Microwave power amplifiers

A microwave power amplifier having application in multiple beam phased antenna array systems including a biasing means connection to the base of a microwave transistor being responsive to radio frequency signals applied to the amplifier to automatically adjust the transistor bias level to maintain constant amplifier gain. The biasing means includes two voltage regulators with their outputs capacitively coupled via a fixed resistor having a value determined by the characteristics of the transistor, the base of the transistor being connected to the biasing means at the junction of the resistor and the capacitive coupling of one of the regulators. The mode of operation of the power amplifier gives rise to a highly efficient linear system while under effective Class B (non-linear) bias.

BACKGROUND OF THE INVENTION 
The invention relates to microwave power amplifiers and, in particular, 
L-Band microwave transistor power amplifiers with a dynamically efficient 
biasing arrangement and having application in multiple beam phased antenna 
arrays. 
In an active multiple beam phased antenna array system each antenna element 
(or elemental sub-array) is driven by a dedicated power amplifier. A 
requirement of such a system is that the power can be flexibly reallocated 
between different beams and that the power amplifiers have to be capable 
of operation over a wide dynamic range whilst simultaneously maintaining a 
very high degree of gain and phase tracking. 
In order to maintain reasonable high levels of efficiency in phase array 
systems it is necessary to operate amplifiers in their non-linear regions. 
Additionally, to provide for a wide signal dynamic (20 dB minimum) the 
most efficient bias modes (Class B or C) are not normally contemplated, 
instead the designer resorts to "overrun" Class A amplifiers (Class A/B). 
It is an object of the present invention to provide a power amplifier with 
dynamically efficient biasing and capable of linear operation whilst under 
effective Class B (non-linear) bias. 
SUMMARY OF THE INVENTION 
The invention provides a microwave power amplifier including a microwave 
transistor and transistor biasing means connected to the base of the 
transistor and responsive to radio frequency signals applied to the 
amplifier to automatically adjust the transistor bias level to maintain 
constant amplifier gain, the biasing means including first and second 
capacitively coupled voltage regulators, the base of the transistor being 
connected to the first regulator via a fixed resistor and a first section 
of the capacitive coupling and to the second regulator via voltage 
switching means and a second section of the capactive coupling.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The principle of operation of the microwave power amplifier according to 
the present invention is that the biasing of the amplifier's microwave 
transistor is automatically adjusted by the incident radio frequency (rf) 
power level in order to keep the gain of the amplifier constant. This mode 
of operation, i.e., a dynamically variable transistor biasing arrangement, 
gives rise to a highly efficient and linear system. 
The bias levels in terms of the transistor base/emitter voltage, V.sub.be, 
and the transistor collector current, I.sub.c, necessary to produce a 
constant gain function over a dynamic range of approximately 20dB are 
illustrated in FIG. 1 of the drawings. The collector current, I.sub.c, is 
a function of both the base emitter forward bias and the rf input drive 
level. 
As shown in FIG. 1, the slope of the V.sub.be versus I.sub.c 
characteristics is linear at lower drive levels followed by an inflection 
which is approximated by a plateau region shown dotted. 
The necessary V.sub.be versus I.sub.c characteristics for constant gain 
illustrated in FIG. 1 can be readily transposed to a base current, 
I.sub.b, instead of collector current, I.sub.c, relationship. In so doing, 
it is clearly evident that the linear sloping region corresponds to a 
V.sub.be source of finite output impedance, determined by that slope, and 
the idealised flat region to zero impedance. 
The simplest circuit topology for a power amplifier that is capable of 
realising such a function in a controllable manner is illustrated in FIG. 
2 of the drawings. 
As shown in FIG. 2, the topology consists of two identical regulators 1 and 
2 with their outputs connected via capacitor decoupling means 3, a 
resistor 4 and capacitor decoupling means 5; the resistor 4 being of a 
value determined from the V.sub.be versus I.sub.c characteristics 
illustrated in FIG. 1. The base of the microwave transistor, as depicted 
in FIG. 2 by the base/emitter voltage, V.sub.be, is connected to the 
regulator 2 via the capacitor decoupling means 5 and via the resistor 4 
and capacitor decoupling means 3 to the regulator 1. 
The quiescent point of the power amplifier (V.sub.b ; I.sub.c) is set by 
the regulator 1 and the regulator 2 is set to a voltage V'.sub.b, 
corresponding to the plateau "knee" illustrated in FIG. 1. 
