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 (I
DSS
)
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.
V
SUPPLY
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).
V
SUPPLY
V
CC
RF
INPUT
C1
L1
R1
Q1
R2
R3
R4
Q2
R5
C2
C3
RF
OUTPUT
V
CC
RF
INPUT
C1
L1
R1
Q1
R2
R4
Q2
R5
C2
C3
RF
OUTPUT
PACKAGED DEVICE
R3
Figure 2. Improved Darlington Pair InGaP HBT
Bias Scheme
PACKAGED DEVICE
Figure 1. Darlington Pair InGaP HBT Bias Scheme
AN3100
1
Freescale Semiconductor, Inc., 2005. All rights reserved.
RF Application Information
Freescale Semiconductor
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
