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Номер произв AN-0002
Описание Biasing Circuits and Considerations
Производители ANADIGICS
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AN-0002 Даташит, Описание, Даташиты
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
0.6
0.4
0.2
0.0
0
2
m2
46
Vdd
8
m1
Vdd=1.200
Vgg=0.000
Ids.i=0.869
m2
Vdd=7.000
Vgg=-1.100
Ids.i=0.337
10
Figure 1. IV characteristics of a typical MESFET device.
05/2003
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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
R1
Vref
Id
R3
Q2
Vdd
DUT
R2 Vgg
R4
Negative
Supply
Figure 2. Simple constant current circuit for use with MESFET amplifiers.
In this circuit a reference voltage is established by the simple resistive voltage divider consisting of R1
and R2. The voltage across R1 should be equal to the voltage drop across R3 plus the emitter-base
voltage of Q2 (typically 0.6 - 0.7 Volts). The actual current flowing through the PNP transistor is quite
modest, typically only 1 mA. The circuit constantly adjusts the gate voltage of the amplifier in order to
maintain the voltage at the base of Q2 such that it equals the reference voltage. This has the effect of
holding the current through R3 constant.
The designer must also consider some tradeoffs concerning the available voltages and the choice of
resistors such as R3. As an example, consider the RFS1003 with a desired operating point of 7V, 400
mA. Since the RFS1003 has a desired operating voltage of 7V, it is generally desirable to restrict R3 to
small values. Selecting R3 to be 1 ohm, and neglecting the 1 mA flowing through Q2 would require a
supply voltage of approximately 7.4 volts. This also sets the voltage at the base of Q2 to be
approximately 6.3 Volts, and allows the resistor values in the voltage divider to be easily calculated.
Typically the voltage divider would be set to have approximately 1 mA of current flowing through it.
The value of R4 can be calculated by knowing that the typical gate voltage for operation is –1.1 Volts.
Assuming that the negative voltage available is –2 V, and setting the current flowing through Q2 to 1 mA
results in a value of approximately 900 Ohms.
This biasing circuit works well for room temperature operation, but has the disadvantage that the base-
emitter voltage of Q2 will change with temperature, even if the reference voltage is held constant. This
can be overcome by the addition of another PNP transistor to form a matched pair. It is important that
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AN-0002 Даташит, Описание, Даташиты
AN-0002
these transistors be well matched, operating at the same bias, and in the same thermal environment.
The schematic of Figure 3 is a refinement of the basic circuit that incorporates temperature compe nsation
as well as several other features. The user does not need to implement all of the functions, but they are
shown here for illustrative purposes.
The main features of this circuit are:
Constant Operating Current
Reduction of Part -to-Part Variation
Temperature Compensation
Negative Voltage Generator
Power Sequencing (Negative before positive)
Positive
Supply
Q3
R1 R3
R 5 Vdd
Q1 Q2
DUT
Vgg
R2 R4
+5V
ON/OFF
CONTROL
+5V
U2
MAX881R
C1+ IN
C1
C1-
C3
NC
NEG OUT GND
C2
/POK
OUT
/SHDN
FB
C4
Figure 3. Schematic for a Negative Bias Generator.
The Maxim MAX881 voltage inverter is shown in its standard configuration. This provides the negative
voltage necessary for the gates of the amplifier. The Maxim part works from supply voltages (V) of 2.5 to
05/2003
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