JPH04367106A - High frequency amplifier circuit for fm - Google Patents

Abstract

PURPOSE:To improve image interference characteristic while maintaining the high degree of selection in the high frequency amplifier circuit for FM. CONSTITUTION:On the output side of a pi type tuning circuit 9 in the front stage, a gate set to the high impedance of an impedance conversion element 10 consisting of an FET is connected. The input signal is converted into low impedance. It is inputted from the source to the pi type tuning circuit 13 in the rear stage.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present application relates to a high frequency amplification circuit in an FM radio receiver.

0002

2. Description of the Related Art Conventionally, in order to improve the image interference characteristics of FM radio receivers, a method has been adopted in which the number of stages of high frequency tuning circuits is increased.

0003

[Problem to be Solved by the Invention] However, increasing the number of stages in a high frequency tuning circuit means increasing the number of length tuning coils, but generally when printed wiring is done on a board, the coil element is relatively large. This requires space, and an increase in the number of tuning coils is not desirable in terms of printed wiring.
Not only is it unsuitable for high frequency circuits in M radio receivers, but also because high frequency tuning circuits naturally consume energy, increasing the number of stages requires increasing the number of amplifier circuits to compensate for this. was there.

0004

[Means for Solving the Problems] The present invention effectively utilizes the selectivity characteristics of a tuning circuit to improve the above-mentioned conventional disadvantages. An impedance conversion element having a high impedance when viewed from the side is connected, and a π-type later-stage tuning circuit is connected to the low-impedance side of the impedance conversion element, and this latter-stage tuning circuit is connected to the input end of the frequency conversion circuit. Further, in the above configuration, an impedance conversion element is connected between the downstream tuning circuit and the input terminal of the frequency conversion circuit, and the high impedance side is connected to the output of the downstream tuning circuit. and its low impedance side is connected to the input end of the frequency conversion circuit.

[0005]

[Operation] After the antenna input signal is selected by the tuning circuit in the previous stage, it is input to the impedance conversion element, but the input signal is resonated at a high impedance in the tuning circuit in the previous stage, so the output of the circuit is A high degree of selectivity is maintained by inputting this into an impedance conversion element set to a high impedance. Since the signal converted to a low impedance in the impedance conversion element is input to the subsequent tuning circuit, it is selected again without any loss due to matching.

[0006]

[Embodiment] An embodiment of the present application will be described below in detail based on the drawings. In FIG. 1, a band-pass type antenna is connected to an antenna input terminal 1, and includes a tuning coil 2 and a pair of tuning variable capacitance diodes 3a and 3b. A tuning circuit 4 and an attenuation circuit 5 consisting of a pair of pin diodes are connected to each other,
A MOSFET is connected to its output terminal via a coupling capacitor C1.
The first gate g1 of a high frequency amplifying element 6 consisting of
It is connected to a π-type high frequency tuning circuit 9 consisting of. A gate g of an impedance conversion element 10 consisting of an FET is connected to the output end of the high frequency tuning circuit 9, and
A tuning coil 11 and a pair of tuning variable capacitance diodes 12a, 12b are connected to the source S via a coupling capacitor C3.
A π-type rear-stage high frequency tuning circuit 13 consisting of
The FM front end IC (for example LA1175 manufactured by Sanyo Electric) 17 is connected to the fifth terminal which is the input terminal of the frequency conversion circuit 14, and the eighth and ninth terminals which are the output terminals of the frequency conversion circuit 14 are connected to the intermediate frequency It is connected to the primary coil of the transformer T, and the secondary coil is connected to the intermediate frequency amplification circuit 15 via a series circuit of a resistor R1 and a filter F.
The secondary side of the intermediate frequency transformer T is connected to the 12th terminal, which is the input terminal of the intermediate frequency transformer T, and the 15th terminal, which is the output terminal thereof, is connected via the circuit 15. A sixth terminal is connected to the tenth terminal, which is the input terminal of the gain control circuit 16, and outputs a drive signal to the attenuation circuit 5 through the circuit 16. The second gate g2 of the MOSFET is connected to the thirteenth terminal, which is the signal output terminal, and the Lo/DX terminal 18 is connected to this.

