GB2143694A

GB2143694A - Radio-frequency amplifier

Info

Publication number: GB2143694A
Application number: GB08415585A
Authority: GB
Inventors: Yoshinobu Yamaguchi
Original assignee: Alps Electric Co Ltd
Current assignee: Alps Alpine Co Ltd
Priority date: 1983-07-15
Filing date: 1984-06-19
Publication date: 1985-02-13
Also published as: DE3425950A1; GB2143694B; JPS6019220U; GB8415585D0; JPH051132Y2; DE3425950C2

Abstract

A radio-frequency amplifier having its input side well isolated from its output side comprises a radio-frequency amplifier circuit and a printed-wiring board. The RF amplifier circuit includes a first capacitor (39) coupled between an input terminal (20) and a grounding terminal (21) on the input side for compensation of frequency characteristics. The RF amplifier circuit also includes a second capacitor (40) coupled between an output terminal (22) and a grounding terminal (23) on the output side for similar purposes. The printed-wiring board has electrically conductive paths for connecting various grounding terminals, and includes an electrically conductive path (54) which exhibits inductance at high frequencies. The equivalent circuit of the amplifier when the input terminal is viewed from the output terminal forms a trap circuit in the form of a bridged-T network. <IMAGE>

Description

SPECIFICATION Radio-frequency amplifier Field of the invention The present invention relates to a radio frequency amplifier and, more particularly, to a radio frequency amplifier whose equivalent circuit taken when the input side is viewed from the output side constitutes a trap circuit in the form of a bridged-T network to greatly attenuate signals transmitted from the output side to the input side.

Background of the invention The technical background of the invention is described with reference to Fig. 1, where a booster mixer 1 acting as a television receiver facility is shown. In this mixer, the video signal received by an antenna 2 is supplied to a mixer 11 via an input terminal 3 connected to the antenna, a filter 4, a booster 5, a distribution network 6, an attenuator 7, a booster 8, an attenuator 9, and a booster 10. An RF modulator 12 supplies a signal to the mixer 11 via an input terminal 13 connected to the modulator, so that this input signal is mixed with the video signal fed from the booster 10. Then, the resultant output signal is furnished to a television receiver 15 through an output terminal 14 connected to the TV receiver.

The video signal received by the antenna 2 is also fed to an attenuator 16 by the action of the distribution network 6, and then the signal is supplied to an output terminal 19 for picture recording via a booster 17 and an attenuator 18. The attenuators 7 and 9 serve to hold the input signals applied to the boosters 8 and 10, respectively, to their appropriate levels. Also, these attenuators hold the signal at the output terminal 14 to an appropriate level.

In the booster mixer 1 designed as described above, a portion of the signal from the RF modulator 12 leaks into the antenna 2 after passing through the mixer 11, the booster 8 and the filter 4.

Then, the signal is radiated into the air and may become a disturbing wave. The boosters 5, 8, and 10 also act to suppress such a leakage, because in general a booster or amplifier is a directional device. That is, it amplifies any signal flowing from the input side to the output side, but it attenuates any signal traveling from the output side to the input side.

Similarly, in case where a videotape recorder is connected to the output terminal 19 for picture recording, the locally generated signal from the tuner incorporated in the recorder leaks into the input terminal 3 connected to the antenna. The attenuator 16 and the booster 17 are used to suppress this leakage, but they may be omitted depending on the performance of the tuner used.

The leakage of the signal from the RF modulator 12 into the input terminal 3 in this booster mixer 1 is now discussed. In order to hold down the level of the leaked signal below the value required by the regulations of FTZ or FCC, the isolation of the modulator input terminal 13 connected to the antenna from the antenna input terminal 3 is required to be greater than 60 dB.

Ordinarily, the signal level at the output terminal 14 to the television receiver is so set as to be higher than the signal level at the antenna input terminal 3 by several decibels. Accordingly, in the booster mixer shown in Fig. 1, the gain per booster lies roughly in the range from 0 dB to 1 dB. The isolation per booster achieved by the prior art technique is slightly in excess of 20 dB under the condition that an attenuator is inserted in the preceding stage to make the gain equal to 0 dB.

