Amplifiers – With semiconductor amplifying device – Including plural amplifier channels
Reexamination Certificate
2002-04-30
2004-06-15
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including plural amplifier channels
C330S285000
Reexamination Certificate
active
06750721
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to power amplifiers and, more specifically, to a circuit and method for adjusting the voltage at the commonly connected ballasted ends of a plurality of base ballasted HBTs forming a power amplifier when the voltage appearing at one of the bases of the plurality of HBTs drops below a threshold.
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. HBT's can draw substantial base current during high power operation. Since multi-finger power devices suffer from thermally related current collapse they cannot be operated without ballast resistors on either the emitter or base. Emitter ballasting is not practical for power amplifiers because of the high emitter currents and small resistor values, so base ballasting must be used. Because an individual HBT is a very small device, it is required to be paralleled with multiple HBTs to achieve high power operation required for most power amplifier applications.
Accordingly, HBT power amplifiers such as those used in radio frequency (RF) applications employ multiple small devices connected in parallel. As noted above, each of these small HBT devices require a ballast resistor to be connected to its base before being connected to the other HBTs forming the power amplifier (PA). The resistive ballasting of individual cells keeps parallel HBT fingers from entering thermal collapse. Additionally, a capacitor may be used to bypass the base resistor to preserve high frequency gain or the RF signal may be fed to the base connections through a separate capacitor. For purposes of illustration, the figures contained herein will illustrate the principal using capacitor bypassed ballast resistors although those skilled in the art will realize that this embodiment of the invention will work the same regardless of the connection of the RF capacitors feeding the base connection.
FIG. 1
shows a typical multi-fingered base ballasted Power Amplifier (PA) circuit. A plurality of HBTs
110
, each ballasted with a resistor
130
/capacitor
120
are connected in parallel. For each small HBT device, first ends
130
a
,
120
a
of a resistor
130
and a capacitor
120
are connected to the base
185
of the individual HBT devices and the other ends
130
b
,
120
b
of the resistor and the capacitor become the input
180
of each base ballasted HBT device
190
. For purposes of simplicity, when two or more base ballasted HBT devices
190
are “connected in parallel”, their collectors
160
share a first common node, their emitters
170
share a second common node connected to ground, and the inputs
180
share a third common node. A radio frequency signal is received at the input
140
and connected to the commonly connected inputs
180
of the base ballasted HBT devices
190
. The commonly connected collectors
160
that are connected to a voltage source
155
produce an amplified RF output
150
.
Due to the base current requirements, a biasing circuit
195
is usually included. Typical biasing circuits with RF decoupling components neglected for simplicity are shown in
FIGS. 2 and 3
.
FIG. 2
shows a diode biasing circuit
200
. The base of an HBT device
210
is connected to the collector and the collective inputs
180
of the of the base ballasted HBT devices
190
of FIG.
1
. The emitter is connected to ground. The first end
220
b
of a reference resistor
220
is connected to the collector and base while the second end
220
a
of the reference resistor
220
is connected to a reference voltage
230
.
FIG. 3
shows a preferred current mirror biasing circuit
300
. A current mirror is formed by HBT devices
310
and
320
. The collector of the first HBT device
310
is connected to a voltage source
350
, its emitter is connected to the base of the second HBT device
320
and its base is connected to the collector of the second HBT device
320
. The emitter of the second device is connected to ground. And finally, the first end
330
b
of a reference resistor
330
is connected to the base of the first HBT device
310
and the collector of the second HBT device
320
while the second end
330
a
of the reference resistor is connected to a reference voltage
340
.
Although not exhaustive, these biasing circuits are typical of those employed in the industry although other types of biasing circuits are contemplated and may be used with the invention. The biasing circuits try to keep the current through the power device constant with variations in temperature and reference voltages. Although either of these biasing circuits or others could be used, current mirror biasing is typically preferred and will be used in the discussion.
Using the current mirror of
FIG. 3
as the Bias of
FIG. 1
, the reference voltage
340
and the reference resistor
330
form a constant current source which is mirrored by the first HBT device
310
and the second HBT device
320
. If no ballast resistors
130
were required, the current mirror would be adequate up to the limits of the HBT devices
310
and
320
. However, with ballast resistors and during high power operation, the current mirror is unable to keep the voltage on the bases
185
of the individual HBT power device cells
110
constant because of the drop on the ballast resistor. In power operation when more HBTs are connected in parallel, increased base current is required from the current mirror. This strain on the current mirror results in increased voltage drops across the ballasting resistors
130
resulting in the voltage at the base of the individual HBT devices
110
to droop, limiting linearity and maximum output power.
What is required is an improved HBT power amplifier circuit that doesn't effect the quiescent point at lower output powers, but comes into play when higher powers are being generated that effectively prevents this drooping from occurring.
REFERENCES:
patent: 5670912 (1997-09-01), Zocher
patent: 6333677 (2001-12-01), Dening
patent: 6448859 (2002-09-01), Morizuka
patent: 6515546 (2003-02-01), Liwinski
patent: 6529079 (2003-03-01), Apel et al.
Freescale Semiconductor Inc.
Nguyen Khanh Van
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