JPS6019184B2 - Audio FM demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an audio FM demodulator in which an audio FM detection circuit, a differential emphasis circuit, and an output circuit are integrated into an integrated circuit.

FIG. 1 shows a circuit diagram of a television receiver using a conventional integrated circuit element for audio FM demodulation.

In the conventional integrated circuit device 1, the audio FM detection circuit 2
An emitter follower circuit consisting of a transistor 3 and a resistor 4 receives the output of the emitter follower circuit, and the output is applied to a low frequency amplification output section 5 to drive a speaker 6. Resistor 7 is a load resistance of audio FM detection circuit 2, and also has a resistance with capacitor 8.
It constitutes an enhancement circuit and also acts as a base bias resistor for transistor 3. When the integrated circuit element 1 having such a configuration is used in a television receiver, the vertical deflection circuit section 9 and the horizontal deflection circuit section 10
DC power is supplied to the integrated circuit element 1 from a power supply 11 that has a common power source 11.

Therefore, a ripple voltage due to the current of the deflection circuit sections 9, 10, etc. is present at point A. When the power supply voltage with this ripple voltage superimposed is supplied to the integrated circuit element 1, the resistor 7
As mentioned above, is also the base bias resistance of the transistor 3, so this ripple voltage component is applied to the base of the transistor 3 and appears as a ripple voltage at the output point B of the integrated circuit element 1. It is amplified by the low frequency amplification output section 5 and outputted from the speaker 6 to produce deflection current sound, especially vertical deflection current sound (hereinafter referred to as V sound).
generates a harsh noise. That is, even a small ripple voltage is amplified and is perceived as a large V sound. In order to eliminate this V sound, in a television receiver using a conventional integrated circuit element 1, a resistor 12 shown in FIG. A decoupling circuit consisting of a capacitor 13 and a capacitor 13 was provided.

However, since the frequency of the vertical deflection current is low (60Hz in Japan and the United States), the capacitor 13 needs to have a large capacity, which is not only expensive but also large in size.
There was also a drawback that there were large restrictions on component placement. Therefore, it is an object of the present invention to provide an audio FM demodulator that is implemented as an integrated circuit element and does not require a decoupling circuit for removing ripples in the power supply voltage.

As mentioned above, the V sound appears at point A in FIG. 1 and is caused by the pull voltage component appearing at the base of the transistor 3 through the resistor 7 when the integrated circuit element 1 is also supplied.

Therefore, if the 1'' Z pull voltage component can be prevented from appearing at the base of the transistor 3, such V noise will not occur. To do this, the value of the resistor 7 can be increased, but as mentioned above, this resistor 7 serves as the DC and AC load for the audio FM detection circuit 2, and at the same time serves as the base bias resistance of the transistor 3. Therefore, there are also restrictions on the manufacturing method of integrated circuit elements, and it is not possible to make them extremely large. That is, in integrated circuit elements, it is difficult to produce resistors with high resistance and high resistance. Furthermore, since the resistor 7 and the capacitor 8 also constitute a differential defense circuit, the accuracy of the resistance value is also required. in this way,
In the audio FM demodulator using the conventional integrated circuit element 1, the resistor T is the load of the audio FM detection circuit 2, is also the base bias resistor of the transistor 3, and is also the resistor for de-enphasis. Because it had three functions: , the degree of freedom in designing this resistance value was restricted. FIG. 2 shows a voice F according to an embodiment of the present invention.
1 is a circuit diagram of a television receiver using an M demodulator.

That is, in this embodiment, the load of the audio FM detection circuit 22 that performs FM detection of the audio intermediate frequency signal in the integrated circuit element 21 is replaced by the constant current source circuit 23, and the detected output signal is based on the constant current source circuit 23. is supplied from a constant current source circuit 23 to the base of a first emitter follower circuit consisting of a transistor 24 and a resistor 25 to which is connected. Furthermore, the resistor 26 of the de-emphasis circuit is connected to the emitter on the output side, and de-emphasis is performed by the capacitor, and the de-emphasis output audio signal is connected to the output terminal of the de-emphasis circuit. The signal is supplied to a second emitter follower circuit consisting of a transistor 27 and a resistor 28, the base of which is connected to the transistor 27, and is output from the emitter of the transistor 27 to an output point B. The output audio signal is amplified by a low frequency amplification output section 5, and V sound interference is removed by driving a speaker 6. More specifically, a ripple voltage is similarly generated in the power supply at point A due to the currents in the deflection circuit sections 9 and 10 as described above.

The power supply voltage including this ripple voltage component is applied to the integrated circuit element 2.
However, since the load of the audio FM detection circuit 22 and the base bias supply source of the transistor 24 are the constant current source circuit 23, the above-mentioned ripple voltage component does not appear at the base of the transistor 24. That is,
Since the constant current source circuit 23 has a very large alternating current internal impedance, this corresponds to setting the resistance value of the resistor 7 shown in FIG. 1 to a very large value. The ripple voltage component in transistor 2
This is because it is prevented from appearing at the base of 4. Note that since the constant current source circuit 23 has a low DC internal impedance, there is no problem in supplying DC current to the audio FM detection circuit 22 and supplying bias current to the transistor 24. Furthermore, since a de-emphasis circuit consisting of a resistor 26 and a capacitor 8 is connected to the emitter on the output side of the first emitter-follower circuit consisting of a transistor 24 and a resistor 25, even if the constant current source circuit 23 is used. There is no problem with de-emphasis operation.

