Amplifier protection circuit

The protection circuit includes a micro strip line whose length is equal to a fourth of the wavelength of an amplification signal frequency on a board and whose one side is connected to the input side of a low noise amplifier, a diode whose one side is connected to the other side of the micro strip line and whose other side is grounded, and a capacitor which is connected to the point where the micro strip line is connected to the diode and whose impedance is sufficiently small at the amplification signal frequency. In this configuration, the diode has virtually no effect to the amplification signal component because the impedance of the protection circuit for the amplification signal frequency viewed from the contact point becomes sufficiently high. In addition, for static electricity charged on the antenna connected to the input terminal, the diode is virtually connected directly to the input side of the low noise amplifier because the micro strip line is almost zero in resistance.

BACKGROUND OF THE INVENTION 
The present invention relates to a protection circuit for a low noise 
amplifier used, for example, in a receiver on the ground for receiving at 
the ground microwave signals from communication satellites, broadcast 
satellites, etc. 
With the quick development of information network systems, the demand of 
satellite communication systems is sharply increasing and their frequency 
bands are going toward higher frequencies. As high frequency field effect 
transistors, Schottky barrier type field effect transistors (MESFET: 
MEtal-Semiconductor junction FET) using compound semiconductors such as 
GaAs, are put to practical use. In addition, in order to realize smaller, 
lower cost and higher performance systems, the first amplification stage 
of the down converter circuit, where high frequency signal is converted to 
low frequencies, has recently designed as an MMIC (Microwave Monolithic 
Integrated Circuit). 
GaAs-used FETs are superior devices, having low noises even at very high 
frequencies such as microwave bands, but they have a defect to be weak in 
electrostatic breakdown as compared with silicon devices. For a long time, 
efforts have been made to protect a circuit from such electrostatic 
breakdown. For a low noise amplifier, however, its noise figure is greatly 
governed by the noise figure of the input circuit. Therefore, there is no 
practical low noise amplifier with a protection circuit capability because 
adding an extra circuit to the input circuit may deteriorate the noise 
figure. 
Moreover, it has so far been usual that such a low noise amplifier is 
connected to a parabolic antenna whose primary radiator mostly has a wave 
guide structure. Therefore, it has been possible to prevent the 
electrostatic breakdown only by carefully treating the low noise amplifier 
(or the circuit including it). This may be one reason why this kind of 
study has not been so active. 
Meanwhile, flat antennas have begun to be put to practical use as ground 
antennas to receive microwave signals from communication satellites or 
broadcast satellites. A flat antenna, consisting of many antenna elements 
located in a flat plane, gathers signal power from each element via 
conductive wire and provides them as a single output. In the early stage, 
microwave reception flat antennas were greatly inferior to parabolic 
antennas both in cost and performance. These days, however, they have 
reached the level of practical use by flourishing study of micro strip 
antennas since the late 1970s as well as the improved performance of 
microwave-use printed circuit boards. 
It is also expected that the advance of MMIC (Monolithic Microwave 
Integrated Circuit) technology will promote the spread of flat antennas 
because smaller antennas are preferred. 
On the contrary, how to connect a flat antenna to a reception system has 
not necessarily been studied sufficiently. For example, there is a method 
based on a conventional technology in which a wave guide, which is an 
ordinary microwave propagation means, is used as a means for connecting 
them both. However, this method makes it difficult to achieve total 
miniaturization and weight reduction. Thus, advantages of the 
miniaturization of the reception systems and the flat shape of the 
antennas are not sufficiently utilized. 
On the other hand, connecting a flat antenna directly to a reception system 
is difficult. This is because a flat antenna has an antenna pattern 
(metal) printed film sandwiched between insulators. Thus, the antenna may 
be charged greatly enough to damage the reception system. The present 
invention intends to solve such a problem. 
SUMMARY OF THE INVENTION 
To solve this problem, the protection circuit of the present invention 
comprises a micro strip line whose length is equal to a fourth of the 
wavelength of an amplification signal frequency on a used board and whose 
one side is connected to the input side of a low noise amplifier, a diode 
whose one side is connected to the other side of the micro strip line and 
whose other side is grounded, and a capacitor which is connected to the 
point where the micro strip line is connected to the diode and whose 
impedance is sufficiently small at the amplification signal frequency. 
