Two loop automatic level control for power amplifier

The power supply potential applied to the final stage of an RF amplifier is odulated by means of an adaptive power supply. The power supply potential applied to the final RF amplifier is controlled by a first control loop in accordance with an error signal derived from a comparison of a signal corresponding to the weighted sum of the magnitude of the supply voltage applied and the current drawn by the final amplifier and the amplitude of the modulating signal. Additionally, an automatic level control circuit controls the level of the RF output signal in a second control loop in accordance with a comparison between the magnitude of the RF output of the amplifier and the amplitude of the modulating signal.

BACKGROUND OF THE INVENTION 
This invention relates to power amplifiers for electrical signals and more 
particularly to a signal modulated power amplifier having high efficiency 
of operation over a wide range of operating conditions. 
RF amplifiers whose DC potential voltages are continuously varied to match 
signal level requirements for maintaining high efficiency operation 
throughout a predetermined range of input signal levels are generally 
known. One such example is disclosed in U.S. Pat. No. 3,413,570 entitled, 
"High Efficiency RF Power Amplification With Modulation Signal Controlled 
On-Off Switch Varied Amplifier DC Potentials", issued to W. B. Bruene, et 
al. on Nov. 26, 1968. There both non-linear and linear amplifiers are 
operated at a relatively high efficiency by modulation of the RF amplifier 
DC supply potentials which are caused to vary continuously in accordance 
with the amplitude of the modulating input signal. 
Another such example is disclosed in U.S. Pat. No. 4,320,350 entitled, 
"Sliding Power Supply For RF Power Amplifier", by Michael J. Drapac. In 
that invention the power supplied by a sliding power supply to a power 
amplifier is indirectly controlled by a single feedback loop which 
operates in accordance with the load impedance voltage standing wave ratio 
of the amplifier. 
Accordingly, it is an object of the present invention to provide improved 
control of an RF amplifier for maintaining power amplifier efficiency at a 
relatively high level. 
It is another object of the present invention to provide an improvement in 
power supply and signal modulation of an RF amplifier for providing 
improvement in efficiency over a wide range of load, line voltage and 
envelope power. 
Still a further object of the present invention is to provide a two loop 
feedback control of an RF amplifier for providing an improvement in 
amplifier efficiency. 
SUMMARY OF THE INVENTION 
The foregoing as well as other objects are achieved by a two loop 
combination of an adaptive power supply feedback control loop for an RF 
power amplifier where the power supply control is effected through an 
error signal obtained from a comparison of a signal corresponding to the 
weighted sum of the magnitudes of the amplifier's supply voltage and 
current driven by the amplifier and the amplitude of the modulating 
signal, and an automatic level feedback control loop which amplitude 
modulates the power amplifier in accordance with a comparison between the 
amplitude of the modulating signal and the level of the RF output signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like reference numerals refer to like 
components, attention is first directed to FIG. 1. There reference numeral 
10 designates an RF signal source which is to be modulated. The output 
from the RF signal source 10, which may be either of the pulse or CW type, 
is coupled to a driver amplifier 12 where it is amplitude modulated by 
means of an automatic level control circuit 14. The amplitude modulated RF 
signal is coupled to a final RF amplifier stage 16 where it is coupled as 
an RF output to suitable antenna means, not shown, via a directional 
coupler 18. 
The automatic level control circuit 14 is incorporated in a feedback 
control loop which is responsive to a feedback signal derived from the RF 
output signal level and the modulating signal f(t). In FIG. 1 the 
modulating signal f(t) is shown applied to terminal 20 while the RF signal 
level is detected by an RF signal level sensor 22 coupled to the 
directional coupler 18. The modulating signal is applied to a summing 
junction 24 where there is also applied a DC bias potential +V coupled to 
terminal 26. The modulation signal with the DC bias applied is coupled as 
a composite reference signal V.sub.ref to the automatic level control 
circuit 14 along with the output of the RF level sensor 22. The control 
circuit 14 operates to generate an error signal in a well known manner, 
such as by differencing the two inputs to the automatic level control 
circuit 14, which then operates to vary the gain, for example, of the 
driver amplifier 12 to amplitude modulate the RF signal applied from the 
source 10. 
The subject invention more importantly includes another feedback control 
loop involving the modulation signal f(t) applied to terminal 20 and an 
adaptive DC power supply 26 which outputs a variable supply potential 
V.sub.a to the final stage 16 of the RF amplifier in accordance with an 
error signal derived from the comparison between a signal corresponding to 
the weighted sum of the magnitude of the final amplifier load current 
I.sub.O and supply voltage V.sub.a and the amplitude of the modulating 
signal f(t). The error signal for controlling the adaptive power supply 26 
is generated by means of a comparator circuit 30 which is coupled to a 
current sensor 32 and a signal corresponding to the magnitude of the 
modulating signal. As shown in FIG. 1, the modulating signal is applied to 
the comparator 30 by means of an offset circuit 34 which is adapted to 
apply a DC bias to the modulating signal by a predetermined amount in the 
same fashion that the summing junction 24 applies a DC bias to the 
modulating signal for developing the reference voltage V.sub.ref for the 
automatic level control circuit 14. 
