Amplifiers – With semiconductor amplifying device – Including protection means
Reexamination Certificate
1999-11-19
2001-08-21
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including protection means
C330S310000, C330S20700P
Reexamination Certificate
active
06278328
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a protection circuit for preventing transistor breakdown during overvoltage output operation in a power amplifier having a GaAs heterojunction bipolar transistor (HBT) a Si bipolar transistor.
2. Description of Related Art
Monolithic microwave integrated circuits (MMIC) and modules (hybrid ICs and multichip modules) using GaAs metal-semiconductor field effect transistors (MESFET), GaAs high electron mobility transistors (HEMT), or GaAs-based HBTs are widely used in power amplifiers for mobile communications devices. Of these, GaAs-based HBTs offer the following three advantages over conventional FETs, and are therefore considered promising as power module elements for future mobile communication devices. That is, a GaAs-based HBT:
1) does not require a negative gate bias voltage, and can therefore be used for simple power supply operations;
2) can be used for on/off operations without an analog switch on the drain side, similarly to a Si-MOSFET;
3) has a high output power density, enabling a particular rated output power to be achieved from a device that is smaller than a FET power amplifier with comparable output power.
HBT power amplifiers using these particular properties of HBTs are starting to be used in 2 W to 4 W high output mobile telephones conforming to Europe's GSM (Global System for Mobile Communications) standard, a 900 MHz system that is the most widely used mobile telephone service in Europe.
FIG. 10
is a typical circuit diagram for an HBT power amplifier
500
used in a GSM mobile telephone. Referring to
FIG. 10
, input terminal
501
is the input terminal for RF signals to be power amplified. Output terminal
502
is the output terminal for amplified signals. Transistors
503
to
505
are heterojunction bipolar transistors for signal amplifying. Transistors
506
to
511
are bias HBTs. Terminals
512
to
514
are terminals for applying collector bias voltages Vcl to Vc
3
. Terminal
515
is the supply terminal for the power supply voltage Vcc. Terminal
516
is the supply terminal for power control voltage Vpc for controlling the base voltage of amplifying transistors
503
to
505
using bias transistors
506
to
511
. As shown in the figure, this HBT power amplifier
500
also comprises resistors
517
to
542
, capacitors
543
to
552
, and microwave lines
553
to
557
.
A typical HBT power amplifier has a high low-frequency gain and is more susceptible to low frequency oscillation than an FET power amplifier. To prevent this low frequency oscillation, HBT power amplifier
500
has an RC feedback circuit comprising resistor
521
and capacitor
544
disposed between the collector and base of transistor
503
; an RC feedback circuit comprising resistor
525
and capacitor
545
disposed between the collector and base of transistor
504
; and an RC feedback circuit comprising resistor
529
and capacitor
548
disposed between the collector and base of transistor
505
.
A current of approximately 2 A flows to circuits in a mobile telephone based on the GSM standard. A means such as a regulator or cut-off circuit that operates when an overvoltage is applied could be provided between the power terminal and battery as a means of protecting transistors in the HBT power amplifier
500
. However, such cut-off circuits and regulators are typically large-scale, high power consumption devices. It is therefore not possible to dispose a cut-off circuit or regulator between the battery and supply terminal of the HBT power amplifier
500
, and the supply terminal is directly connected to an internal battery.
It is also desirable to provide an isolator between the output terminal of the HBT power amplifier
500
and a downstream circuit (such as an antenna). This isolator is used for suppressing variations in the load curve of HBT power amplifier
500
when the load impedance of a downstream circuit varies. This isolator, however, is typically comparable to the chip size of the HBT power amplifier
500
, and is therefore omitted in GSM standard mobile telephones because of the high demand for smaller mobile telephones.
If the supply voltage rises to a particular level above the recommended operating conditions (3 V to 3.6 V) during battery recharging, for example, rises to 4.5 V to 5.5 V, and the load impedance of downstream circuits varies greatly from the normal 50 &OHgr; level, the load characteristic curve of the last transistor
505
in a HBT power amplifier
500
for GSM devices described as noted above may fluctuate excessively, causing the peak collector voltage Vce to exceed the breakdown voltage and transistor
505
to fail.
The last transistor
505
can fail as a result of load fluctuations during battery charging as described below, but failure can also result from an overvoltage being applied to the collector of the last transistor
505
.
It should also be noted that a feedback circuit
600
having a diode
604
as shown in
FIG. 11
has also been used to resolve the above-noted problems relating to an overvoltage supply in applications other than mobile telephones and other mobile devices, particularly in the field of optical communications. The feedback circuit
600
shown in
FIG. 11
is used primarily in the signal amplifier
601
disposed in the front end of an optical communications receiver. When an overvoltage supply is input the diode
604
becomes conductive, thereby reducing the power applied to the signal input terminal
602
of the signal amplifier
601
to prevent signal amplifier
601
failure, wave distortion, and overload output.
High output power amplifiers producing 1 W or more are used in GSM standard mobile telephones. Providing a feedback circuit
600
as described above to the input/output terminals of such a high output power amplifier, however, cannot effectively protect the internal transistors of the power amplifier, and in particular cannot protect the last transistor
505
. This is because failure and thermal damage to transistors inside the amplifier is due not primarily to an overvoltage supply being input to the amplifier, but is due to the peak collector voltage Vce exceeding the breakdown voltage due to load fluctuations during overloaded operation of transistors in the amplifier.
The load fluctuation characteristic during overload operation of the transistors when a tuner
560
produces a variable load impedance at the output terminal
502
of the HBT power amplifier
500
is considered next below.
FIG. 12
shows the second and third stage circuits of a HBT power amplifier
500
to which this tuner
560
is connected.
FIG. 13
shows the load curve and Ic-Vce curve of the last transistor
505
when tuner
560
is connected. Point Al on the graph indicates the normal collector bias voltage V
c3-1
(typically 3.2 V) applied to terminal
514
when the base current is I
b2
. Curve cl is the load curve when the load impedance of the tuner
560
is normal (that is, 50 &OHgr;) and the voltage standing wave ratio (VSWR) is 1:1. Curve c
2
is the load curve when there is a mismatch, that is, the load impedance deviates from the normal impedance (50 &OHgr; in this case) and the VSWR is 8:1 to 10:1. Such a mismatch occurs when, for example, a mobile telephone comprising this HBT power amplifier
500
passes a highly conductive object, such as a steel utility pole.
As will be known from comparing curves c
1
and c
2
, if the standing wave ratio of the output terminal is increased and the output mismatch is increased, load curve fluctuation will increase and the peak collector voltage Vce will approach the transistor breakdown voltage area
1
indicated by a circle line in the figure.
FIG. 14
shows the load curve and Ic-Vce curve of the last transistor
505
when the collector bias voltage applied to terminal
514
is a voltage V
c3-2
(5.0 V in this example) higher than V
c3-1
(3.2 V as noted above). Point A
2
on the graph indicates the collector bias voltage V
c3-2
applied to terminal
514
when the base current is I
b2
. As is curve
Asada Tomoyuki
Inoue Akira
Suzuki Satoshi
Yamamoto Kazuya
Choe Henry
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Pascal Robert
LandOfFree
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