaP HBT)
• GaAs Heterostructure Field Effect Transistor (HFET)
The required biasing methods for the different circuit
schemes are described in this application note.
GPA CIRCUIT DESIGN METHODS
Freescale’s InGaP HBTs are designed using one of two
different circuit methods. The low power GPAs (P1dB from 12
to 24 dBm) are designed using a Darlington Pair (Fig. 1). The
Darlington Pair is biased when voltage is applied to the
collector of discrete devices Q1 and Q2. Resistor R1 is used
for negative feedback of the amplifier but is also part of the
voltage divider with R2 to establish the base bias on Q1.
VSUPPLY
R6
HBT devices are current - driven; therefore, Freescale
recommends that designers use a constant current source to
minimize the impact of shifts in supply voltage and shifts in the
temperature of the operating environment. Deviations from
the optimal current can impact both power and linearity
performance. A series resistor between the voltage supply
and collectors of the Darlington is the easiest way to emulate
a constant current source (R6 in Fig. 1). This is necessary for
the MMG3001NT1, MMG3002NT1 and MMG3003NT1
devices.
Because the RF output of the Darlington Pair is also used
for the DC bias, an RF choke is required (L1) to connect the
voltage supply to the output. RF coupling capacitors may also
be required on the RF input and RF output because the input
and output of the devices are DC coupled.
Since the release of these first three devices, Freescale has
developed a method to eliminate the need for an external
resistor and to enable the devices to operate directly from a
positive 5 Volt supply. This approach has exceptional current
stability over temperature and has a pending patent. All
Darlington HBT products with the exception of the
MMG3001NT1, MMG3002NT1 and MMG3003NT1 use this
approach (Fig. 2).
VSUPPLY
VCC
RF
INPUT
Q1
VCC
C3
L1
Q1
R2
R3
R4
C2
Q2
R2
R3
R4
RF
OUTPUT
R1
C1
RF
OUTPUT
R1
C1
RF
INPUT
C3
L1
R5
C2
Q2
R5
PACKAGED DEVICE
Figure 2. Improved Darlington Pair InGaP HBT
Bias Scheme
PACKAGED DEVICE
Figure 1. Darlington Pair InGaP HBT Bias Scheme
 Freescale Semiconductor, Inc., 2005. All rights reserved.
RF Application Information
Freescale Semiconductor
AN3100
1
The second circuit method is used on the intermediate
power amplifiers (P1dB ranging from 27 to 34 dBm). These
are designed with a MMIC that contains a discrete device, Q1,
with an integrated current mirror to drive the base (Fig. 3).
This active bias approach means that the bias current has
minimal shift with normal supply voltage deviations over the
specified operating temperature range. R6 in Fig. 3 is an
external dropping resistor that is required to establish the
reference voltage on the current mirror that drives the bias of
Q1.
The HFETs are discrete devices that operate directly from
a 5 Volt supply voltage (Fig. 4). The DC blocking capacitor that
is integrated in the feedback loop prevents the gate voltage
from being established with R1 and R2; therefore, the HFET
devices operate at IDSS when 5 Volts are applied to the drain.
L1 is again required as an RF choke as well as the RF coupling
capacitors, C2 and C3.
VSUPPLY
C4
VCC
R6
L1
C6
L1
Vref
RF
INPUT
R1
C5
C2
Q2
R4
C2
Q3
R2
R1
C3
R2
PACKAGED DEVICE
RF
OUTPUT
R3
RF
INPUT
Figure 4. HFET Bias Scheme
C4
Q1
C3
C1
Q1
R5
RF
OUTPUT
C1
PACKAGED DEVICE
Figure 3. Intermediate Power InGaP HBT Bias Scheme
The reference voltage (Vref) is different for each device
based on its size. The data sheets for each device list the
specific reference voltage required for optimal bias current. L1
is required to prevent the DC supply line from improperly
loading the RF output. RF coupling capacitors (C3 and C4 in
Fig. 3) are also required.
The third circuit approach in GPAs is used for the HFET
devices. Bias of this type of device is very similar to the
Darlington circuit technology.
SUMMARY
The GPA lineup from Freescale is designed to operate from
a single positive voltage supply, which makes them easy to
use. Designers using these devices should be careful to bias
the devices correctly using the appropriate method for the type
of device used. If the current is set too low, linearity and power
will degrade. If the current is set too high, there is some risk
of compromising reliability.
The techniques outlined here are a guide to the bias
approaches for the different technologies and products
available from Freescale. The data sheets for each device
should be followed to achieve optimal performance from all
GPAs.
AN3100
2
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NOTES
AN3100
RF Application Information
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AN3100
Document Number: AN3100
Rev. 0, 3/2005
4
RF Application Information
Freescale Semiconductor