JPS63167529A - High frequency amplifier circuit - Google Patents

Abstract

PURPOSE:To improve the image disturbing ratio by providing a trap circuit comprising a parallel resonance circuit in series with a high frequency transistor (TR) receiving an output from an antenna input stage of a high frequency amplifier circuit so as to make the reception sensitivity constant. CONSTITUTION:An output from the antenna input circuit is inputted to a terminal 1 and fed to a gate of an FET 3 of a high frequency amplifier circuit via a capacitor. A trap circuit 18 comprising a parallel resonance circuit consisting of a coil 16 and a capacitor 17 is connected to the source of the FET 3 and an image frequency f2 is trapped from the output of the FET 3 of the high frequency amplifier circuit. In using the high frequency amplifier for a front end part of the receiver, the image disturbing ratio is much improved in comparison with a conventional receiver and decreased to a degree nearly neglecting the image disturbance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field of the Invention] The present invention relates to a high frequency amplifier circuit designed to improve the reception characteristics of a superheterodyne receiver, and is particularly suitable for an upper heterodyne type receiver. This is a high frequency amplification circuit.

[Problem that the invention seeks to solve]

FIGS. 5 to 7 are circuit diagrams schematically showing a front end section of a conventional superhetero gain receiver including a high frequency amplification circuit and a tuning circuit. Throughout the drawings, l is a terminal to which an input signal from the antenna input circuit is supplied, 2 is an output terminal that derives the output obtained via the high-frequency amplification circuit and the tuning circuit, and 3 is a field-effect transistor for high-frequency amplification (hereinafter referred to as , referred to as FET), 4.5
6 is a tuning circuit in which a coil and a capacitive element such as a variable capacitance diode are connected in parallel, and 6 is a terminal to which a tuning voltage is applied. In FIG. 5, a series resonant circuit 9 consisting of a coil 7 and a capacitor 8 is connected to the output stage of the tuned circuit 5.
The figure shows a coil 11 and an antenna capacity 1110 at the signal input stage.
A parallel resonant circuit 12 is connected by. In FIG. 7, a filter circuit 15 consisting of a resistor 13 and a capacitor 14 is connected to the output stage of the tuning circuit 5.

In general, superhetero gain receivers have the disadvantage that the load QL of the tuning circuit decreases as the receiving frequency becomes higher, resulting in worsening of image disturbance characteristics. Additionally, as shown in Figure 4 (a), the gain of a high frequency amplifier circuit generally increases in the high range due to the frequency characteristics of the tuning circuit 4.5, making it impossible to maintain a constant gain over the entire receiving frequency range. .

Incidentally, the horizontal axis in FIG. 4 shows the receiving frequency f, and the vertical axis shows the gain. As is clear from (a) in Figure 4, the gain increases at the image frequency f2, so the unnecessary image frequency is amplified and the image interference ratio changes.
The disadvantage is that the ratio is poor.

In order to improve these problems, a series resonant circuit 9 consisting of a coil 7 and a capacitor 8 is installed in the output stage as shown in FIG.
Trap the image frequency by providing
The image interference ratio has been improved. Figures 6 and 7
In the figure, the gain frequency characteristics of the high frequency amplifier circuit are adjusted by providing a parallel resonant circuit 12 or a filter circuit 15 at the input stage or the output stage. Also, if you try to improve both characteristics, you will have to combine the circuits shown in Figures 5 and 6 or 7, which will complicate the configuration of the front end section of the high frequency amplifier circuit, increase the number of parts, and make it expensive. There was a drawback.

[Purpose of the invention]

The present invention has been made to improve the above-mentioned problems, and its main objectives are to improve the image interference ratio and to equalize the gain of the output of the tuning circuit connected to the high-frequency amplifier circuit with respect to the receiving frequency. The object of the present invention is to provide a high frequency amplification circuit consisting of a convenient circuit that can be easily implemented.

[Embodiments of the invention]

FIGS. 1 and 2 are circuit diagrams showing an embodiment in which a tuning circuit is connected to a high frequency amplifier circuit according to the present invention, and is suitable for an upper hetero gain type radio receiver. The output from the antenna input circuit is input to the terminal 1, and is supplied to the gate electrode of the FET 3 of the high frequency amplifier circuit via the co-capacitor.

In Fig. 1, the source electrode of FET 3 is connected, or as shown in Fig. 2 (the drain electrode of FET 3 is connected to the coil 16 and the capacitor 1).
A trap circuit 18 consisting of a parallel resonant circuit consisting of 7 is connected to trap the image frequency f2 from the output of the FET 3 of the high frequency amplification circuit. In addition, F
The tuning circuits 4 and 5 at the subsequent stage of the ET3 have the same configuration as the circuit shown in the conventional example, so a description thereof will be omitted.

Now, the gain frequency characteristics and image interference ratio of the high frequency amplifier circuit of the present invention will be explained with reference to FIGS. 3 and 4 which show the gain frequency characteristics.

