JPS5930350B2 - Band switching circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an FMAM tuner for stereo equipment.
The present invention relates to a band switching circuit that can smoothly perform band switching of signal processing circuit means having different types of bands, and is also suitable for implementation in a semiconductor integrated circuit.

To switch the bands of signal processing circuit means having different bands, for example, an FMAM tuner of a stereo device, conventionally, the power supplies of the FM and AM circuit blocks and the signals to be passed are switched.

By the way, in conventional band switching circuits for performing such band switching, both the FM circuit block and the AM circuit block, including the power supply terminals, are made independent, and the power supply terminals for both circuit blocks are switched and passed. A configuration is adopted in which a switch circuit is provided to switch the signals to be used. For this reason, not only is it inevitable that the number of switch circuits will increase, but if both circuit blocks are integrated on a single semiconductor substrate to obtain one system circuit block semiconductor circuit, both circuit blocks Two external terminals are required to supply power to operate the device, which limits the number of external terminals that can be used for other functions. Furthermore, when switching between bands, if the DC fluctuation components of the respective band signals are transmitted to the speaker, a switching shock sound is generated, which causes auditory discomfort.
In order to prevent the generation of such shock sounds, it is necessary to add a circuit system that can cut off the passage of audio signals until the circuit system reaches a stable level. There was also the inconvenience of complicating the process. The present invention has been made in view of the above-mentioned inconveniences and problems that existed in the conventional band switching method.The present invention realizes the common use of terminals and can perform band switching while keeping the number of terminals to the minimum necessary. The present invention provides a band switching circuit that can suppress the occurrence of shock noise during switching.

The configuration and operation of the band switching circuit of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an FMAM switching circuit according to an embodiment of the present invention. In the illustrated circuit, an FM intermediate frequency signal is applied to terminal 1, and an AM signal is applied to terminal 2 from an antenna. The FM intermediate frequency signal applied to the terminal 1 is converted into a low frequency signal through the FM intermediate frequency amplification/width amplifier 3 and the FM detector 4, and is turned into a low frequency signal by the signal changeover switch 5.
When the FM side contact a is selected, the signal passes through the signal changeover switch 5, and the stereo demodulation circuit section includes a demodulator 6 and a demodulator 8 having an FMAM detection signal input terminal 7;
The signal is output to output terminals 17 and 18 as a left and right stereo audio signal or a monaural audio signal via a differential connection type switch circuit section constituted by resistors 9 to 16. Further, the AM signal applied to the terminal 2 is transmitted to the high frequency amplifier 1.
It is amplified by 9. Further, the output of the high frequency amplifier 19 is mixed with the signal generated by the local oscillator 21 in a mixing amplifier 20, and is passed through an AM intermediate frequency filter 22 to be mixed with the signal generated by the local oscillator 21.
It is extracted as an intermediate frequency signal. This AM intermediate frequency signal is intermediate frequency amplified by an AM intermediate frequency amplifier 23 and then detected by an AM detector 24. The detection output from the AM detector 24 is generated at the b-side contact of the switch 5, and when the switch 5 is switched from the FM side to the AM side and the b-side contact is selected, the switch 5 and the stereo demodulation circuit section are It passes through and is taken out as a monaural signal from terminals 17 and 18. In the figure, 25 is an AMAGC signal application terminal, 26 is an oscillation coil of the local oscillator 21, and 27 is a constant current source circuit. Note that switch 284 is connected to switch 5. When the b-side contact is selected, it operates to supply the power supply voltage applied to the terminal 29 to the mixing amplifier 20 and the local oscillator 21.By the way, when performing AM reception in such a basic circuit block, Switches 5 and 28 select contact B.15d, which is the AM side contact. By selecting this contact, power supply voltage is applied to AM local oscillator 21 from terminal 29 through contact d and oscillation coil 26. Note that the terminal The voltage at 29 is approximately the same value as the voltage at the collector of transistor 30.