In operation, the control loop involving the automatic level control 
circuit 14 and the control loop involving the adaptive power supply 28 
maintains amplifier efficiency at a high level over a wide range of 
operating conditions such as load, primary line voltage and envelope 
power. This is achieved primarily by the adaptive power supply control 
loop of FIG. 1, which causes the load line of the power supply 28 to 
follow the modulating signal f(t) as shown by the current vs. voltage 
diagram of FIG. 2. There reference numeral 36 denotes the power supply 
load line which has zero voltage and current intercepts of 2V.sub.a and 
2I.sub.0, respectively. With no modulating signal applied, an operating 
point 38, for example, for an output voltage V.sub.a causes a current 
I.sub.0 to be drawn by the final RF amplifier 16. The effect of the 
modulating signal f(t), however, causes the error control signal output 
from the comparator 30 to shift the load line 36 laterally in either 
direction from the position shown in FIG. 2. It is this lateral shift 
which comprises the major improvement over prior art power amplifiers 
which include variable power supplies for providing an enhancement for 
improvement in the efficiency of the amplifier. The combined effect of the 
two control loops, however, form an even greater enhancement of control of 
the overall amplifier efficiency. 
While the embodiment shown in FIG. 1 is adapted for an amplitude modulated 
power amplifier, it is within the scope of one skilled in the art, when 
desirable, to also utilize FM modulation techniques. Also while a current 
sensor 32 is shown, the adaptive power supply control loop could be 
implemented utilizing a voltage sensor. Additionally, the adaptive power 
supply 28 may be configured of a switching type supply shown and 
described, for example, by the aforementioned Bruene, et al. patent. 
Referring now to FIG. 3, shown therein are the specific circuit details of 
the adaptive power supply control loop of FIG. 1. As shown in FIG. 3, 
circuitry for the current sensor 32, the comparator 30 and the offset 
circuit 34 are disclosed together with the addition of a pair of gain 
setting summing amplifiers 40 and 42 consisting of operational amplifiers 
44 and 46. With respect to the current sensor 32, it is comprised of a 
series resistor 48 connected between the adaptive power supply circuit 28 
and the final RF amplifier stage 16 and across which are coupled two 
resistive voltage dividers consisting of resistors 50, 52 and 54, 56. The 
common connection or intermediate circuit node 58 between resistors 50 and 
52 is connected to the plus (+) input of the operational amplifier 40 
while the common connection 60 between resistors 54 and 56 is connected to 
the negative (-) input of the amplifier 40. A gain setting feedback 
resistor 45 is also connected from the output to the negative input. 
Accordingly, the amplitude of the current I.sub.0 is measured in terms of 
the voltage drop across the resistor 48. The comparator circuit 30 is 
shown in FIG. 3 comprised of a differencing operational amplifier 62 
having its negative (-) input connected to the output of the gain setting 
summing amplifier 40 which along with the circuitry 32 develops an output 
signal V.sub.1 proportional to the weighted sum of the supply voltage 
V.sub.a and the load current I.sub.0. The plus (+) input to the 
differencing amplifier 62 is connected to the output of the gain setting 
amplifier 42 which includes a gain setting feedback resistor 47 and a 
resistor 49 referencing the negative (-) input to ground. The output of 
operational amplifier 46 comprises an amplified voltage V.sub.2 
proportional to the modulating signal f(t) which is applied to terminal 20 
and connected to the offset circuit 34 by means of a coupling capacitor 
66. A DC bias or offset is provided by a resistive voltage divider 
consisting of a pair of resistors 68 and 70 connected in series between a 
DC voltage terminal 70 to which is applied a power supply potential +V and 
ground. The offset voltage applied to the modulating signal is the 
magnitude of the DC voltage appearing at circuit junction or intermediate 
circuit node 72. Accordingly, the modulating signal f(t) offset by the DC 
voltage appearing at circuit junction 72 is applied to the plus (+) input 
of the gain setting amplifier 42. 
Thus what has been shown and described is the combination of two control 
loops which provide an improved power level of control having a relatively 
high efficiency. 
While the present invention has been shown with a certain degree of 
particularity, it should be noted that the foregoing detailed description 
has been made by way of illustration and not of limitation and 
accordingly, all alterations, changes and modifications coming within the 
spirit and scope of the invention as defined in the appended claims are 
herein meant to be included.