JPS6380603A - High frequency amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent oscillations from occurring regardless of variation in the impedance at an input terminal by combining two right-angled phase hybrids with amplifier circuits. CONSTITUTION:The right-angled phase hybrid ecrcuits HB1 and HB2 are used and a 1st port P1 of a 1st hybrid circuit HB1 is used as the input terminal; and the input terminals of the two amplifier circuits A1 and A2 are connected to a 2nd port P2 and a 3rd port P3 having a mutual 90 deg. phase difference, and a terminating element Z0 is connected to a 4th port P4, i.e. an isolation port. Further, the output terminals of the amplifier circuits A1 and A2 are connected to the two ports P3 and P2 of a 2nd hybrid circuit HB2 which have a mutual 90 deg. phase difference, the terminating element Z0 is connected to the isolation port P4, and an output terminal is led out of another port P1. Consequently, the circuit never oscillates even if the impedance at the input terminal varies.

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention] [Industrial Application Field] The present invention relates to a high frequency amplification circuit for amplifying an electrical signal having a frequency of, for example, a sub-microwave band, and particularly relates to measures for preventing oscillation.

[Prior art] For example, when receiving radio waves with a frequency of several QHz, the high frequency amplification circuit of the receiver uses gallium arsenide (GaAs).
A FET is used as an active element, ie, an amplification element. However, this type of active element has a relatively large input impedance and output impedance (approximately several Ω), so this type of high-frequency amplification circuit is susceptible to noise generated internally and noise arriving from the outside due to electromagnetic induction. In particular, if there is an impedance mismatch, oscillation is likely to occur due to the influence of reflected waves, and if zero oscillation occurs, the high frequency amplifier circuit will not operate normally.

When configuring a receiver, an antenna having an impedance of, for example, 50Ω is connected to the input terminal of the high frequency amplification circuit. Since the input impedance of the high frequency amplifier circuit is large, a matching circuit is generally inserted between the antenna and the input terminal of the high frequency amplifier circuit to prevent reflected waves from occurring.

[Problems to be Solved by the Invention] However, even when a matching circuit is present, if a large change occurs in the impedance of the antenna, mismatching occurs and the high frequency amplifier circuit oscillates.

In general communication equipment, the impedance of an antenna is approximately constant; however, in the case of an antenna mounted on a car, for example, when the car passes near a metal object, the impedance changes.

Furthermore, if the antenna is removable, when the antenna is removed from the receiver, the input end of the high frequency amplifier circuit becomes high impedance, causing mismatch.

When such a mismatch occurs, oscillation is likely to occur in conventional high frequency amplifier circuits.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high frequency amplifier circuit that does not cause oscillation regardless of the presence or absence of a change in impedance at an input terminal.

[Structure of the Invention] [Means for Solving the Problems] In order to achieve the above object, the present invention uses a coupling element generally called a quadrature hybrid (hereinafter abbreviated as hybrid), and The first port of the hybrid is connected to the input terminal i, the input terminals of the two amplifier circuits are connected to the second port and the third port, which have a phase difference of 90 degrees from each other, and the fourth port, that is, the isolated Connect the termination element to the port. Further, each output terminal of the amplifier circuit is connected to two ports of the second hybrid having a phase difference of 90 degrees from each other, a termination element is connected to the isolated port, and an output signal is taken out from the other port. .

[Operation] Each S parameter in the 3 (dB) quadrature hybrid HH shown in FIG. 2 is expressed by the following equation (1).

...(1) However, a1 to a4 and b1 to b4 indicate signal voltages in each arrow direction at each port of the hybrid shown in FIG.

From the above equation (1), the following equation holds true.

b 1 = a 2/ff -j (a 3//l')
”・(2a)b2=al//;i' j(a
4/A') ”・(2b)b3= j(al/f
f)+a4//ff...(2c)b4-
j(a2//l')+a3/'ff...-(
2d) In other words, when a predetermined signal is input to port P1,
.

Signals of the same level with one phase different from each other by 90 degrees appear at ports P2 and P3, and no signal appears at port P4. In this case, port P4 is generally called an isolated port.

