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.
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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. 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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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
R5
R3
Vdd
Q1
Q2
DUT
Vgg
R2
R4
+5V
U2
MAX881R
C1+
C1
C1-
C3
NC
IN
C2
+5V
NEG OUT
GND
/POK
OUT
ON/OFF
CONTROL
C4
/SHDN
FB
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
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5.5 volts. The capacitors listed in the bill of materials (BOM) are those used by Maxim in their application
circuit. Other choices are possible, depending upon the ripple requirements. In this application the
output voltage was set to -2V.
The Shutdown pin is active low (pull it to ground for the circuit to operate).
Power sequencing is very important in order to assure that the amplifier is not overstressed. From the IV
curves it can be observed that if the drain voltage were to be applied first, while the gate voltage
remained at 0V, then the current through the device would be at its maximum, roughly double the nominal
value. Instead, the negative voltage should be applied first, which holds the FETs pinched off to keep
power dissipation low until the drain voltage reaches its desired value. R5 is a pull-up resistor that holds
FET Q3 off when the circuit is off (Shutdown = High). The Maxim part monitors the output voltage during
power-up, and sets /POK low when the negative voltage has reached 92% of its final value. Setting /POK
low turns on FET Q3, which safely powers up the rest of the circuit.
Example: Biasing the RFS1006
If the input voltage is 7.8V, and the desired voltage is 7.0V for the RFS1006, with an average operating
current of 520 mA, then the resistor, R3, should be 1.5Ω. This value does dissipate some power (0.42W
in this example). This resistor should be W and also 1% tolerance, if possible, since the bias current
will change directly with the resistor value.
Resistor R2 sets the bias current through Q1, which is the supply voltage minus the drop across R3 and
the V
EB
(0.7V) of Q2. With a bias of 1 mA, and neglecting base currents, the closest standard value of R2
is 6.34 kΩ. Selecting a bias current of 1 mA for Q2 determines the value of R4 (909Ω is the closest 1%
standard value).
Transistor Q1 forms part of the temperature compensation/current mirror. Biasing it to 1 mA, and setting
the base voltage equal to that of Q2, the voltage drop across R1 should be the same as that across R3.
This resistor should be 806Ω.
In some cases only a maximum of 7V will be available to run the ANADIGICS power amplifier. Since
resistor R3 will introduce a voltage drop, it should be made as small as possible, on the order of 0.1
Ohms. This will cause only a negligible voltage drop of approximately 50 mV, allowing the amplifier to
achieve nearly its full output power. Resistor R1 will need to be resized accordingly. The disadvantage
to this approach is that it is more sensitive to the resistor values.
In some cases it is desirable to vary the operating current of the circuit. In production this may be
achieved by changing the value of R1. In a lab environment, this can be achieved through the use of a
small potentiometer in series with R1.
Conclusion
A simple method for achieving consistent amplifier operation has been presented. For additional
information contact the factory.
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Table 1. Bill of Materials for Additional Functions Illustrated in Figure 3.
Ref Des
C1, C2, C3
C4
Q1/Q2
Q3
R1
R2
R3
R4
R5
R6
U2
Value
1 uF
4.7 uF
N/A
N/A
806 ?
6340 ?
1.5 ?
909 ?
10 K?
1 M?
N/A
Description
1206, 16 V, low ESR ceramic Cap
1206, 10 V, low ESR ceramic Cap
SOT-363, Dual 3906 PNP Transistor
HEXFET Power Mosfet, Low On Resistance
0603, 1/16W, 1%
0603, 1/16W, 1%
2512, 1 W, 5%
0603, 1/16W, 1%
0603, 1/16W, 1%
0603, 1/16W, 1%
10 uMAX, Low -Noise Bias Supply
Part No.
Manufacturer
Quantity
EMK316BJ105KF
Taiyo Yuden
3
LMK316BJ475KL
Taiyo Yuden
1
MMDT3906TR-ND Diodes Incorporated
1
IRLML6401TR- ND International Rectifier
1
ERJ-3EKF8060V
Panasonic
1
ERJ-3EKF6341V
Panasonic
1
ERJ-1TYJ1R5U
Panasonic
1
ERJ-3EKF9090V
Panasonic
1
ERJ-3EKF1002V
Panasonic
1
ERJ-3EKF1004V
Panasonic
1
MAX881REUB
Maxim
1
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