Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2003-04-28
2004-10-26
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S296000
Reexamination Certificate
active
06809593
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to power amplifiers and, more specifically, to a device and method for controlling the bias of a power amplifier.
BACKGROUND OF THE INVENTION
Radio Frequency (RF) power amplifiers are used as components in many communication devices, including many wireless communication devices, such as base stations and mobile devices such as cell phones. Hetero-junction bipolar transistor (HBT) power amplifiers are a specific type of power amplifier used 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 V
REF
130
. This configuration requires that a separate voltage V
REF
130
(different from the battery voltage V
BAT
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, R
REF
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, V
REF
230
, would need to be greater than is desirable for mobile communication applications and the solution is therefore not viable.
In certain applications, RF power amplifiers are placed within feedback control loops to provide for power control. A measurement of the output RF power delivered by the RF power amplifier vs. the input voltage will often indicate a steep slope condition where the RF power amplifier output changes very rapidly with respect to changes in the input voltage. When an RF power amplifier presents the steep slope condition instability in the power control loop and other undesirable overall RF power amplifier breakdown conditions may result. Thus, it would be desirable to provide an RF power amplifier device that addresses the steep slope condition while maintaining high performance operation.
Accordingly, there is a need for an improved RF power amplifier device and method of operation.
REFERENCES:
patent: 5497125 (1996-03-01), Royds
patent: 6396347 (2002-05-01), Lie et al.
patent: 6639465 (2003-10-01), Samelis et al.
Gilsdorf Benjamin R.
Newman David A.
Freescale Semiconductor Inc.
Toler Larson & Abel, LLP
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