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15
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Bias Scheme for NBB-Series Amplifiers
Bias Scheme for NBB-Series Amplifiers
Introduction
RFMD’s 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:
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TECHNICAL NOTES
AND ARTICLES
(
V
CC
–
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
AN0014
Typical Bias Configuration
R
BIAS
4
C
BLOCK
IN
1
2
3
V
D
L
CHOKE
VCC
(optional)
OUT
C
BLOCK
Figure 2. Typical bias configuration for the NBB-series amplifier.
The device is current-controlled due to the nature of the circuit. The simplest current source which can be
connected is a resistor (RBIAS) and voltage supply (VCC) as shown. Two blocking capacitors (CBLOCK) are
shown to prevent DC-loading of the circuit by an adjacent component.
Alternatively, a current steering circuit (see Sedra and Smith,
Microelectronic Circuits,
2nd Edition, page 508) may be
used to bias the amplifier as shown in far right of Figure 3. Such circuits are commonly used to bias various stages of an
IC. The circuit uses one positive power supply (V
CC
). The DC reference current I
REF
is generated in the branch that con-
sists of the diode-connected transistor, Q
1
, resistor R
1
, and the diode-connected transistor Q
2
. Assuming all transistors
have high current gain and hence the base currents are negligibly small, then the reference current is given by:
V
CC
–
V
BE1
–
V
BE2
I
REF
= -----------------------------------------------
R
1
Eq. 2
Diode-connected transistor Q
1
forms a current mirror with Q
3
. Thus Q
3
will supply a constant current I
CC
equal to I
REF
.
Transistor Q
3
can supply this current to any load as long as the voltage that develops at the collector does not exceed
that at the base (V
CC
-V
BE3
).
+V
CC
Q1
+V
CC
Q3
+V
CC
I
CC
I
CC
R
BIAS
I
REF
R1
I
CC
Q2
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TECHNICAL NOTES
AND ARTICLES
Figure 3. The family of NBB-series amplifiers is biased using a current source (left).
The simplest current source is a resistor connected to a voltage source (middle). Alternatively, a current
steering circuit (right) may be used as well to produce a constant current (see Sedra and Smith,
Microelectronic Circuits,
2nd Ed., page 508).
RF Choke Selection
The RF choke in series with the bias resistor is recommended as the bias line will effectively load the output of the ampli-
fier. When considering power delivered from the amplifier to a load, it is useful to model the output of the amplifier as a
Thevenin equivalent: a voltage V
th
with internal impedance Z
O
(50Ω for a matched amplifier). When an amplifier is driven
in a 50Ω load, only half the Thevenin voltage (V
th
/2) appears across the load resistor. If a RF choke is not used then the
voltage across the load reduces to V
th
xR
BIAS
/(2R
BIAS
+50). The power deliver to the load is reduced by the reduction in
the voltage which appears across the load resistor. Taking this ratio, the reduction in power (in dB) by not using an RF
choke can be expressed by:
P
REDUCTION
15-24
=
(
V
th
⁄
2
)
-
20
×
LOG
-------------------------------------------------------------
V
th
R
BIAS
⁄ (
2R
BIAS
+ 50
)
Copyright 1997-2002 RF Micro Devices, Inc.
Eq. 3
AN0014
(
2R
BIAS
+ 50
)
-
P
REDUCTION
= 20
×
LOG
----------------------------------
2R
BIAS
Eq. 4
In the case of the NBB-300 with a bias resistor of 120Ω (V
CC
=10V), the reduction in power delivered to a 50Ω load is
1.64dB.
In order to prevent loading of the output of the amplifier, a reactance of five to 10 times the characteristic impedance is
desired. At upper microwave frequencies where lumped element chokes are not available, a microstrip bypass circuit is
desirable. Such as choke circuit would consist of a 90° high-impedance line with a short-circuit radial stub. If the toler-
ances are an issue with the short-circuit stub, a short circuit may be provided from a suitable capacitor instead. In such
instances the self-resonance frequency of the capacitor must be considered.
