INA-12063-BLK HP [Agilent(Hewlett-Packard)], INA-12063-BLK Datasheet - Page 11

• Manufacturer Part Number: INA-12063-BLK
• Description: 1.5 GHz Low Noise Self-Biased Transistor Amplifier
• Manufacturer: HP [Agilent(Hewlett-Packard)]

If the CAD analysis indicates
there is a potential instability
issue (K < 1 and/or B1
frequency) as in Case 2 or Case 3
above, then some stability
countermeasures will be needed.
There are four basic techniques
for handling potential instability:
(a) Live with it. If the source and
load impedances that will be
presented to the amplifier are
well defined, the finesse
approach of using stability circles
may be used. Stability circles
(calculated by a program such as
Touchstone) are plotted on a
Smith chart and define regions of
loads that could cause a circuit to
oscillate. An amplifier is safe
from oscillation if the expected
amplifier terminations lie well
outside of the unstable regions on
both the input and output imped-
ance planes. Since the possibility
of oscillation could exist at any
frequency for which the
INA-12063 has gain, stability
circles must be checked at
frequencies over a wide
frequency range when this
method is used.
(b) Resistive feedback. The use of
resistive feedback is often used to
create stable, wideband, amplifi-
ers. While effective in stabilizing
active devices, this method will
not be considered here since a
significant penalty is often paid in
degraded NF, less gain, and
lowered output power
performance.
(c) Lossless feedback. Reactive
feedback elements can also be
used to stabilize amplifiers. The
INA-12063 already incorporates
one type of reactive feedback in
the emitter of the RF transistor,
with a resulting improvement in
stability. Further use of the
0 for any
lossless feedback technique is not
suggested for most INA-12063
amplifier applications since this
method adds considerable design
complexity as well as additional
parts count and board space to
the circuit.
(d) Resistive loading. Resistive
loading can be used at either the
input or output of the INA-12063
to create an unconditionally
stable amplifier. This is the brute-
force method of ensuring stabil-
ity. It is fairly fail-safe and is also
the simplest to implement. The
addition of a resistive element to
either the amplifier input or
output creates RF loss which
manifests itself as lower gain plus
either increased NF (if the
resistance is added to the input)
or lower output power (if the
resistance is placed at the
output.)
In keeping with the goals of low
cost (i.e., circuit simplicity), the
resistive loading method is the
technique suggested for produc-
ing an unconditionally stable
amplifier for most applications of
the INA-12063.
The resistive loading can be
applied in either series or shunt
and can be added to either the
input or output of the amplifier.
The choice of series or shunt
resistive load may be dictated by
whether the real part of the
output impedance of the amplifier
device is greater or less than
50
a shunt resistor when the ampli-
fier impedance is >50
series resistor for the case of
>50
the overall impedance closer to
50
In some cases, excessive voltage
drop across the stabilizing
resistor due to the DC current
. The logical choice is to use
, thus simplifying the match.
. This technique will bring
6-126
and a
into the device may preclude the
use of the series configuration.
Shunt resistance is usually the
most straightforward solution to
implement since it can be easily
bypassed to ground with a
capacitor without disturbing the
bias.
For gain or buffer stages requir-
ing maximum output power, the
loading is applied to the amplifier
input. If the performance goal is
low noise figure, the resistive
loading is implemented on the
output side of the INA-12063 as
shown in Figure 11.
Figure 11. Shunt Stabilizing Resistor
for LNA.
A simple manual optimization
may be used to determine a
starting value for the stabilizing
resistor. By adding a shunt
resistor to the output of the
INA-12063 in the circuit file used
in the previous stability analysis,
K may be observed while adjust-
ing the value of the resistor. The
shunt resistor should be the
highest value that will adequately
stabilize the circuit.
The three possible cases resulting
from the stability analysis will
now be considered.
Case 1 (K>1 over the entire
frequency range) is always the
hoped for situation since it is the
easiest to deal with. If K is greater
than unity by a comfortable
margin, then no further action is
needed at this point.
INPUT
RF
INA-12063
STABILIZING
RESISTOR
OUTPUT
RF