JPH0423967B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a television tuner that widens the bandwidth of an intermediate frequency tuning circuit with little loss.

(Prior Art) FIG. 3 shows a circuit diagram in which a VHF mixing circuit, an intermediate frequency tuning circuit, and an amplifier circuit for amplifying an intermediate frequency signal are combined in a conventional television tuner. In FIG. 3, transistors 1 and 7 are connected in cascade. An emitter resistor 2 and a bypass capacitor 3 are interposed in parallel between the emitter of this transistor 1 and the ground, and the collector is connected to a power supply terminal 26 via a power supply resistor 4, and the transistor 7 is connected via a DC blocking capacitor 5. It is connected to the Emitsuta. transistor 7
The emitter is further grounded via an emitter resistor 6, and the base is grounded via a bias resistor 8 and a bypass capacitor 9 in parallel, and is also connected to a power supply terminal 26 via another bias resistor 10. This circuit constitutes a VHF mixing circuit. Further, the collector of the transistor 7 is connected to one end of the intermediate frequency tuning coil 12 and also connected to the base via the intermediate frequency tuning capacitor 11. This intermediate frequency tuning coil 12
The other end is grounded via another intermediate frequency tuning capacitor 13 and connected to a power supply terminal 26 via a power supply resistor 15. Then, an intermediate frequency tuning coil 12 and an intermediate frequency tuning capacitor 11,1
3 constitutes a π-type intermediate frequency tuning circuit. Furthermore, the connection between the tuning coil 12 and the tuning capacitor 13, which are the output ends of the intermediate frequency tuning circuit, is connected to the emitter of the transistor 21 via a coupling capacitor 16. The emitter of this transistor 21 is further grounded via an emitter resistor 17, and the base is grounded via a bias resistor 18 and a bypass capacitor 19 in parallel, and is also connected to a power supply terminal 26 via another bias resistor 20. . Furthermore, the collector of this transistor 21 is connected to a power supply terminal 26 via a feeding resistor 22 and to an intermediate frequency signal output terminal 24 via a DC blocking capacitor 23. Note that 25 is a bypass capacitor. The transistor 21 and the like constitute a base-grounded transistor amplifier circuit, which functions as a buffer amplifier for intermediate frequency signals.

(Problems to be Solved by the Invention) Here, a high frequency equivalent circuit diagram of the above-mentioned FIG. 3 is shown in FIG. 4. In FIG. 4, the same elements as those in FIG. 3 are given the same reference numerals, and redundant explanation will be omitted.

In FIG. 4, an output resistor 27 and an output capacitor 28 of a VHF mixing circuit are connected to the input side of an intermediate frequency tuning circuit consisting of an intermediate frequency tuning coil 12 and intermediate frequency tuning capacitors 11 and 13. Also,
An input inductance 29 and an input resistance 30 of a transistor amplifier circuit are connected to the output side of the intermediate frequency tuning circuit via a coupling capacitor 16.

By the way, the value of the input inductance 29 in the common base type transistor amplifier circuit is extremely small in the intermediate frequency signal band (60 MHz or less) and can be ignored. Therefore, as shown in FIG. 5, the output side of the intermediate frequency tuning circuit is grounded via the coupling capacitor 16.
In FIG. 5, 31 is the equivalent capacitance of the intermediate frequency tuning capacitor 11 and the output capacitance 28 of the VHF mixing circuit.

As a result, coupling capacitor 16 is connected in parallel to intermediate frequency tuning capacitor 11 and functions as an intermediate frequency tuning capacitor.
The coupling capacitor 16 also connects the output impedance of the intermediate frequency tuning circuit to the transistor 2.
In order to match the low human input impedance of 1, it is set to a large capacitance value and also functions as an impedance matching circuit. Since a capacitor with a large capacitance value is connected to the intermediate frequency tuning circuit in this way, there is a problem that the selectivity Q of the intermediate frequency tuning circuit becomes higher than necessary and the bandwidth is narrow.

As a countermeasure for this, coupling capacitor 16
It is not preferable to reduce the capacitance of the transistor 21 for the reasons mentioned above, and reducing the capacitance causes a large loss when transmitting the intermediate frequency signal to the transistor 21. Furthermore, although it is possible to widen the bandwidth of the intermediate frequency tuning circuit by reducing the resistance value of the feed resistor 15 due to the damper effect, this still causes a large loss of the intermediate frequency signal due to the feed resistor 15, which is not practical.

An object of the present invention is to solve the above-mentioned problems of the conventional television channel.
To provide a television tuner capable of widening the bandwidth of an intermediate frequency tuning circuit with little loss.

