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AN-0002

ANADIGICS
Part Number AN-0002
Manufacturer ANADIGICS
Description Biasing Circuits and Considerations
Published May 17, 2014
Detailed Description MESFET Amplifier Biasing AN-0002 Biasing Circuits and Considerations for GaAs MESFET Power Amplifiers Summary In order ...
Datasheet PDF File AN-0002 PDF File

AN-0002
AN-0002

Overview
MESFET Amplifier Biasing AN-0002 Biasing Circuits and Considerations for GaAs MESFET Power Amplifiers Summary In order to properly use any amplifier it is necessary to provide the correct operating environment, especially the DC bias.
This application note outlines some of the considerations for biasing MESFET amplifiers.
Items considered herein are: • Constant current operation, • Temperature compensation of the biasing network, and • Power sequencing of the applied voltages.
Overview The I-V curves of Figure 1 represent a typical MESFET device in a common source configuration.
For a typical device operating in Class A the desired current is 50% of the maximum current for any particular part.
Typical MESFET devices are depletion mode, meaning that the highest drain-source current occurs for a gate voltage of approximately zero (Vgg ~ +0.
5 V).
As the gate voltage becomes more negative, the device current drops and eventually approaches zero at the pinch-off voltage.
The two main variables in the production of MESFET power amplifiers are the maximum current and the pinch-off voltage.
Since the operating voltage is assumed to be fixed by the available voltages in the system, it is the drain current that should be monitored and controlled in order to provide consistent performance from unit to unit.
1.
0 m1 0.
8 Ids.
i, A 0.
6 m1 Vdd=1.
200 Vgg=0.
000 Ids.
i=0.
869 m2 m2 Vdd=7.
000 Vgg=-1.
100 Ids.
i=0.
337 0 2 4 6 8 10 0.
4 0.
2 0.
0 Vdd Figure 1.
IV characteristics of a typical MESFET device.
05/2003 http://www.
Datasheet4U.
com AN-0002 The schematic of Figure 2 is a simplified representation of a circuit that provides constant drain current bias for MESFET amplifiers.
Since MESFETs are voltage controlled, the amount of gate current is quite low.
An exception to this condition occurs under conditions of high-level RF drive where the gate current increases and eventually changes sign.
Positive Supply Id R1 R3 Vdd Q2 DUT Vref R2 R4 Vgg Negative Supply Figure 2.
Simp...


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