NBB-301 RFMD [RF Micro Devices], NBB-301 Datasheet - Page 2

• Manufacturer Part Number: NBB-301
• Description: Bias Scheme for NBB-Series Amplifiers
• Manufacturer: RFMD [RF Micro Devices]

15
Figure 2. Typical bias configuration for the NBB-series amplifier.
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
sists of the diode-connected transistor, Q
have high current gain and hence the base currents are negligibly small, then the reference current is given by:
Diode-connected transistor Q
Transistor Q
that at the base (V
Figure 3. The family of NBB-series amplifiers is biased using a current source (left).
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
in a 50
voltage across the load reduces to V
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:
15-24
AN0014
I
P
REF
load, only half the Thevenin voltage (V
REDUCTION
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.
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).
3
=
can supply this current to any load as long as the voltage that develops at the collector does not exceed
V
-----------------------------------------------
CC
CC
-V
–
=
BE3
V
20 LOG
BE1
R
1
).
1
–
forms a current mirror with Q
V
th
BE2
with internal impedance Z



------------------------------------------------------------- -
V
th
Copyright 1997-2002 RF Micro Devices, Inc.
I
CC
xR
th
Typical Bias Configuration
IN
R
1
BIAS
BIAS
, resistor R
/(2R
C
V
BLOCK
I
th
CC
th
CC
2R
/2) appears across the load resistor. If a RF choke is not used then the
BIAS
). The DC reference current I
1
2
+V
BIAS
R
1
+50). The power deliver to the load is reduced by the reduction in
BIAS
, and the diode-connected transistor Q
CC
+
4
2
50
O
3
. Thus Q
3
(50
V



I
D
REF
Q1
for a matched amplifier). When an amplifier is driven
Q2
(optional)
C
+V
L
R
CHOKE
BLOCK
BIAS
3
R1
CC
will supply a constant current I
OUT
VCC
REF
Q3
+V
I
CC
CC
is generated in the branch that con-
2
. Assuming all transistors
CC
equal to I
Eq. 2
Eq. 3
REF
.