(Means for Solving the Problem) In order to achieve the above object, the television tuner of the present invention has an intermediate frequency signal outputted from an intermediate frequency tuning circuit including a tuning capacitor between the output end and ground. It is configured to be input to the emitter of a common base type transistor amplifier circuit via only a coupling capacitor and a resistor in series, and the resistor is provided at a stage before the emitter resistor connected between the emitter and ground. It is being

(Function) Since a coupling capacitor and a resistor are interposed in series between the intermediate frequency tuning circuit and the common base type transistor amplifier circuit, the coupling capacitor and the resistor are connected as a load on the output side of the intermediate frequency tuning circuit. If the resistor and coupling capacitor are replaced with an equivalent parallel resistor and capacitor, the value of the equivalent parallel capacitor will be smaller than the value of the conventional series capacitor, and the tuning It acts to lower the Q of the circuit. Furthermore, since an equivalent parallel resistance is also added, this also reduces the Q of the tuned circuit. Therefore, the bandwidth of the intermediate frequency tuning circuit can be widened. Moreover, the resistance value of this resistor may be small, and the loss can be suppressed to a small value.

(Description of Embodiments) Hereinafter, embodiments of the television channel of the present invention will be described with reference to FIGS. 1 and 2. 1st
The figure shows the television channel of the present invention.
FIG. 2 is a circuit diagram of an embodiment in which a VHF mixing circuit, an intermediate frequency tuning circuit, and an amplifier circuit for amplifying an intermediate frequency signal are combined, and FIG. 2 is a high-frequency equivalent of FIG. 1, especially in an intermediate frequency signal band. It is a circuit diagram.

In FIGS. 1 and 2, circuit elements that are the same as those in FIGS. 3 to 5 are given the same reference numerals, and redundant explanations will be omitted.

1 differs from that shown in FIG. 3 in that a resistor 14 having a small resistance value is interposed between the other end of the intermediate frequency tuning coil 12 and the coupling capacitor 16.

With this configuration, the high frequency equivalent circuit diagram of the television tuner of the present invention is as shown in FIG.
A resistor 14 and a coupling capacitor 16 are connected in series as a load to the output side of the intermediate frequency tuning circuit.

Therefore, even if the capacitance of the coupling capacitor 16 is large, the influence of the resistor 14 on the selectivity Q of the intermediate frequency tuning circuit can be reduced, and the bandwidth of the intermediate frequency tuning circuit can be widened. Moreover, the resistor 14 may have a small resistance value, and the loss due to the resistor 14 can be suppressed to a small value. Note that since the capacitance of the coupling capacitor 16 can be increased, the loss due to the coupling capacitor 16 can be reduced.

(Effects of the Invention) As explained above, in the television tuner according to the present invention, a coupling capacitor and a resistor are interposed in series between an intermediate frequency tuning circuit and a common base type transistor amplifier circuit. Therefore, a coupling capacitor and a resistor are connected in series as a load on the output side of the intermediate frequency tuning circuit, and this resistance reduces the influence of the large capacitance of the coupling capacitor on the selectivity Q of the intermediate frequency tuning circuit. can be reduced, and the bandwidth of the intermediate frequency tuning circuit can be widened. Furthermore, the resistance value of this resistor only needs to be small, resulting in an excellent effect of suppressing loss to a small level.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment in which a VHF mixing circuit, an intermediate frequency tuning circuit, and an amplifier circuit for amplifying an intermediate frequency signal are combined in a television tuner according to the present invention, and FIG. FIG. 3 is a high-frequency equivalent circuit diagram of FIG. 1 in a frequency signal band, and FIG. 3 shows a conventional television tuner in which a VHF mixing circuit, an intermediate frequency tuning circuit, and an amplifier circuit for amplifying the intermediate frequency signal are combined. FIG. 4 is a high frequency equivalent circuit diagram of FIG. 3, and FIG. 5 is a high frequency equivalent circuit diagram of FIG. 3, particularly in an intermediate frequency signal band. 11, 13: intermediate frequency tuning capacitor, 12:
Intermediate frequency tuning coil, 14: resistor, 16: coupling capacitor, 21: transistor.

Claims (1)

[Claims] 1 π equipped with a tuning capacitor between the output end and ground
The intermediate frequency signal output from the type intermediate frequency tuning circuit is input to the emitter of the common base type transistor amplifier circuit through only a coupling capacitor and a resistor in series, and the resistor is A television tuner characterized in that the television tuner is provided before an emitter resistor connected between the emitter and ground.