FIG. 3 shows the gain frequency characteristic of the output from the output terminal 2 obtained through the tuning circuit with respect to the receiving frequency f. The horizontal axis of the figure shows the frequency f, and the vertical axis shows the gain of the output from the tuned circuit.

fl is the reception frequency, fo is the local oscillation frequency, and f2 is the image frequency. Figure 3 (a) is the same as Figure 1 and 2.
This is a diagram showing the gain frequency characteristics of the high frequency amplifier circuit when the trap circuit 18 is not added to the FET 3 in the figure, and as is clear from (a) of FIG. 3, the high frequency amplifier circuit also amplifies the image frequency 12, This shows that the image interference ratio is poor. On the other hand, by providing a trap circuit 18 consisting of a parallel resonant circuit in series with the FET 3, a gain frequency characteristic as shown in FIG. 3(b) can be obtained. The gain on the high frequency side from the receiving frequency f is reduced compared to the conventional one or the circuit in which the trap circuit 18 is removed from the FET 3 of the present invention, indicating that the image frequency f2 is sufficiently trapped. Therefore, by using the high frequency amplification circuit of the present invention in the front end section of a receiver, the image interference ratio can be significantly improved compared to conventional receivers, and the image interference can be reduced to a negligible level. I understand.

Furthermore, the gain frequency characteristics of the output of the high frequency amplifier circuit according to the present invention will be explained. FIG. 4(a) shows the gain frequency characteristic when the trap circuit 18 is removed from the embodiments shown in FIGS. 1 and 2, which causes a difference between the highest and lowest receiving sensitivities and deteriorates the receiving sensitivity. In contrast, the high frequency amplifier circuit of the present invention is provided with a trap circuit consisting of a parallel resonant circuit at either the source or drain of FET 3 of the high frequency amplifier circuit, as shown in FIGS. 1 and 2. .

It has a gain frequency characteristic that attenuates the image frequency f2 as shown in FIG. 4(b).

Now, as is well known, the admittance Y of the trap circuit 18 consisting of a parallel resonant circuit is expressed as follows, where the capacitance of the capacitor l7 is C, the coil 16 has its own series resistance r, and the inductance is L. .

1-ω” Lc+jωrc r +jωL −・−・・−・−・・・・・−−−−−−(11 At the resonance frequency ω of the image frequency f2. Set it as follows.

Resonant frequency ω. When (2) is 2, Admitan Y is Y
--r, and the impedance shows a substantially minimum value at the image frequency f2. In this way, the trap circuit 18 has a characteristic of attenuating the image frequency f2.

As in the present invention, the trap circuit 1 is connected in series to the FET 3.
By connecting 8, the gain in the receiving frequency range higher than 1 is attenuated and the image frequency f2 is sufficiently removed, resulting in a gain frequency characteristic as shown in (b) in Fig. 3, but the characteristics in the high frequency region are as follows. , is determined by the gain frequency characteristics as shown in (b) of FIG.

Also, by attenuating the gain from the low to high frequencies of the receiving frequency as shown in (b) in Figure 4, the output from FET3 has a gain frequency characteristic that greatly attenuates the image frequency f2. The gain of the output from the tuning circuit, that is, the output from the output terminal 2, can be made uniform with respect to the receiving frequency, as shown in (c) of FIG. Therefore, according to the receiver using the high frequency amplifier circuit of the present invention, it is possible to make the reception sensitivity uniform in the low and high frequencies of the reception band, and to obtain good reception characteristics.

Note that the transistors of the high frequency amplifier circuit may be constructed of field effect type or bipolar type transistors as in the embodiment. The configuration of the tuning circuit after the high-frequency amplifier circuit is as follows:
Although the configuration is similar to the conventional example shown in FIGS. 5 to 7, the configurations of the high frequency amplifier circuit and the tuning circuit are not limited to the tuning circuits shown in FIGS. 1 and 2.

〔Effect of the invention〕

According to the high frequency amplification circuit of the present invention, the high frequency amplification circuit only has a trap circuit made of a parallel resonant circuit in series with the high frequency transistor to which the output from the antenna input stage of the high frequency amplification circuit is supplied. It is possible to make the gain of the output from the subsequent tuning circuit uniform from the low frequency range to the high frequency range, thereby making it possible to make the receiving sensitivity constant and to improve the image interference ratio at the same time.

Therefore, by using the high frequency amplifier circuit of the present invention, good reception characteristics can be achieved with fewer parts than conventional ones.
It is possible to provide an inexpensive receiver.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment in which a high frequency amplification circuit of the present invention and a tuning circuit are connected to the subsequent stage, and Fig. 2 is a circuit diagram showing an embodiment in which the high frequency amplification circuit of the present invention and a tuning circuit are connected to the subsequent stage. FIG. 3 is a circuit diagram showing another embodiment, FIG. 3 is a diagram showing gain frequency characteristics for explaining the present invention, and FIG. 4 is a diagram showing gain frequency characteristics with respect to reception frequency. 1, Z, S: terminal, 3: field effect transistor, 3.4: tuned circuit, 16: coil.

Claims (1)

[Claims] The high frequency amplification circuit to which the input signal from the antenna input circuit is supplied includes a tuning circuit that tunes to a predetermined reception frequency, and the transistor of the high frequency amplification circuit to which the input signal from the antenna input circuit is supplied; A high frequency amplification circuit characterized by comprising parallel resonant circuits connected in series.