Bias Resistor Selection
The output voltage of the amplifier (V
D
) varies as a function of both the bias current (I
CC
) and the temperature. The vari-
ation of device voltage versus current is supplied on each data sheet. From this data, a coefficient may be calculated for
the change in V
D
versus current (see Figure 4). Notice that all coefficients are positive, indicating that the device voltage
increases with increasing current. A large bias resistor is desirable because it reduces the variation in bias current,
reducing the change in important amplifier parameters such as P1dB and IP3. Selecting a large bias resistor, R
BIAS
,
requires selecting a higher voltage supply (V
CC
) to maintain the desired bias current (I
CC
). The current steering circuit
(see Figure 3) provides a steady current and minimizes variations in amplifier parameters as well.
Table 1. Summary of the coefficient of the change in device voltage (V
D
) versus amplifier current (I
CC
).
The data is calculated from the plot provided with each datasheet. The positive coefficient indicates
that the device voltage increases with increasing current.)
Typical Device Voltage Variation with Current,
NBB Amplifier Model Number
δV
D
/δI
CC
(in V/A)
NBB-300
NBB-301
NBB-400
NBB-401
NBB-410
NBB-500
+4
+4
+7
+3
Device voltage (V
D
) decreases with increasing temperature as shown in Figure 5. An average rate of change of the
device voltage versus temperature is calculated and provided in the table. The device voltage can be expressed as a
function both current and temperature as follows.
δV
D
δV
D
-
-
V
D
(
I
CC
,
T
)
=
V
O
+
-----------
×
I
CC
+
---------
× (
T
–
T
O
)
δT
δI
CC
Table 2. Summary of the dependence of the amplifier output voltage (V
D
) versus temperature.
The coefficient of the change in device voltage (V
D
) versus ambient temperature is calculated from the
tabular data. The negative coefficient indicates that the device voltage decreases with increasing current.
Device Voltage (V
D
) versus Temperature
Model
Amplifier
Temp. Coef.
Number
Current (mA)
(mV/°C)
-45°C
+25°C
+85°C
NBB-300
NBB-301
NBB-400
NBB-401
NBB-410
NBB-500
50
50
65
35
4.03
4.09
4.19
4.12
3.86
3.90
4.00
3.94
3.70
3.74
3.88
3.78
-2.75
-2.80
-2.85
-2.70
Eq. 5
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TECHNICAL NOTES
AND ARTICLES
Copyright 1997-2002 RF Micro Devices, Inc.
15-25
AN0014
Equation 5 may be substituted into Equation 1 and solved for I
CC
to get:
I CC
=
δV
D
-
V
CC
–
---------
× (
T
–
T
O
)
δT
-----------------------------------------------------------
-
δV
D
-
R
BIAS
+
-----------
δI
CC
Eq. 6
Equation 6 may be differentiated with respect to temperature to get an expression which is useful to observe the effect of
the bias resistor (and supply voltage) selected by the user:
δV
D
-
–
---------
δI
CC
δT
----------- = --------------------------------------
-
δT
δV
D
-
R
BIAS
+
-----------
δI
CC
Eq. 7
For example, an NBB-400 biased from a 5V supply (V
CC
) using a 22Ω resistor (R
CC
) will exhibit the following current
change with respect to temperature,
δI
CC
–
(
– 2.8
)
----------- = ------------------- = 0.108mA
⁄ °C
-
-
δT
(
22 + 4
)
Eq. 8
which translates into a variation of 14mA over the temperature range from -40°C to +85°C. By comparison, if a 12V sup-
ply rail is with a 162Ω bias resistor, then a change in current with respect to temperature reduces to 0.016mA/°C. This
translates into a current variation of only 2.2mA over the same operating temperature range.
15
TECHNICAL NOTES
AND ARTICLES
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Copyright 1997-2002 RF Micro Devices, Inc.
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