A voltage V determined by the division ratio of resistors 31 and 32 is supplied from the emitter of the transistor 30 to the base of a transistor 33 constituting a differential amplifier. On the other hand, a voltage lower than the voltage applied to the terminal 35 by the base-emitter voltage of the emitter-follower transistor 36 is applied to the base of the transistor 34 constituting the differential amplifier through the division ratio of the resistor 37, resistors 38, 39, and diode 40. A bias voltage VO determined by is supplied. Therefore, for the voltage when the AM reception state is selected. If it is chosen to be sufficiently low, transistor 33 will be conductive, while transistor 34 will be in a low state. A current from a constant current source constituted by the transistor 41 and the resistor 42 flows to the resistor 43 via the transistor 33, and the transistor 44 becomes conductive. On the other hand, no current flows through the resistor 45 because the transistor 34 is temporarily disconnected.
6 is broken. Further, when the transistor 44 becomes conductive, a current flows through the resistor 47, and a voltage drop occurs across the resistor 47. Transistors 481-48 whose collectors are connected to each bias supply point of the FM signal system. This voltage is applied to the switch circuit section 49 composed of transistors 481-48. are conductive, so F
The bias supply point of the M signal system is the transistor 48, ~48
It is grounded through n and its potential becomes approximately zero volts. On the other hand, when the FM reception state is selected, the supply of power supply voltage to the oscillation coil 26 is cut off by the action of the switch 28 described above, so that the transistor 30 is briefly cut off.
The base voltage V1 of the transistor 33, which was obtained by dividing the emitter potential by the resistors 31 and 32, decreases.
That is, the relationship between the base voltages of the transistors 33 and 34 is V, which is opposite to that during AM reception. 〉〉V, and the transistor 34 becomes conductive, while the transistor 33 becomes in a state of being turned off. Therefore, all the current from the constant current source flows through the resistor 4.
5, which causes transistor 46 to conduct and transistor 44 to turn on. That is, current flows through the resistor 50 and a voltage drop occurs. Transistors 511 to 5 whose collectors are connected to the bias supply point of the AM signal system
Since this voltage is applied to the switch circuit section 52 composed of transistors 51 to 51n, all of the transistors 51 to 51n are conductive, and the bias supply point of the AM signal system is connected to the transistors 51 to 51n.
It is grounded through ~51n and its potential becomes approximately zero volts. According to the present invention, the bias supply to the FM signal system and the AM signal system is selectively switched in accordance with the above circuit operation, and furthermore, this switching operation is performed by the AM oscillation coil 26.
AM system and FM system
It is no longer necessary to provide independent power supply terminals for each of the systems, and the number of terminals is therefore reduced by one. Further, in the band switching circuit of the present invention, the occurrence of shock noise during FMAM switching is suppressed by the functions of the circuit parts described below. That is, although it has already been mentioned that the transistor 44 is conductive under the AM reception state, the power supply voltage applied to the oscillation coil 26 through the switch 28 is also applied to the filter transformer 22 via the resistor 53 and the capacitor 54. It is also supplied as the power supply voltage for the mixing amplifier 20. Further, this power supply voltage is also applied to the base of the transistor 55.

Therefore, in the AM reception state, a voltage approximately equal to the collector voltage is supplied to the base of the transistor 65. Therefore, a voltage V2 obtained by dividing the emitter voltage of transistor 55 by resistors 56 and 57 is applied to the base of one transistor 58 constituting the differential amplifier. The voltage V applied to the base of the transistor 34 is applied to the base of the other transistor 59 constituting the differential amplifier. is applied. Therefore, under AM reception conditions, the above voltages V2 and V.

If the magnitude of voltage 2 is selected so that the relationship V2>>VO holds between
The transistor 8 becomes conductive, while the transistor 59 becomes almost off. Under such circuit conditions, the transistor 60 and the resistor 61
Most of the current in the constant current source circuit configured by the transistor 58 flows to the transistor 58, and then to the differential amplifier configured by the transistors 62 and 63. By the way, as already mentioned, during AM reception, the transistor 44 is conductive and the transistor 46 is briefly disconnected, and the transistor 63 for forming the differential amplifier is conductive, while the transistor 6
2 becomes in the off state, and the current flowing through the transistor 58 passes through the transistor 63 and exits to the power supply. Therefore, the resistor 6 connected to the collector of the transistor 62
No current flows through the transistor 4, and the transistor 65, to which the voltage generated across the resistor is applied as a base bias voltage, remains in an OFF state. a transistor 67 whose base is connected to the collector of the transistor 65 via a resistor 66;
It is once again in a disconnected state, and the voltage at point A, to which its collector is connected via resistor 68, is maintained at a predetermined voltage. By the way, the voltage VA at point A is determined by the emitter voltage of a transistor 73 whose base bias is determined by a constant current circuit consisting of a transistor 70 whose base is connected to a fixed bias supply terminal 69 and a resistor 71 and a resistor 72 and a resistor 74. 75, and this voltage VA is higher than the voltage VB at point B connected to the emitter of transistor 79, whose base bias is determined by a constant current circuit composed of transistor 76 and resistor 77, and resistor 78. Highly selected.

