PFM19030 Datasheet, PDF (14/15 Pages) List of Unclassifed Manufacturers – 1930-1990 MHz, 30W, 2-Stage Power Module Enhancement-Mode Lateral MOSFETs

PFM19030
Test Fixture Active Bias Circuit Principles of Operation
The test fixture operates off of a single voltage supply. It contains two switches and two potentiometers.
The switches provide for independent on/off for the input and output devices of the module. The
potentiometers allow adjustment of quiescent current level of each stage. The adjustments should be made
with no RF applied to the module.
Q1 Die Carrier
Input
Match
Q1
Output
Match
Vsupply
R1 <<Rbias
R1
R1
S1
Vsupply
- _{Differental Amp}
+
Ids_sense
Rbias
Ireference
Principal of Operation of Bias Circuitry
The principal of operation of the fixture bias circuit is demonstrated in the above Figure. The potentiometer
establishes a reference current, and the operational amplifier adjusts gate voltage to maintain that current in
the sense device. The same DC gate voltage is also applied to the main (RF) device. Sense devices are
scaled versions of the main (RF) devices, on the same die (to facilitate temperature tracking). As the
temperature of the die changes due to RF drive (or ambient temperature changes), the operational amplifier
maintains constant current through the Sense FET, and thus constant quiescent bias for the main (RF) FET.
No RF signal is applied to the Sense FET.
There is a separate independent bias circuit for the input (Q1) device and for the output device (Q2) of the
module.
Experience has shown this bias circuit to be a reliable method of maintaining tight control of quiescent
current over operating temperature, and for minimizing the impact of device aging effects on amplifier
performance. However, there are some precautions regarding use of this circuit. The principle of the circuit
is for the differential amplifier (op amp) to adjust gate voltage until the desired current is achieved through
the sense FETs. If the current path is interrupted (thereby not allowing I_ds__sense to flow), the operational
amplifier will increase gate bias in an attempt to increase current, with the possibility that the quiescent bias
current in the RF FET may increase beyond a safe limit (the device may be destroyed). The zener diodes in
the test fixture circuit (D1 and D2, test fixture schematic) are safeguards for prohibiting excessive gate
voltage to be applied to the transistors.
Page 14 of 15 Specifications subject to change without notice. U.S. Patent No. 6,822,321
http://www.cree.com/
Rev. 2