Similarly, when a predetermined signal is input to port P4, signals of the same level appear at ports P2 and P3 with one phase different by 90 degrees, and no signal appears at port P1. Also, when a predetermined signal is input to port P2, baud) - PI
and P4, the same level signals whose phases are 90 degrees different from each other appear, and no signal appears at port P3.Furthermore, when a predetermined signal is input to port P3, ports P1 and P4 appear.
To 4.

Signals of the same level with phases different by 90 degrees appear, and no signal appears at port P2.

Here, consider a case where a signal is input to port P1 of hybrid HB, and amplifier circuits having the same configuration are connected to ports P2 and P3 of hybrid HB, respectively. Port P2
If the reflection coefficient of the amplifier circuit connected to P3 and P3 is 311, then the following equation holds true in FIG.

a2=S11・b2...
・(3a)a 3=S 11 ・b 3
”・(3b) Also, if a load that matches the characteristic impedance of the circuit is connected to the isolated port P4 of the hybrid HB, this port P4 will theoretically be terminated without reflection, and a4 = 0. .

In this case, the following equation holds true from equations (2b) and (2c).

b 2 = a 1 /A' −・(
4a) b 3 = j (a 1 /ff)
”(4b) Therefore, the above formula (3a), the formula (3b)
Using equations (4a) and (4b), the above (2nd
a) By transforming the equation, the following equation is obtained.

b 1 = a2//L' -j (a3/'ff) =
S11・b2/ff-j(Sll・b3//l')=S
11(a LIRl )/ff j(j-511(a
1/ff)/ff)=S11・al/2-8
11・al/2=0・
(5) That is, by connecting a load that matches the characteristic impedance of the circuit to the isolated port P4 of the hybrid HB, the reflected waves a2 and a3 on the ports P2 and P3 sides are absorbed by the load, Its effect does not appear on port P1, so hybrid H
Amplifiers with high input impedance are connected to ports P2 and P3 of B, and a relatively large reflected wave (a2
Even if a3) occurs, the reflected wave will be transmitted to port P1.
+1)ll For example, if the signal on the output side of the amplifier goes around to the input side of the amplifier and is included in the reflected waves a2 and a3, if the gain of the amplifier is large, the reflected wave The levels of waves a2 and a3 increase. In that case, if reflected waves a2 and a3 are transmitted to port P1, and then reflected by an antenna connected to port P1 and transmitted to ports P2 and P3, there is a high possibility that oscillation will occur in the circuit. .

However, in the above configuration, one reflected wave a2 and @3 are not transmitted to port P1, so even if the impedances of port P1 and the elements connected to it are not matched, the reflected waves a2 and @3 are reflected at the input terminal of the amplifier. Since the signal is not reflected again on the antenna side and returned to the input terminal of the amplifier, the above-mentioned oscillation does not occur.

One hybrid HB as described above is used as a first hybrid in the present invention. The second hybrid is used to combine signals that are obtained at the output terminals of two amplifier circuits and have mutually different phases. Also, the second
By using this hybrid, it is possible to achieve matching between the output of the amplifier circuit and the circuit connected thereto, as in the case of the first hybrid.

Other objects and features of the present invention will become clear from the following description of an embodiment with reference to one drawing.

[Embodiment] FIG. 1 shows the configuration of one type of high frequency amplification circuit implementing the present invention. This high frequency amplification circuit is used to amplify signals with a frequency of about several GHz.

HBI and HB2 are coupling elements called 3 dB quadrature hybrid, and in this example, those manufactured by Hirose Electric Co., Ltd. are used.

A receiving antenna is connected to port P1 of one hybrid HBI. A terminating resistor R is installed between the hybrid HBI port (isolated boat) P4 and the ground.
A termination circuit 10 consisting of a TI and a trimmer capacitor CT is connected. Ports P2 and P3 of HBl are connected to input terminals of an amplifier circuit including transistors Q1 and Q2, respectively.

The transistors Q1 and Q2 used in this example are 23-571 manufactured by NEC Corporation.

