Amplifiers – With semiconductor amplifying device – Including particular biasing arrangement
Reexamination Certificate
2002-06-28
2004-06-15
Nguyen, Patricia (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular biasing arrangement
C330S307000, C330S285000
Reexamination Certificate
active
06750722
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to power amplifiers and, more specifically, to a circuit and method for controlling the bias of a hetero-junction bipolar transistor (HBT) power amplifier.
BACKGROUND OF THE INVENTION
Hetero-junction bipolar transistor (HBT) power amplifiers are becoming the standard for cellular applications due to their high power density and reduction in die size. Unfortunately, biasing these transistors with a constant current poses some difficulty. The voltage supply limitation typical to mobile applications combined with a relatively high Vbe of HBT devices make traditional integrated methods unusable.
FIG. 1
shows a typical diode based biasing control of an HBT transistor. A power amplifier Qpa HBT
100
is biased by a diode configured transistor
110
where the base and collector are shorted together and receive a current through a resistor
120
and supply voltage Vref
130
. This configuration requires that a separate voltage Vref
130
(different from the battery voltage Vbat
140
supplied to the collector of the power amplifier
110
) be applied to the diode transistor and the biased base of the power amplifier in order to tightly control the biasing current. This configuration leads to several problems for power amplifier applications in mobile communications. Typically, the power amplifier
100
is N times larger than the diode transistor
110
leading to current stealing. Additionally, Rref
120
needs to be large to provide stability over variations in temperature and process, but needs to be small to provide enough current to properly bias the power amplifier, resulting in a circuit that would require a stable reference which supplies a prohibitively large amount of current and is not a viable circuit for power amplifiers in mobile communications applications.
Another solution, shown in
FIG. 2
, solves the problem of current stealing by using a current mirror with an emitter follower to bias the current supplied to the power amplifier's base. The base of a power amplifier transistor
200
is connected to a base of mirrored transistor
210
and the emitter of a emitter follower transistor
250
. The collector of the mirrored transistor
210
is connected to the base of the emitter follower transistor
250
and is connected to a reference voltage
230
through a reference resistor
220
while the collector of the emitter follower transistor
250
is connected to the battery voltage
240
which is also connected to the collector of the power amplifier transistor
200
through some impedance
270
. However, this type of circuit is not viable because gallium arsenide (GAS) HBT power amplifiers as now used have Vbe's in the order of 1.4 volts while battery voltage supplies are required to be in the range of 2.7 volts. To control the voltage at the base of the power amplifier, the voltage supply, Vref
230
, would need to be greater than is desirable for mobile communication applications and the solution is therefore not viable.
What is needed is a method of controlling the bias of an HBT power amplifier transistor used in RF applications that provides stability over temperature and process.
REFERENCES:
patent: 5216381 (1993-06-01), Botti et al.
patent: 6043714 (2000-03-01), Yamamoto et al.
patent: 6429746 (2002-08-01), Koukkari
patent: 6617928 (2003-09-01), Finlay et al.
Gilsdorf Benjamin R.
Newman David A.
Freescale Semiconductor Inc.
Nguyen Patricia
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