Amplifiers – With semiconductor amplifying device – Including distributed parameter-type coupling
Reexamination Certificate
2000-10-04
2002-01-29
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including distributed parameter-type coupling
C330S054000, C330S295000
Reexamination Certificate
active
06342815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a wideband power amplifier and, more particularly, to a capacitively coupled, wideband MMIC power amplifier that employs an active feedback regulator to provide reduced DC and RF process variation dependence to make the amplifier more easily and readily manufacturable.
2. Discussion of the Related Art
Wideband MMIC power amplifiers are widely employed in the telecommunications industry. An MMIC amplifier which can deliver high power and produce large voltage swings over a wide frequency bandwidth is important for many telecommunication applications, such as high data rate fiber transmission systems, microwave frequency converter applications, etc. For example, there exists a need in the art for a low-cost, wide bandwidth MMIC amplifier that can deliver medium to high power (>12 dBm) at an output voltage greater than 2 volts peak-to-peak over several decades of frequency bandwidth (20 kHz-20 GHz). However, making a semiconductor amplifier that is both wideband and high power is difficult because of certain manufacturing limitations, particularly the limitations of DC and RF process variation dependence.
Various circuit designs have been proposed in the art to make wide bandwidth, high power semiconductor amplifiers. For example, a capacitively coupled distributed amplifier has been proposed that is one of the best known designs for achieving this goal. In one particular known MESFET MMIC design, a series capacitance is employed in connection with the gate of each FET to reduce the effective shunt capacitance used to synthesize the input transmission line of the distributed amplifier to enable a greater distributed amplifier bandwidth without scaling down the size of the power FETs used. It has been shown that this approach could achieve 1 W of output power over a 2-8 GHz bandwidth. This circuit design has been extended to a capacitively coupled HBT distributed amplifier to improve the power added efficiency (PAE) and linearity of the wideband power amplifier. In this modified design, 0.5 W was obtained over a 2-8 GHz bandwidth. In both of the designs, the lower frequency band edge is limited to 2 GHz due to the capacitive coupling technique, thus, preventing them from being used in some telecommunications applications requiring a lower frequency band.
FIG. 1
is a schematic diagram of a conventional capacitively coupled power distributed amplifier
10
that is known in the art. The amplifier
10
includes an input transmission line
12
and an output transmission line
14
, where input inductors
16
are periodically connected in series along the input transmission line
12
and output inductors
18
are periodically connected in series along the output transmission line
14
. The input line
12
and the output line
14
can be traces on a printed circuit board. An input signal, such as a microwave signal, applied to an input node
20
of the input transmission line
12
is electrically coupled into the output transmission line
14
, and is provided at an output node
22
of the output transmission line
14
. An input termination resistor
26
is provided at an opposite end of the transmission line
12
from the node
20
to prevent back reflections on the transmission line
12
that may act to reduce the input signal depending on the relative phase of the reflection. Likewise, an output termination resistor
28
is provided at an end of the output transmission line
14
opposite to the node
22
to prevent back reflections of the output signal on the transmission line
14
.
The amplifier
10
includes a plurality of amplifier stages
32
that are distributed between the inductors
16
and
18
along the transmission lines
12
and
14
, and act to couple electromagnetic energy from the input transmission line
12
to the output transmission line
14
with a certain amount of gain. Each amplifier stage
32
includes an amplifying device
34
that may be an HBT transistor. The amplifier
10
defines a distributed transmission line modeled by the series inductors
16
and
18
and a shunt capacitance C
&pgr;
in the amplifying devices
34
. The bandwidth of the signal being coupled from the input transmission line to the output transmission line
14
is determined by the inductance of the inductors
16
and
18
and the shunt capacitance C
&pgr;
. Although the inductors
18
and the output shunt capacitance of the amplifying devices
34
affects the output power on the output transmission line
14
, it is typically the input shunt capacitance C
&pgr;
of the amplifying devices
34
that affects the overall gain-bandwidth product. Therefore, the practical upper frequency bandwidth limit of the distributed amplifier
10
is usually determined by the cut-off frequency f
ci
of the input distributed transmission line
12
.
The cut-off frequency f
ci
is defined as:
f
ci
=1/(&pgr;L
&Ggr;
C
&Ggr;
) (1)
where L
&Ggr;
is the inductance of the inductors
16
and C
&Ggr;
is the effective shunt capacitance of the amplifier stages
32
. To increase the gain and output power of the amplifier
10
, it is necessary to increase the size of the amplifying devices
34
, or increase the bias current applied to the transistor in the devices
34
. However, when the amplifying devices
34
are biased with more current, a higher input diffusion capacitance is created in the amplifying devices
34
. This diffusion capacitance causes the amplifying devices
34
to appear to have a large shunt capacitance C
&pgr;
, which acts to reduce the cut-off frequency as defined in equation (1). So, as the power output of the amplifier
10
increases, the bandwidth typically decreases.
To overcome this upper bandwidth limitation, it is known in the art to employ a series capacitor
36
in combination with each amplifying device
34
in each stage
32
. The series capacitor
36
acts as a division of the shunt capacitance C
&pgr;
in the amplifying devices
34
that reduces the input capacitance. Because the capacitor
36
is in series with the shunt capacitance C
&pgr;
in the amplifying devices
34
, the effective capacitance of the transconductance of the amplifier
10
can be reduced, thus increasing the upper bandwidth limitation for high transition bias currents. This allows a designer to develop a distributed amplifier with a greater upper bandwidth cut-off frequency. In this design, the effective shunt capacitance C
&Ggr;
is C
bb
C
&pgr;
/(C
bb
+C
&pgr;
), where C
bb
is the capacitance of the capacitor
36
. It is desirable to have a low value for C
bb
to produce a small C
&Ggr;
which allows a greater bandwidth without changing the output characteristics of the amplifier
10
. Thus, a wider bandwidth can be achieved without sacrificing power.
Because the capacitor
36
has an infinite or high impedance at DC or low frequencies, the low end of the frequency bandwidth is limited. To overcome this limitation, it is known to include a shunt resistor
38
in parallel with the capacitor
36
to provide a signal path around the capacitor
36
for low frequency or DC signals. The low frequency performance of the amplifier
10
is determined by:
f=½&pgr;R
bb
C
bb
(2)
where R
bb
is the value of the resistor
38
. It is thus desirable to provide a high resistance for the resistor
38
to get a low frequency response at the lower end of the bandwidth. The lower frequency band edge is approximately determined by the pole produced by the capacitor
36
and the resistor
38
. The resistor
38
allows a bias to the base or gate terminal of the amplifying devices
34
. As will be shown below, a large value for the resistor
38
causes the manufactured amplifier to be more sensitive for variations in process.
A benefit of the capacitive coupling technique as discussed above is that the upper frequency bandwidth can be extended for a given output power, or, for a greater bandwidth, the device periphery can be increased to be obtain higher output power. The net gain is an increase
Harness & Dickey & Pierce P.L.C.
Nguyen Khanh Van
Pascal Robert
TRW Inc.
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