These are field effect transistors (FETs) made of gallium arsenide (GaAs).

The electrical signal appearing at port P2 of hybrid HBI is applied to transistor Q1 via coil L2, amplified, and applied to port P3 of hybrid HB2 via coil L4 and capacitor C3. Further, the electric signal appearing at the baud-P3 of the hybrid HBI is applied to the transistor Q2 via the coil L3, amplified, and applied to the port P2 of the hybrid HB2 via the coil L5 and the capacitor C5.

Port P4 of hybrid HB2 is connected to the termination resistor RTI
is grounded through. The input terminal of an amplifier circuit including a transistor Q3 (23 to 571) is connected to the port P1 of HB2. The electrical signal appearing at port P1 of hybrid HB2 is applied to transistor Q3 via coil L8, amplified, and appears at the output terminal via coil L9 and capacitor C9.

Termination resistor R connected to hybrid HBI, HB2
The resistance values of T1 and RT2 are set to match the characteristic impedance Zo of the circuit.

The trimmer capacitor CT is used to compensate for the reactance of the characteristic impedance of the circuit. In addition, I
C1 is a three-terminal regulator, which supplies a stabilized power supply voltage (5■) to each amplifier circuit.

The circuit of FIG. 1 is simplified in FIG. 3. Al, A2, and A3 shown in FIG. 3 correspond to each amplifier circuit including transistors Ql, Q2, and Q3 shown in FIG. 1, respectively. . The operations of the hybrid HBI and HB2 used in this embodiment are the same as the hybrid HB shown in FIG. 2 described above.

Therefore, the explanation of the previously explained parts will be omitted.

When the gains of the amplifier circuits A1 and A2 are set to 821, the following equation holds true in FIG.

a 5 = S21 ・b 2 = (
6a) a 8 = S21 ・b 3
(6b) By substituting the above equations (4a) and (4b) into these, the following equation is obtained.

a 5-321 ・a 1/'ff ・
...(7a) a8=-j(S21・al/ff)
...17b) Here, al, A2 in Fig. 2
．． A3. A4. bl, b2゜b3 and b4 are
In hybrid HB2, A7. A8.
A5. A6. b7. b8. b5 and b6, the above-mentioned equations (2a) 2 to (2d) are transformed as shown in the following equations.

b 7 = a 8//l' -j (a 5/'ff
) ...(8a) b 8 = a7/ff
-j (a6/ff) ...(8b)b
5 = j (a7/ff) 10 a6/ff - (8
c) b 6 = j (a8/ff) + a5/'f
f − (8d) Then, by substituting the above equations (7a) and (7b) into the above equation (8a), the following equation is obtained.

b7=a8/ff j (a5/ff)=-j (
821・a 1/ff) /I'l j ((821
・a 1/ff)/ff)=-j (S21-a 1/
2) j (S21・al/2)=−j (S21
・a 1) ” (9) In other words,
Two amplifier circuits AI and A2 are provided between the two hybrid HBI and HB2, but the hybrid HBI
The input terminal (Pl) of the hybrid HB2 and the output terminal (Pl) of the hybrid HB2
The gain in the circuit between Pl) is S21, which is equal to the gain of one amplifier circuit, and the phase of the signal changes by 90 degrees in this circuit.

Furthermore, if a reflected wave (A7) occurs at the input terminal of the amplifier circuit A3 connected to the output of the hybrid HB2,
The wave is transmitted to ports P2 and P3 of (b5.b
8), and they are reflected by the outputs of the amplifier circuits A1 and A2, input into the hybrid HB2 again, and transmitted to the port P4 (b6); however, the hybrid H
Port P4 of B2 is terminated by characteristic impedance Zo so as not to generate reflected wave a6. In other words,
The wave b7 of the hybrid HB2 that is cursed by the PO)-PI corresponds to bl shown in the above equation (5), and its level becomes zero. That is, the reflected wave (A7) is generated at the input terminal of the amplifier circuit A3.
Even if this occurs, it will not be reflected again and superimposed on the signal wave b7, and no standing wave will be generated in the wave b7.