[0007] Thus, the input signal input to the antenna input terminal is selected by the antenna tuning circuit 4 and then passed through the coupling capacitor C1 to the first gate g of the MOSFET 6.
1 and is amplified, and the amplified output from its drain is input to a π-type high frequency tuning circuit 9 via a coupling capacitor C2, where it is selected again and the impedance conversion element 10 whose drain is connected to ground. input to gate g. In the above, the input signal is resonated at high impedance in the π-type high frequency tuning circuit 9, and therefore, the output of the circuit 9 is input to the gate g of the impedance conversion element 10 set to high impedance, so it is highly selective. degree can be maintained.

The signal input to the impedance conversion element 10 is converted to a low output impedance without increasing or decreasing the signal voltage, and is output from the source S to the π-type subsequent high frequency tuning circuit 13, so that matching is achieved. After being selected again by the high frequency tuning circuit 13 without causing any loss due to and the ninth terminal respectively, and the output thereof is input to the intermediate frequency amplification circuit 15 from the twelfth terminal through a series circuit of the intermediate frequency transformer T, the resistor R1, and the ceramic filter F, and is amplified, and the amplified output is The signal is output from the 15th terminal to the detection stage.

A part of the wideband intermediate frequency signal that has passed through the intermediate frequency transformer T is inputted from the tenth terminal to the gain control circuit 16 via a series circuit of a resistor R2 and a capacitor C5, and from the circuit 16 to the attenuation circuit 5. An AGC voltage for driving the pin diode and an AGC voltage for controlling the high frequency amplification element 6 are generated, and these AGC voltages are applied to the sixth terminal and the thirteenth terminal.
The antenna input signal is outputted from each terminal, and the attenuation circuit 5 attenuates the signal according to the electric field strength of the antenna input signal, and the high frequency amplification element 6 controls the amplification factor according to the electric field strength.

FIG. 2 shows that in the above-mentioned embodiment, an impedance conversion element 19 consisting of an FET whose drain is connected to ground is further connected to the output side of the second high frequency tuning circuit 13, and its output end is connected to a coupling capacitor C4. Another embodiment is shown in which the output signal is connected to the fifth terminal of the FM front end IC 17 via a High selectivity can be maintained by inputting the input to the gate g of the impedance conversion element 19 whose impedance is set to high, and after being converted to a lower impedance than the source of the impedance conversion element 19,
FM front end IC1 via coupling capacitor C4
Enter 7.

[0011]

[Effects of the Invention] As described above, according to the present application, a π-type tuning circuit with high resonant impedance and high selectivity is adopted as the tuning circuit, and an impedance conversion element is connected between the tuning circuits. Since the output of the tuning circuit in the preceding stage is received at high impedance and outputted at low impedance to the tuning circuit in the succeeding stage, the selectivity in the tuning circuit can be effectively utilized and the image disturbance characteristics can be improved. In addition, since the increase in the number of stages of the tuning circuit can be suppressed, it is effective in downsizing the printed circuit board, and according to the structure of claim 2, the selection performance can be maintained even higher. It has the advantage of

[Brief explanation of the drawing]

[Figure 1] Electrical circuit diagram

[Figure 2] Electrical circuit diagram in another embodiment

[Explanation of symbols]

1 Antenna input terminal 4 Antenna tuning circuit 6 MOSFET 9, 13 π-type high frequency amplification circuit 10, 19 Impedance conversion element 17 FM front end IC

Claims (2)

[Claims] Claim 1: An impedance conversion element having a high impedance as viewed from the tuning circuit side is connected to the output side of a π-type front-stage tuning circuit, and a π-type rear-stage tuning circuit is connected to the low impedance side of the impedance conversion element. 1. A high frequency amplifier circuit for FM, characterized in that a tuning circuit in the latter stage is connected to an input end of a frequency conversion circuit. 2. In claim 1, an impedance conversion element is connected between the downstream tuning circuit and the input terminal of the frequency conversion circuit, and the high impedance side thereof is connected to the output side of the downstream tuning circuit, and the impedance conversion element is connected between the downstream tuning circuit and the input terminal of the frequency conversion circuit. A high frequency amplifier circuit for FM, characterized in that a low impedance side is connected to an input terminal of the frequency conversion circuit.