NBB-500_ Ver la hoja de datos (PDF) - RF Micro Devices
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NBB-500_ Datasheet PDF : 4 Pages
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AN0014
Typical Bias Configuration
CBLOCK
IN
1
4
3
2
VD
RBIAS
LCHOKE
(optional)
CBLOCK
VCC
OUT
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 (VCC). The DC reference current IREF is generated in the branch that con-
sists of the diode-connected transistor, Q1, resistor R1, and the diode-connected transistor Q2. Assuming all transistors
have high current gain and hence the base currents are negligibly small, then the reference current is given by:
IREF
=
V----C----C-----–----V----B---E---1----–-----V----B---E---2-
R1
Eq. 2
Diode-connected transistor Q1 forms a current mirror with Q3. Thus Q3 will supply a constant current ICC equal to IREF.
Transistor Q3 can supply this current to any load as long as the voltage that develops at the collector does not exceed
that at the base (VCC-VBE3).
+VCC
+VCC
Q1
+VCC
Q3
ICC
RBIAS
ICC
IREF
R1
ICC
Q2
15 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 Vth with internal impedance ZO (50Ω for a matched amplifier). When an amplifier is driven
in a 50Ω load, only half the Thevenin voltage (Vth/2) appears across the load resistor. If a RF choke is not used then the
voltage across the load reduces to VthxRBIAS/(2RBIAS+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:
PREDUCTION
=
20
×
L
OG
-V---t--h---R----B---I--A---S-(--V-⁄--(-t--h2---R⁄---2-B--)-I--A---S----+-----5---0----)
Eq. 3
15-24
Copyright 1997-2002 RF Micro Devices, Inc.
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