INA-12063-TR1 Ver la hoja de datos (PDF) - HP => Agilent Technologies

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Fabricante

INA-12063-TR1

1.5 GHz Low Noise Self-Biased Transistor Amplifier

HP => Agilent Technologies 

If the CAD analysis indicates

there is a potential instability

issue (K < 1 and/or B1 ≤ 0 for any

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

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␣ Ω. The logical choice is to use

a shunt resistor when the ampli-

fier impedance is >50␣ Ω and a

series resistor for the case of

>50␣ Ω. This technique will bring

the overall impedance closer to

50␣ Ω, thus simplifying the match.

In some cases, excessive voltage

drop across the stabilizing

resistor due to the DC current

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.

RF

INPUT

INA-12063

RF

OUTPUT

STABILIZING

RESISTOR

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.

6-126

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