In operation of the power amplifier, as the rf drive is increased more base 
current is drawn and hence the voltage V.sub.b of regulator 1 drops in 
linear proportion until it reaches the preset level of regulator 2. Since 
both circuits can only source and not sink current the regulator 2 drops 
in while the regulator 1 drops out, the transition between these two 
states being quite smooth as is the desired function. 
The rf circuit and the associated major biasing components for a power 
amplifier according to the present invention is illustrated in FIG. 3 of 
the drawings. 
As shown in FIG. 3, the emitter of a microwave transistor MWT, for example, 
a bipolar transistor, is connected to earth, the base of the transistor 
MWT is connected to an r.f. input terminal 6 via an input matching 
transformer T1 and a capacitor C1 and the collector of the transistor MWT 
is connected to an r.f. output terminal 7 via an output matching 
transformer T2 and a capacitor C2 and to a collector voltage source, 
V.sub.c, via a 1/4 wavelength (.lambda./.sub.4) bias line L1 and a 
parallel bank of capacitors, C3 to C6. 
The voltage regulators 1 and 2 and the resistor 4 of FIG. 2 are given the 
same reference numbers in FIG. 3 and the capacitor decoupling means 3 and 
5 of FIG. 2 are respectively shown in FIG. 3 as parallel banks of 
capacitors, C7 to C9 and C'7 to C'9. 
The output of the voltage regulator 2 is connected via the parallel bank of 
capacitors C'7 to C'9, a solid state diode switch D1 and the bias line L2 
to the base of the transistor MWT. A capacitor C10 is connected in 
parallel with the capacitor banks C7 to C9 and C'7 to C'9 between one side 
of each of the regulators 1 and 2 and the junction of the resistor 4, line 
L2 and diode D1. 
As shown in FIG. 3 bias is applied to the base of the transistor MWT via 
the line L2 which is r.f. short circuited by the capacitor C10 thereby 
forming a high impedance at the r.f. centre frequency (1.54 GHz in this 
case). At intermediate frequencies (beat frequencies in the case of a 
multi-tone input signal) the bias line is effectively low impedance such 
that the capacitor banks, C7 to C9 and C'7 to C'9, and the resistor 4 are 
in the current path from the bias source to the base of the transistor 
MWT. The linearising action of the resistor 4 is therefore effective at 
these frequencies. 
Selection of the values for the capacitors C7 to C9 is of paramount 
importance to ensure adequate decoupling and hence maintain a flat 
"modulation bandwidth" within the amplifier. 
At high r.f. input signal levels, when the bias source V'.sub.b comes into 
operation, the decoupling provided by the capacitor bank, C7 to C9, is 
insufficient due to the blocking resistor 4 hence the inclusion of the 
capacitor bank C'7 to C'9 in direct line with the voltage (V'.sub.b) 
regulator 2. When regulator 2 is not operational at low drive levels, 
diode switch D1 acts to prevent the capacitor bank C'7 to C'9 from 
nullifying the linearising effect of the resistor 4. If this is not 
effected, then the resistor 4 would only be in circuit at very low 
intermediate frequencies. 
The switching action of the diode D1 is controlled by including it in the 
feed-back sense loop 8 of the regulator 2. 
The actual number and values of the capacitors in each of the parallel 
banks which form the capacitor decoupling means 3 and 5 is not material 
save for the fact that the capacitor filter network is required to 
maintain adequate performance for the application and that this has to be 
duplicated via an r.f. switch in order for the circuit to function 
correctly at all power levels. 
The value of the resistor 4 is matched to the transistor MWT and is 
directly proportional to the devices hfe. Its value is typically in the 
range 5 to 40 ohms. 
The advantage of the microwave power amplifier according to the present 
invention is that it uses two simple fixed regulators, capacitor 
decoupling means and a fixed resistor to produce linear operation while 
under effective Class B (non-linear) bias. Furthermore, the simple 
clamping action of one of the regulators negates the need for a variable 
resistor and ensures that there is no deterioration in performance at high 
r.f. drive levels. 
The main application of the microwave power amplifier according to the 
present invention is in multiple beam phased antenna array systems wherein 
each antenna element (or elemental sub-array) is driven by a dedicated 
power amplifier and, in particular, L-Band microwave transistor power 
amplifiers.