Power Amplifier

27 - 31 GHz

Rev. V2

XP1027-BD

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App Note [1] Biasing -

It is recommended to separately bias each

mA. Separate biasing is recommended if the

amplifier is to be used in a linear application or at

high levels of saturation, where gate rectification

will alter the effective gate control voltage. For

non-critical applications it is possible to parallel all

stages and adjust the common gate voltage for a

Linear Applications -

For applications where the amplifier is being used

in linear operation, where best IM3 (Third-Order

Intermod) performance is required at more than

5 dB below P1dB, it is recommended to use active

gate biasing to keep the drain currents constant as

the RF power and temperature vary; this gives the

best performance and most reproducible results.

Depending on the supply voltage available and the

power dissipation constraints, the bias circuit may

be a single transistor or a low power operational

amplifier, with a low value resistor in series with the

drain supply used to sense the current. The gate

voltage of the pHEMT is controlled to maintain

correct drain current compensating for changes

over temperature.

Saturated Applications -

For applications where the amplifier RF output

power is saturated, the optimum drain current will

vary with RF drive and each amplifier stage is best

operated at a constant gate voltage. Significant

gate currents will flow at saturation and bias

circuitry must allow for drain current growth under

this condition to achieve best RF output power and

power added efficiency. If the input RF power level

will vary significantly, a more negative gate voltage

will result in less die heating at lower RF input drive

levels where the absence of RF cooling becomes

significant. Under this bias condition, gain will then

vary with RF drive

NOTE -

For any application it is highly recommended to

bias the output amplifier stage from both sides for

best RF and thermal performance.

CAUTION -

Make sure to properly sequence the applied

available before applying the positive drain supply

are protected with silicon diodes to limit the applied

voltage.

Bias Arrangement

Layout for reference only – It is recommended to bias output

stage from both sides.

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