JPH0828622B2 - High frequency amplifier circuit - Google Patents

Description

TECHNICAL FIELD The present application relates to a high frequency amplifier circuit in a tuner.

2. Description of the Related Art Generally, in a tuner for AM, in order to improve a selection characteristic and raise an image ratio, a tuning circuit is provided in multiple stages and an attenuation of an input signal due to an increase in the tuning circuit is compensated between tuning circuits. Connecting coupling amplifier circuits is well known.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, when a tuning circuit is made to have multiple stages by using a coupling amplifier circuit, the sensitivity difference becomes large particularly in an AM tuner having a wide reception frequency bandwidth, and the performance of the tuner is rather deteriorated. Therefore, there was a fear that it would occur, so there was a limit to the number of stages in which the tuning circuit could be increased.

Means for Solving the Problems Therefore, in order to improve the image ratio, the present application has a plurality of tuning circuits so as to reduce the difference in sensitivity as much as possible and increase the number of stages of the tuning circuit. Between the tuning circuits, the source of the field effect transistor is connected to an amplifier coupling circuit in which a resistor that sets the operating internal resistance to a large value and a series circuit of a coil that suppresses sensitivity at the higher frequency are connected. To do.

Embodiments Detailed description will be given below with reference to the drawings. FIG. 3 shows a high-frequency amplifier circuit in an AM tuner having a conventional configuration including a wide-band amplifier circuit A and a selection circuit B. In FIG. Antenna terminal, (2) is AGC input terminal,
(3) is a bipolar transistor in which the base is connected to the AGC terminal (2) and the emitter is connected to the drain of the field effect transistor (4), and its collector is connected to the first high frequency transformer (5). (6) is an output terminal to the wide band AGC circuit, and (7) is a small-scale connected in series with the primary coil of the first high frequency transformer (5) so as not to affect the selection characteristics of the first tuning circuit described later. A resistor of resistance, which determines the sensitivity characteristics of the broadband AGC circuit. (8) is a first tuning circuit composed of a secondary coil of the first high frequency transformer (5) and a variable capacitance diode, and (9) is a second high frequency transformer, the primary coil of which has an output terminal to a MIX circuit ( 10) is connected. (11) is a second tuning circuit composed of the secondary coil of the second high frequency transformer (9) and a variable capacitance diode.

Then, the signal input from the antenna circuit to the antenna terminal (1) is input to the wideband amplifier circuit A, and this input signal is controlled by the bipolar transistor (3) whose AGC voltage is input to the base, and the controlled signal is , From the output terminal (6) to the broadband AGC circuit.

On the other hand, the tuning signal selected by the first tuning circuit (8) by the tuning voltage Vt is fed from the tap of the tuning coil to the tuning coil of the second tuning circuit (11), that is, the secondary coil of the second high frequency transformer (9). The tuning signal inputted and further selected by the second tuning circuit (11) is inputted from the primary coil of the second high frequency transformer (9) to the MIX circuit via the output terminal (10).

According to the experiment conducted by the applicant in this high frequency amplification circuit, the image ratio was 48db at 1400KHz.

The present application relates to a high-frequency amplifier circuit having the above-described configuration, in which an operating internal resistance between the front and rear tuning circuits is set to a large value, and an amplification coupling circuit that suppresses sensitivity at the higher frequency side is connected. Is shown in FIG. That is, FIG. 1 shows a case where the amplification coupling circuit C is connected between the first tuning circuit (8) and the second tuning circuit (11) described above. The field effect transistor (1) in which the tuning coil tap of (8) is connected to the gate
A resistor (16) for increasing the operating internal resistance of the transistor (15) and a coil (17) for suppressing sensitivity at the higher frequency side are connected in series to the source of 5), and this series circuit is The second high frequency transformer (9) is connected to the tap of the secondary coil, that is, the tuning coil of the second tuning circuit (11).

 The same reference numerals as those in FIG. 3 denote the same members.

Therefore, according to the present application, the resistor (16) and the coil (1
By the connection of 7), the operating internal resistance of the field effect transistor (15) seen from the tuning coil side becomes large and the deterioration of Q of the tuning circuit is suppressed, so that the sensitivity difference with respect to the frequency change of the tuning circuit can be reduced. This means that, for example, when the resistor (16) is omitted, a part of the tuning coil of the second tuning circuit (11) is short-circuited, and Q is lowered, and the sensitivity difference with respect to the frequency change of the tuning circuit. Can be easily understood from the fact that it will be expanded.

Therefore, even if the number of amplifier circuits is increased, it is possible to reduce the sensitivity difference between the reception frequencies and improve the image ratio.

According to an experiment conducted by the applicant, the image ratio in the circuit shown in FIG. 1 was 61db at 1400KHz.

FIG. 2 shows another embodiment of the present application. Specifically, in the configuration shown in FIG. 1, a resistor (16) for increasing the operating internal resistance of the field effect transistor (15).
And a coil (17) for suppressing high frequency sensitivity are grounded, and the drain of the field effect transistor (15) is connected to the primary coil of the third high frequency transformer (18). By the secondary coil of the third high frequency transformer (18) and the variable capacitance diode, the third tuning circuit (1
9), and the tap of the tuning coil is connected to the tuning coil of the second high frequency transformer (9), that is, the second tuning circuit (11).

With this configuration, however, the tuning circuit is increased by one stage as compared with the case of the first embodiment described above, so that the image ratio can be further improved.

According to an experiment by the applicant, the image ratio in the circuit shown in Fig. 2 was 78db at 1400KHz.

EFFECTS OF THE INVENTION As described above, according to the present application, it is possible to provide a high-performance tuner that can reduce the difference in sensitivity that accompanies a change in the reception frequency to improve the image ratio.

[Brief description of drawings]

1 and 2 are circuit diagrams showing an embodiment of the present application, and FIG. 3 is a circuit diagram of a conventional configuration. In the figure, A is a broadband amplifier circuit, B is a selection circuit, C is an amplification coupling circuit, (5) is a first high frequency transformer, (7), (16).
Is a resistor, (8) is a first tuning circuit, (9) is a second high frequency transformer, (11) is a second tuning circuit, (15) is a field effect transistor, (18) is a third high frequency transformer, and (19). ) Is a third tuning circuit.

Claims (1)

[Claims] 1. A resistor having a plurality of tuning circuits, a resistor for setting a large operating internal resistance of a source of a field effect transistor between the tuning circuits in the preceding and following stages, and a coil for suppressing sensitivity at a higher frequency. A high-frequency amplifier circuit characterized by being connected with an amplification coupling circuit in which a series circuit of is connected.