2
MOTOROLA RF APPLICATIONS REPORT
NEW BIASING CIRCUIT
The new biasing circuit is shown in Figure 2.
Figure 2. New Biasing Circuit
V
SUPPLY
I
Q
V
G
V
D
R1
R2
V
BIAS
VOLTAGE
REGULATOR
V
ref
The bias bridge is adjusted to set the quiescent current as
specified at ambient temperature. Laser trimmable resistors
can be used in mass production for that purpose.
Thermal tracking is introduced to compensate the
quiescent current variations versus temperature. A diode in
the bias bridge compensates the LDMOS temperature
coefficient. We have now:
R2
R1
R2
V
G
=
V
ref
+V
D
(Equation 3)
Assuming no reference voltage variation versus bias
voltage and temperature (dV
ref
= 0), we have:
(Equation 4)
dVg = dVd
Thermal tracking:The thermal compensation is now
independent of R1 and R2 values and thus independent of the
manufacturing spread of transistor’s threshold voltage.
Bias voltage variations:As the bias voltage is now
regulated, we can see from equation 3 that the bias voltage
variations are no longer applied on the gate. This results in a
constant quiescent current at a given temperature.
Note that in this case the bias voltage can be directly
connected to the power supply voltage.
IMPLEMENTATION
This new biasing circuit is already implemented in three
Motorola RF LDMOS power modules designed for GSM base
station applications. The MHW910 (10 W GSM900),
MHW1810 (10 W GSM1800) and MHW1910 (10 W GSM1900)
modules take advantage of this solution.
This new biasing circuit allows keeping this optimum bias
point in every temperature and bias voltage combination. One
is now able to get the best performances, especially within the
whole temperature range.
Another advantage from a customer standpoint is that these
new power modules no longer require external bias regulation
circuitry for optimum performances.
The voltage regulator used in the MHW910 and MHW1810
is the LP2951. This IC provides the required performances of
a very low output voltage temperature coefficient and a very
good input voltage variations rejection.
Another feature of the LP2951 is the availability of the
shutdown input which allows a logic level signal to turn–off or
turn–on the regulator output.
This biasing circuit can be used in the same way for discrete
LDMOS transistors as well. Special attention must be taken
regarding the DC and RF decoupling.
This circuit guarantees best possible performance by
maintaining a constant quiescent current regardless of
temperature, bias voltage and gate threshold voltage
variations.
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the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and
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data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals”
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