Because the diode is separated by the micro strip line whose length is 
equal to a fourth of the amplification frequency wavelength, it has 
virtually no effect to the amplification signal component. For static 
electricity, meanwhile, the diode is virtually connected directly to the 
input side of the low noise amplifier because the 1/4 wavelength micro 
strip line is almost zero in resistance.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a first embodiment of the present invention. A protection 
circuit 10 includes a micro strip line 2, a diode 3 and a capacitor 4. The 
micro strip line is made of gold or the like. The micro strip line 2, 
connected to the input side of a low noise amplifier 1, is designed to be 
equal in length to a fourth of the wavelength of a signal frequency 
amplified by the low noise amplifier 1 (hereafter referred to as the 
amplification signal frequency). The actual length of the fourth 
wavelength of the amplification signal frequency depends on the material, 
thickness, etc., of a board. To the other side of the micro strip line 2, 
the anode of the diode 3 is connected. For example, this diode 3 can be 
formed by shortcircuiting the drain and source terminals of an MESFET. In 
this case, the gate of this FET is used as an anode. The cathode of the 
diode 3 (when an MESFET is used to form the diode 3, a terminal commonly 
connected to the drain and source) is grounded. To the diode 3, the 
capacitor 4 is connected in parallel. Its capacitance is such that the 
impedance is sufficiently low at the amplification signal frequency. 
Therefore, at the amplification signal frequency, the capacitor 4 side of 
the micro strip line 2 is almost shortcircuited and the impedance at the 
contact point 6 before the capacitor side by a 1/4 wavelength, becomes 
infinitely high. If the capacitance of the capacitor 4 is not to lower its 
impedance sufficiently at the amplification signal frequency, the 
impedance of the protection circuit 10 viewed from the contact point 6 
does not become high enough to be disregarded, resulting in degenerating 
the low noise characteristics. When the impedance of the protection 
circuit 10 viewed from the contact point 6 is ideally infinite, the noise 
figure of the low noise amplifier 1 is degenerated only by the 
transmission loss along the micro strip line 2. 
Specifically, the capacitance C of the capacitor 4 is set as follows. That 
is, the impedance Zc of the capacitor is expressed by Zc=1/2.pi.f.sub.0 C, 
where f.sub.0 is the amplification signal frequency. The impedance Zc 
should be 1/10 or less of the input/output impedance of the amplifier. 
Since the input/output impedance of an amplifier is normally about 
50.OMEGA., the impedance Zc should be 5.OMEGA. or less, that is, 
Zc=1/2.pi.f.sub.0 C&lt;5. Thus, in the present invention, the capacitance C 
of the capacitor 4 is set as C&gt;1/10.pi.f.sub.0. 
The above mentioned embodiment assumes an MIM (Metal Insulator Metal) type 
as the capacitor 4. This can be replaced by an open stub 7 in length to 
the micro strip line 2, as shown in FIG. 2 (second embodiment). In this 
configuration, the impedance at contact point 6, a 1/2 wavelength before 
the edge of the open stub 7, becomes infinitely high because the edge is 
open. 
If the antenna connected to the input terminal 5 is charged reversely, the 
diode 3 should be connected reversely as well, as shown in FIG. 3 (third 
embodiment). Because the charging polarity depends on the material of the 
antenna connected to the input terminal 5 and the environment, the 
polarity of the diode 3 must be determined appropriately. 
Meanwhile, a protection circuit of a fourth embodiment shown in FIG. 4 does 
not depend on the charging polarity of the antenna connected to the input 
terminal 5. This configuration is formed by connecting another diode 8 in 
reverse direction to and in parallel to the diode 3 of the first 
embodiment. This makes it always possible to pull out charges from the 
antenna connected to the input terminal 5, regardless of the charging 
polarity. 
A fifth embodiment shown in FIG. 5 is useful when the withstand voltage of 
the diode is not sufficient or when the low noise amplifier 1 is biased 
deeper than the forward voltage of the diode and the input stage is not 
D.C. cut by a capacitor. In this embodiment, additional diodes are used. 
That is, in this embodiment, the diodes 3 and 8 of the fourth embodiment 
are replaced by two series of multiple diodes 12 and 11, respectively. 
Note that although this embodiment is based on the fourth embodiment where 
two diodes are oppositely connected, increasing the number of diodes is 
also similarly effective for the first to third embodiments where one 
direction diode is used. 
As described above, in the protection circuit of the present invention, the 
diode has virtually no effect to the amplification signal component 
because the diode is separated by the micro strip line whose length is 
equal to a fourth of the amplification frequency wavelength, and for 
static electricity, the diode is virtually connected directly to the input 
side of the low noise amplifier because the 1/4 wavelength micro strip 
line is zero in resistance. Therefore, this protection circuit can realize 
a low noise amplifier, which is resistant to electrostatic breakdown, 
without degenerating the noise characteristics.