Therefore, in order to make the isolation of the antenna input terminal from the modulator input terminal 13 greater than 60 dB, provision of three or more boosters is needed, thus placing certain limitations on the reduction in the cost to manufacture the booster mixer and on the miniaturization.

Summary of the invention It is the main object of the present invention to provide a booster, or radio-frequency amplifier, which is excellent in isolation, whereby it is free of the foregoing problems.

In summary, the invention resides in a radio frequency amplifier which comprises: a radio-frequency amplifier circuit including a first capacitor for compensating frequency characteristics and a second capacitor for compensating frequency characteristics, the first capacitor being coupled between an input terminal and a grounding terminal on the input side, the second capacitor being coupled between an output terminal and a grounding terminal on the output side; and a printed-wiring board including an electrically conductive connection path connected to the grounding terminals of the first and second capacitors, a grounding path connecting together said grounding terminal on the input side and said grounding terminal on the output side, and an electrically conductive path connecting together the connection path and the grounding path and forming an inductance; the equivalent circuit of the radio-frequency amplifier when said input terminal is viewed from said output terminal constituting a trap circuit in the form of a bridge.

Brief description of the drawing Figure 1 is a block diagram of a booster mixer using conventional RF amplifiers; Figure 2 is a circuit diagram of an RF amplifier according to the present invention; Figure 3 is an equivalent circuit diagram representing the circuit shown in Fig. 2; Figure 4 is a view showing a printed-wiring board on which the circuit of Fig. 2 is disposed; Figure 5 is an equivalent circuit diagram representing an RF amplifier that consists of the circuit of Fig. 2 and the printed-wiring board of Fig. 4; Figure 6 is a block diagram of a booster mixer using RF amplifiers according to the invention; and Figure 7 is a graph showing measured values of the isolation of the booster mixer shown in Fig. 6 and measured values of the isolations of booster mixers using conventional RF amplifiers, for comparison purposes.

Detailed description of the invention Referring to Fig. 2, there is shown a circuit diagram of a radic-frequency amplifier circuit which is used in a radio-frequency amplifier embodying the invention and which is installed on a printed-wiring board. The circuit includes an input terminal 20, a grounding terminal 21 on the input side, an output terminal 22, a grounding terminal 23 on the output side, a terminal 24 connected to a power supply for obtaining the power voltage, a transistor 25, bias resistors 26, 27, 28, 29, a feedback resistor 30, a load resistor 31, capacitors 32 and 33 for blocking the flow of alternating current, bypass capaci tors 34, 35, a capacitor 36, a coil 37, and a choke coil 38. These are the fundamental circuit components of the amplifier, and their values are so set that it functions as a broadband amplifier.Added to this radio-frequency amplifier circuit are a printed-wiring board (described later) and capacitors 39 and 40, both of which are used to constitute a trap circuit and to compensate the characteristics at high frequencies.

An equivalent circuit representative of the radiofrequency amplifier circuit of Fig. 2 is shown in Fig.

3, where the fundamental circuit section of the amplifier is indicated by reference numeral 41. This simplified circuit consists of the capacitors 39 and 40 for compensating the characteristics at high frequencies, and the circuit section 41. The capacitor 39 is placed between the terminals 20 and 21 on the input side, while the capacitor 40 is inserted between the terminals 22 and 23 on the output side.

Referring next to Fig. 4, there is shown the print pattern of the printed-wiring board 42 on which the radio-frequency amplifier circuit shown in Fig. 2 is formed. The components placed on the board are indicated by the same reference numerals as in Fig. 2. A transistor 25 is placed in the portion surrounded by the broken line. The printed-wiring board 42 is provided with electrically conductive paths 43 through 51. Further, an electrically path 52 is connected to the grounding terminals of the bias resistor 26, the capacitor 39 for compensating the characteristics at high frequencies, the bypass capacitor 35, the bias resistor 28, the capacitor 40 for compensating the characteristics at high frequencies, and the load resistor 31.Another electrically conductive path 53 is connected to the grounding terminal 21 on the input side, the grounding terminal 23 on the output side, and the grounding terminal of the capacitor 36. The path 52 is connected with the path 53 through an electrically conductive path 54 which is formed so as to exhibit inductance at high frequencies.