By the way, it is considered that the input impedance of the amplifier circuits At and A2 connected to ports P2 and P3 of the hybrid HBI includes a reactance component. In this case, if these reactance components are not compensated for,
A reflected wave a2. generated by the amplifier circuit At, A2. The phase of a3 no longer matches the above theoretical phase. Then, a reflected wave b1 is generated at the port P1 of the hybrid HBI. However, if a reactance element is inserted into each amplifier circuit to compensate for the reactance component, the number of components will increase, and l
! The number of i1m locations also increases.

Therefore, in this embodiment, as shown in FIG.
A trimmer capacitor CT (
Semi-fixed) is connected. By including one reactance component in the termination circuit 10, port P4 can be matched with two amplifier circuits Al and A2 that include reactance components. Therefore, in this example, each amplifier circuit AI,
A2 is not provided with a circuit to compensate for the reactance component.

Note that in FIG. 1, capacitors C6 and C10 are used to correct the phase of the signal.

In the high frequency amplifier circuit shown in FIG. 1, the following characteristics were actually obtained.

Gain: 31.5 [dB] Noise level (NF): 0.93 [dB] Input side voltage standing wave ratio (VSI): 1.36 Output side voltage standing wave ratio (VSWR): 1.12 11th! I
It has been confirmed that this high-frequency amplifier circuit operates stably without oscillation, not only when an antenna with a predetermined impedance is connected to the input terminal, but also when the input terminal is left open (infinite impedance). Ta.

In the above embodiment, a 3 dB quadrature hybrid was used as the hybrid means, but the configuration may be modified as long as it has at least four ports and the phase relationship of each port satisfies the predetermined requirements of the present invention. Even if they are different, they can be used as the hybrid means of the present invention.

[Effects] As described above, according to the present invention, even if the impedance of the input terminal changes, oscillation does not occur in the circuit.
For example, if the present invention is implemented in a device where the impedance of the input terminal is likely to change, such as a high frequency amplifier circuit of a receiver mounted on an automobile, favorable results can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an electrical circuit diagram illustrating one type of high frequency amplification circuit embodying the present invention. FIG. 2 is a block diagram showing one hybrid means. FIG. 3 is a simplified block diagram of the circuit shown in FIG. 1. 10: Termination circuit (first termination means) CT: Trimmer capacitor (reactance element) HBI:
3dB quadrature hybrid (first hybrid means) HB2: 3dB quadrature hybrid (second hybrid means) ANT: antenna (input means) RT2: termination resistor (second termination means) A1: amplifier circuit (I A2: Amplification circuit (second amplification means) A3: Amplification circuit (output means)

Claims (5)

[Claims] (1) having at least four ports, transmitting a signal applied to the first port to the second and third ports with a phase difference of substantially 90 degrees from each other, and transmitting a signal applied to the fourth port; to the second and third ports with a phase difference of substantially 90 degrees from each other, and the signal applied to the second port is transmitted to the first and fourth ports with a phase difference of substantially 90 degrees from each other. , a first hybrid means for transmitting a signal applied to the third port to the first and fourth ports with a phase difference of substantially 90 degrees from each other; having substantially the same function as the first hybrid means; a second hybrid means; an input means connected to a first port of the first hybrid means; a first termination means connected to a fourth port of the first hybrid means; a first amplification means having an input terminal connected to a second port of the means and an output terminal connected to a first port of the second hybrid means; an input terminal to a third port of the first hybrid means; a second amplifying means having a terminal connected thereto and having an output terminal connected to a fourth port of the second hybrid means; a second terminating means connected to a second port of the second hybrid means; and an output means connected to the third port of the second hybrid means. (2) The high frequency amplifier circuit according to claim 1, wherein the first termination means includes a resistance element and a reactance element. (3) The high frequency amplifier circuit according to claim (2), wherein the reactance element is a variable reactance element whose reactance changes. (4) The high frequency amplification circuit according to claim 1, wherein the first amplification means and the second amplification means have the same configuration. (5) The first amplification means and the second amplification means include field-effect transistors made of gallium arsenide. The high frequency amplification circuit according to item 3) or item (4).