Furthermore, these constant current source circuit 23 and transistor 24
The first emitter-follower circuit consisting of the resistor 25 and the resistor 26, the second emitter-follower circuit consisting of the transistor 27 and the resistor 28 can be configured with constants suitable for integration, and can be used as the integrated circuit element 21. Since it can be configured easily, there is no need to increase the number of peripheral circuits. FIG. 3 shows a specific example of the structure of the integrated circuit element 21 and its peripheral circuits.

In FIG. 3, the audio intermediate frequency signal output from the limiter amplifier 100 is a differential peak type audio FM signal.
It is applied to the input of the detection circuit 22 (the base of the transistor Q, 2), and at the same time it is phase-shifted by the external phase shift circuit 101 and applied to the other input of the audio FM detection circuit 22 (the base of the transistor Q, 7). be done.

The frequency characteristics of each input are shown in Figure 4, and these are the diode characteristics between the bases and emitters of transistors Q, 3, Q, and 6, and the capacitors C4 and C4, each having a small capacity (approximately one stage PF). Peak detection is performed by C5, which is applied to the differential amplifier of transistors Q, 4, Q, 5 to calculate the difference, and a demodulated detection output signal appears at the collectors of transistors Q, 5. capacitor C6
is a small capacity (about 30PF) capacitor for removing audio intermediate frequency components. Since capacitors C4, C5, and C6 are all of small capacity, they can easily be connected to the integrated circuit element 21.
created within. A current mirror type constant current source circuit 23 is configured by the transistors Q22 and Q22 on the Q side, and this constant current source circuit 23 serves as a load for the audio FM detection circuit 22, and the detection is carried out from the transistor Q side to the base of the transistor 24. Output signal and base.

A bias voltage is supplied. In FIG. 3, a constant current source circuit is connected as a load to the collectors of transistors Q and 5 of the audio FM detection circuit 22, and the remaining transistors Q, 2, Q, 3, Q, 4, Q, 6, Q , 7 are directly supplied with power.

In this way, it is common practice in differential amplifier circuits to supply power only to the output transistors through the load and directly to the other transistors. Even if power is directly supplied to parts other than the circuit, there will be no problem since it is a differential amplifier configuration. That is, in this circuit configuration, the bases of transistors Q, 2 and Q, 7 are in phase in terms of DC and low frequency through the coil of the phase shift circuit 101, and the bases of transistors Q, 3 and Q, 6 are in phase. Resistors R, 6 and R, 9
It is connected to a common power supply through a common power supply to be in phase with respect to DC and low frequencies, and is further connected to the emitters of transistors Q, 3 and Q, 6, and is connected to a common power supply for DC and low frequencies other than the detected signal. Therefore, the ripple voltage component of the power supply appears as a common-mode component at the base of each pair of transistors Q, 2 to Q, 7, and the common-mode component cancellation effect of the differential amplifier causes the ripple voltage component of the power supply to appear in the output signal as a common-mode component. No ripple voltage component appears. Note that the transistor Q,
The ripple voltage component of the power supply is applied to the collector of transistor Q4, whereas the ripple voltage component of the power supply is not applied to the collectors of transistors Q and 5 due to the action of the constant current source circuit 23. , the output signal of the differential amplifier changes only by changes in the voltage difference between the bases of transistors Q,4 and Q,5 and changes in the current value of transistors Q,9, and changes in the voltage at the collector of transistors Q,4. Therefore, the ripple voltage component of the power supply does not appear in the output audio signal of the collectors of the transistors Q and 5.

As described above, according to the audio FM demodulator of the present invention,
A decoupling circuit for V-sound removal is not required, which is very advantageous in terms of the number of parts, manufacturing man-hours, and cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a television receiver using a conventional audio FM demodulator, FIG. 2 is a circuit diagram of a television receiver using an embodiment of the audio FM demodulator of the present invention, and FIG.
The figure is a specific circuit diagram thereof, and FIG. 4 is a frequency characteristic diagram of its input. 8...Capacitor, 21...Integrated circuit element, 22...Audio FM detection circuit, 23...
・Constant current source circuit, 24, 27...transistor, 25,
26, 28...Resistance. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. An FM detection circuit to which a constant current source circuit is connected as a load and performs FM detection of an audio intermediate frequency signal; and an FM detection circuit whose base is connected to the constant current source circuit on the output side of the FM detection circuit, A first emitter to which a detection output signal is supplied from the constant current source circuit to this base.
a follower circuit, a day emphasis circuit which is connected to the emitter of the first emitter-follower circuit and is made up of a resistor and a capacitor; and a base is connected to the output terminal of the day emphasis circuit and the audio signal is taken out from the emitter. and a second emitter follower circuit, the FM detection circuit other than the capacitor, the constant current source circuit, the first and second emitter follower circuits, and the day emphasis circuit are included in an integrated circuit element. The capacitor is externally connected to the integrated circuit element, and the constant current source circuit and the first and second emitters are connected to each other.
An audio FM demodulator characterized in that a power supply terminal of a follower circuit is connected to a power supply containing a ripple voltage.