Telecommunications – Transmitter – With feedback of modulated output signal
Utility Patent
1998-08-07
2001-01-02
Hunter, Daniel S. (Department: 2749)
Telecommunications
Transmitter
With feedback of modulated output signal
C455S127500, C330S297000
Utility Patent
active
06169885
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of radio communications and more particularly to power amplifier control circuits.
BACKGROUND OF THE INVENTION
In cellular radiotelephones according to the prior art, a power amplifier is used to amplify transmit signals to be transmitted from an antenna. As shown in
FIG. 1
, the power amplifier PA provides amplified transmit signals to the duplexor
23
for transmission from the antenna
21
. The duplexor separates the amplified transmit signals from receive signals which are received from the antenna
21
and provided to the receiver
25
. The transmission and reception of radiotelephone communications using the power amplifier, the duplexor, the receiver, and the antenna of
FIG. 1
will be understood by those having skill in the art.
More particularly, the power amplifier PA can be a depletion mode n-channel GaAs FET power amplifier (GaAs PA) which uses a positive battery voltage +V
BAT
and a negative bais voltage −V
BIAS
for operation wherein the negative bias voltage −V
BIAS
is less than the battery ground voltage. Furthermore, the power amplifier PA may be damaged or destroyed if the positive battery voltage +V
BAT
is applied to the power amplifier before the negative bias voltage is applied to the power amplifier. Accordingly, the power amplifier is isolated from the positive battery voltage by the series switch Q
1
which can be a p-MOSFET switch, wherein the source of the p-MOSFET switch is coupled to the positive battery voltage, the gate of the p-MOSFET switch is coupled to the node N
2
, and the drain of the p-MOSFET switch is coupled to the power amplifier.
As shown, the p-MOSFET switch Q
1
can be activated using the control circuit including resistors R
1
, R
2
, and R
3
, and the switch Q
2
. In particular, the system controller
27
generates a logical high control signal voltage on node N
1
coupled to the gate of the switch Q
2
when the system controller determines that the positive battery voltage should be applied to the power amplifier. The resistor R
3
pulls the node N
1
to ground when the system controller output is in a high impedance state such as during power up. The switch Q
2
acts as a level converter, converting logic signals (such as 0 V and 3.3 V low and high control signals) to battery control signals (0 V and +V
BAT
low and high control signals).
When the positive control signal voltage is applied to the node N
1
, the switch Q
2
couples the node N
2
to ground so that the gate of the p-MOSFET switch Q
1
is grounded through the resistor R
2
. Accordingly, the gate-to-source voltage V
GS
for the p-MOSFET switch Q
1
is set to approximately −V
BAT
causing the p-MOSFET switch Q
1
to turn on. This couples the positive battery voltage to the power amplifier (PA) which can be modeled as a 10 ohm load from the switch Q
1
drain to ground.
Alternately, the switch Q
2
is turned off when the output of the system controller is at either a logical low state or a high impedance state so that the p-MOSFET switch Q
1
gate is pulled to the positive battery voltage through resistors R
1
and R
2
. The gate-to-source voltage V
GS
is thus zero, causing the p-MOSFET switch Q
1
to be turned off, thereby isolating the power amplifier from the positive battery voltage +V
BAT
.
The system controller is generally implemented as an application specific integrated circuit (ASIC) which may include a microcontroller running system firmware, and the control signal on node N
1
is thus generated in accordance with the system firmware. In particular, the system firmware is designed to enable the p-MOSFET switch Q
1
before transmitting but after the negative bias voltage −V
BIAS
has been applied to the power amplifier. Furthermore, the negative bias voltage −V
BIAS
may also be switched under firmware control to provide power savings when the radiotelephone is not transmitting.
Non-destructive operation of the power amplifier thus relies on proper sequencing of the system firmware and proper operation of the system controller to provide that the negative bias voltage is applied to the power amplifier before the positive battery voltage is applied to the power amplifier. Improper sequencing of the system firmware (caused by so-called firmware bugs, for example), however, can result in power amplifier failures. Corruption of the system controller (implemented as an ASIC) caused by system transients could also cause power amplifier failures.
Furthermore, the performance of the control circuit of
FIG. 1
may be reduced as radiotelephones are powered by batteries having lower voltages. In particular, the gate of the p-MOSFET switch Q
1
is switched between 0 V (turn on) and +VBAT (turn off). With 0 V applied to the gate, the gate-to-source voltage V
GS
is equal to −V
BAT
, so that V
GS
during “turn on” is reduced with reduced battery voltages. Moreover, “on” resistances for the p-MOSFET switch Q
2
increase with reduced battery voltages. Typical drain-to-source “on” resistances R
DS(on)
for a p-MOSFET switch are illustrated in
FIG. 2
for different gate-to-source voltages V
GS
as a function of drain currents I
D
.
As shown in
FIG. 2
, the drain-to-source “on” resistance R
DS(on)
increases significantly as the magnitude of the gate-to-source voltage is reduced. For example, at a drain current I
D
of −1 A, R
DS(on)
increases from approximately a normalized 1 unit of resistance when V
GS
is −4.5 V to approximately a normalized 1.75 units of resistance when V
GS
is −3.5 V. A normalized resistance of 1 unit may be 120 Mohn, for example, in which case a normalized resistance of 1.75 units is equal to 210 mohm. The switch may thus operate with a higher “on” resistance when used in radiotelephones powered by lower voltage batteries, and the “on” resistance will increase further as the battery discharges, so that radiotelephone performance is further decreased.
The increased switch “on” resistance compounds the difficulty of providing sufficient drain current through the power amplifier to maintain a desired radiofrequency (RF) output power. The increased switch “on” resistance also increases power loss between the battery and the PA drain, thereby reducing battery life and increasing heat generation. Furthermore, the use of lower voltage batteries generally requires higher drain currents I
D
to maintain a sufficient RF output power from the power amplifier, while an increased on resistance tends to reduce the PA drain voltage thus requiring higher drain current to maintain PA power output. For example, a 3 V battery may be insufficient to adequately enable the switch Q
1
of
FIG. 1
to obtain sufficient RF output power from the power amplifier. Accordingly, it may be difficult to maintain adequate power amplifier performance with lower voltage batteries. While MOSFET switches with lower “on” resistances may be available, these lower on resistance switches may increase the cost of the radiotelphone.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide radio devices having improved performance.
It is another object of the present invention to provide improved control circuits for radio power amplifiers.
It is still another object of the present invention to provide power amplifier control circuits which can improve power amplifier reliability.
These and other objects are provided according to the present invention by a radio including a power amplifier which is isolated from the positive battery voltage by a switch wherein the switch is activated in response to a transmit activation signal when a negative bias voltage is applied to the power amplifier. A lockout circuit, however, prevents activation of the switch when the negative bias voltage is not coupled to the power amplifier. By preventing activation of the switch when the negative bias voltage is not coupled to the power amplifier, damage to the power amplifier can be reduced. In addition, the switch can be activated by c
Walukas Joel James
Yoder Scott
Ericsson Inc
Hunter Daniel S.
Myers Bigel & Sibley & Sajovec
Tran Pablo
LandOfFree
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