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
ANADIGICS [ ANADIGICS, INC ]
