AN0014
15
AN0014
Bias Scheme for NBB-Series Amplifiers
Bias Scheme for NBB-Series Amplifiers
Introduction
RFMD鈥檚 NBB-series amplifiers are monolithic integrated circuits (IC's) using InGaP/GaAs HBT technology. The
NBB-series uses a Darlington-pair transistor configuration with bias and feedback resistors properly selected to deter-
mine the gain, input and output match and bias (both voltage and current) parameters. A schematic representation of the
NBB-series amplifier is shown in Figure 1.
+V
CC
NBB-Series
Amplifier
R
CC
C1
L1
R
F
C2
Q1
R
BB
R
BIAS
Q2
C3
Figure 1. Schematic representation of Darlington-pair feedback amplifier used in the NBB-series amplifiers.
(On-chip components are shown inside the dotted outline.)
The amplifier may be analyzed as a two-port network with the input (left) and output (right) as shown in Figure 1. The
output node is also used to provide bias to the amplifier through the bias network (top), which is shown in the figure. The
packaged part has a total of four ports: input, output and bias, and two ground connections for minimization of ground
inductance.
Bias Circuit Topology
The input voltage of the amplifier is fixed by the base-emitter voltage of Q
1
and Q
2
, and the output voltage is determined
by the voltage divider established by R
BB
and R
F
. Hence, the output voltage is a designed parameter and the amplifier is
controlled by the current supplied to the output node. Thus, the amplifier is biased using a current source rather than a
voltage source. The simplest current source is a resistor (R
CC
) connected to a voltage source (V
CC
) as shown in
Figure2.
The current source, comprising voltage source V
CC
and resistor R
BIAS
, should be selected such that the designed ampli-
fier voltage (V
D
) appears at the output with the desired current (I
CC
) flowing into the amplifier. The governing relationship
for the bias circuit shown in Figure 2 is:
15
TECHNICAL NOTES
AND ARTICLES
(
V
CC
鈥?/div>
V
D
)
-
I
CC
= ---------------------------
R
BIAS
Eq. 1
The recommended amplifier current (I
CC
) and design value of the amplifier voltage (V
D
) for each NBB-series amplifier is
specified on the data sheet. Hence, if a V
CC
is selected, then the desired R
CC
may be calculated from Equation 1. A bias
resistor table is provided with each datasheet which is calculated using this method.
Copyright 1997-2002 RF Micro Devices, Inc.
15-23
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