An equivalent circuit representative of the radiofrequency amplifier consisting of the RF amplifier circuit shown in Fig. 2 and the printed-wiring board shown in Fig. 4 is shown in Fig. 5. The fundamental amplifier circuit section 41 can be regarded as an impedance predominated by its resistance component when the input side is viewed from the output side. In general, this equivalent circuit is identical with a trap circuit in the form of a bridged-T network that is used in the video IF amplifier circuit of a television receiver. Thus, the RF amplifier shown in Figs 2 and 4 forms a trap circuit when the input side is viewed from the output side. Hence, the absorbing and attenuating effect of the trap circuit increases the isolation further.

Referring next to Fig. 6, there is shown a booster mixer 55 employing a radio-frequency amplifier according to the invention. This booster mixer includes two radio-frequency amplifiers 56 and 57 and other components which are indicated by the same reference numerals as in the case of Fig. 1.

By controlling the values of the capacitors 39 and 40 for the RF characteristic compensating amplifiers 56 and 57, the frequency at which absorption and attenuation take place is so adjusted as to fall within the frequency range of the RF modulator 12.

Measured values of the isolation of the modulator input terminal 13 from the antenna input terminal 13 from the antenna input terminal 3 of this booster mixer 55 are shown in Fig. 7, together with measured values of that of a booster mixer using conventional RF amplifiers, for comparison purposes. Curve 58 represents the isolation of the booster mixer 55 shown in Fig. 6. Curve 59 represents the isolation of a conventional booster mixer employing two radio-frequency amplifiers. Curve 60 represents the isolation of a conventional booster mixer using three radio-frequency amplifiers. As can be understood from this graph, the isolation of the booster mixer using the novel radio-frequency amplifiers abruptly increases in the used frequency range in the vicinity of 600 MHz and exceeds -60 dB, thus satisfying the requirement of FTZ.On the other hand, the conventional booster having two RF amplifiers is unable to fulfill the requirement and so three or more RF amplifiers are necessitated to meet the requirement.

As can be seen from the description thus far made, the novel RF amplifier provides an improved isolation of the input side from the output side.

Consequently, the two novel amplifiers used in a booster mixer can serve the same function as three conventional amplifiers, whereby reducing the cost of the booster mixer and contributing to its miniaturization. In addition, since the mixer needs a less number of amplifiers, it can enjoy the advantage of improved noise factor The novel RF amplifier can be, of course, applied to any device requiring a high isolation, in addition to booster mixers.

While a preferred embodiment has been described, variations thereto will occur to those skilied in the art within the scope of the present inventive concepts which are delineated by the fol

Claims

lowing claims. CLAIMS

1. A radio-frequency amplifier comprising: a radio-frequency amplifier circuit including a first capacitor for compensating frequency characteristics and a second capacitor for compensating frequency characteristics, the first capacitor being coupled between an input terminal and a grounding terminal on the input side, the second capacitor being coupled between an output terminal and a grounding terminal on the output side; and a printed-wiring board including an electrically conductive connection path connected to the grounding terminals of the first and second capacitors, a grounding path connecting together said grounding terminal on the input side and said grounding terminal on the output side, and an electrically conductive path connecting together the connection path and the grounding path and forming an inductance; the equivalent circuit of the radio-frequency amplifier when said input terminal is viewed from said output terminal constituting a trap circuit in the form of a bridge.

2. A radio-frequency amplifier as set forth in claim 1, wherein the frequency at which signals are absorbed or attenuated by said first and second capacitors is adjusted.

3. A radio-frequency amplifier substantially as hereinbefore described, with reference to Figures 2 to 7 of the accompanying drawings.