JPS6128405Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an intermediate frequency amplification circuit used in an electronically tuned tuner of a television receiver.
The present invention relates to an intermediate frequency amplifier circuit that stabilizes output waveform characteristics by correcting variations caused by transistors.

Conventionally, the output waveform of the electronic tuning tuner of a television receiver has a monopeak as shown in Fig. 1a, a double hump characteristic as shown in Fig. 1c, and a waveform as shown in Fig. 1b. It is unstable, unlike the desired standard properties as shown in . Therefore, this becomes a cause of variations in image quality of television receivers. The cause of this is the variation in the input capacitance Cin of the buffer amplifier transistor and post amplifier transistor of the electronically tuned tuner, and this input capacitance
The variation in Cin is determined by the transition frequency T of the above two transistors or the DC current amplification factor h.
There is a correlation with _FE , which is shown in Figure 2. Second
In the figure, the dashed line A is the characteristic of the buffer amplifier transistor, and the solid line B is the characteristic of the post amplifier transistor. These characteristics had the disadvantage of affecting the output waveforms as shown in FIGS. 1a and 1c.

This idea was made in view of the above circumstances.
It is an object of the present invention to provide an intermediate frequency amplification circuit that corrects variations in input capacitance of transistors and stabilizes output waveform characteristics using a simple circuit.

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 3 shows an embodiment of the present invention, in which the input end of a cascode mixer circuit 1 is connected to the signal input end 20, and the input end of an output tuning circuit 2 is connected to the output end of the cascode mixer circuit 1. Ru. The output terminal of this output tuning circuit 2 is connected to the base of a buffer amplifier transistor 3. The emitter of the transistor 3 is grounded via a resistor 7,
A collector is connected to the base of a post-amplifier transistor 4 via a DC blocking capacitor 13. The base of this transistor 4 is grounded via a resistor 9 and connected via a resistor 10 to the positive terminal of a DC power supply 15 whose negative terminal is grounded. Connected to the collector. The emitter of the transistor 4 is connected to the tuner output terminal 5 via a DC blocking capacitor 17 and a resistor 12.
grounded through. The collector of the transistor 4 is grounded via a capacitor 16 and connected to the base of the transistor 3 via a resistor 6. The collector of the transistor 4 is connected to the positive terminal of a DC power supply 15 via a resistor 11, and the positive terminal of the DC power supply 15 is grounded via a capacitor 14.

That is, it is assumed that a transistor with a standard DC current amplification factor h _FE is used as the buffer amplifier transistor 3, and a transistor with a large DC current amplification factor h _FE and a transistor with a small DC current amplification factor h FE is used as the post amplifier transistor 4. Now, post amplifier transistor 4
When the DC current amplification factor h _FE is large, a large amount of current flows, so the voltage drop due to the resistor 1 becomes large and the collector potential of the transistor 4 decreases. Therefore, the resistor 6 has a higher resistance than the collector of the transistor 4.
Since a voltage is supplied to the base of the transistor 3 through the transistor 3, the base potential of the transistor 3 decreases, and the emitter current I _E flowing through the transistor 3 decreases. When this emitter current I _E decreases, the input capacitance Cin of the transistor 3 increases as shown in FIG. That is, when the DC current amplification factor h _FE of the transistor 4 is large, the input capacitance of the transistor 4 decreases, but conversely, the input capacitance of the transistor 3 increases, so that the influence on the output waveform is canceled out.

On the other hand, when the DC current amplification factor h _FE of the transistor 4 is small, the input capacitance of the transistor 4 increases, but the input capacitance of the transistor 3 decreases, and the influence on the output waveform is canceled out.

As described above, according to the present invention, it is possible to provide an intermediate frequency amplifier circuit for an electronically tuned tuner that corrects variations in input capacitance of transistors and stabilizes output waveform characteristics using a simple circuit.
Therefore, it is particularly suitable for use in television receivers.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the output waveform of a conventional electronic tuning tuner, Fig. 2 is a diagram showing the input capacitance characteristics of a transistor in a conventional intermediate frequency amplifier circuit, and Fig. 3 is a circuit diagram showing an embodiment of the present invention. , FIG. 4 is a characteristic diagram showing an example of input capacitance with respect to emitter current of a transistor. 1...Cascode mixer, 2...Output tuning circuit, 3...Buffer amplifier transistor, 4...
...Transistor for post amplifier, 6...Resistor, 1
3...DC blocking capacitor, 15...DC power supply.

Claims (1)

[Scope of utility model registration request] A buffer amplifier transistor whose emitter is grounded and whose base is supplied with the output from the mixer circuit;
a common-collector post-amplifier transistor whose base is connected to the collector of the transistor via a capacitor; a resistor connected between the collector of this transistor and the base of the buffer amplifier transistor; An intermediate frequency amplification circuit for an electronically tuned tuner, comprising a resistor connected between a collector and a DC power source, and means for taking out an output from the emitter of the post amplifier transistor.