The voltages at points A and B are applied as differential base inputs to a differentially connected switch circuit section consisting of transistors 9 to 16 constituting the output stage of the stereo demodulation circuit section. That is, as shown in the figure, transistors 9, 12, 1
The voltage at point A is applied to the bases of transistors 3 and 16, while the voltage at point B is applied to the bases of transistors 10, 11, 14 and 15.

As already explained, during AM reception, the voltage VB at point B is selected to be lower than the voltage VA at point A, so transistors 9, 12, 13, and 16 are conductive while
Transistors 10, 11, 14, and 15 are turned off, and the AM signal input to the stereo demodulation circuit through switch 5 passes through transistors 9 and 16 and is applied to terminals 17 and 18.

By the way, when I switched switches 5 and 28 to the FM side,
The voltage supplied via the power supply switch 28 suddenly drops to a value close to zero volts.

Therefore, the bias supplied from the transistor 30 decreases, and one of the transistors 80 and 81 that constitutes the transistors 33, 44, and 63 and the differential amplifier, the transistor 80, suddenly turns off. Also, the voltage V1 applied to the base of the transistor 33 and the voltage applied to the base of the transistor 34 by switching the switch.
The size relationship is V. 〉〉V, pair of transistors 33 and 34, transistors 43 and 44, and transistor 6
2, 63 and transistors 80, 81 are in the operating state A.
The transistors 34, 46, 62 are reversed from when receiving M.
and 81 are conductive. Furthermore, transistor 82 also becomes conductive. Also, the bias supplied to mixing amplifier 20 does not immediately go to zero volts when the switch is toggled, but decreases as the charge on capacitor 54 is discharged through resistor 83 and transistor 82. Due to the presence of this discharge circuit section, the transistor 58
In a differential amplifier consisting of
8 is in a conductive state. transistor 60
The current flowing through the constant current circuit formed by the resistor 61 and the resistor 61 flows to the resistor 64 through the transistor 58 and the transistor 62 during the time T1 during which the transistor 58 is conductive.

Therefore, the transistor 65 whose base is connected to the connection point between the transistor 62 and the resistor 64 also becomes conductive for the time T1. Furthermore, the transistor 67 whose base is connected to the transistor 65 also becomes conductive for the time T1, and the voltage VA at the point A becomes approximately at the potential of the ground point for the time T1. Therefore, the voltage VA at point A and the voltage V at point B
The magnitude relationship of B is also reversed, and the relationship of VA is established over time T1, and among transistors 9 to 16 forming the differential type switch circuit section, transistors 10, 11, and 14
, 15 are conductive, and the FM signal input to the stereo demodulation circuit section by switching the switch 5 is transmitted to the transistor 10
and 15 and flows to the power supply line. That is, only DC current flows through the terminals 17 and 18 from the time the switch is turned until the time T has elapsed. By the way, when the time T, elapses, the transistor 5
On the other hand, the transistor 59 becomes conductive, and the current flowing through the resistor 64 disappears, so that the transistors 65 and 67 return to the cut-off state.

However, there is a capacitor 84 between point A and ground.
is connected, the potential at point A rises according to the charging time constant determined by capacitor 84 and resistors 74 and 75, and when the predetermined time T2 has elapsed, the voltage at point A
becomes higher than the voltage VB at point B. Therefore, the operating state of the transistors 9 to 16 forming the differential switch circuit section is such that the signal is passed through the terminals 17 and 18 after the time T2 has elapsed since the transistors 65 and 67 were finally disconnected. When this state is established, FM audio signals are taken out to terminals 17 and 18. That is, the FM signal to the output terminals 17 and 18 is blocked from the time the switch is switched from the AM side to the FM side until the time T1+T2 has elapsed, suppressing the occurrence of shock noise. In addition, F
When the M reception state has settled down, the transistors 33, 44, 63, 80, 58, 6
5 and 68 are turned off, while transistor 34
, 46, 62, 81, 59, and 82 are kept conductive, so that the transistor 9 of the differential switch circuit section
, 12, 13, 16 are conductive, transistors 10, 11,
14 and 15 are held in a closed state. Next, when the switches 5 and 28 are switched from the FM side to the AM side, the power supply voltage is applied to the oscillation coil 26 at the same time as the switch is switched.

Therefore, transistors 33, 44, 63 and 80 are rendered conductive, while transistors 34, 46, 62, 81 and 82 remain conductive. By the way, the bias of the mixing amplifier 20 does not become the power supply voltage instantaneously, but increases according to the charging time constant of the resistor 53 and capacitor 54 from the time of switching the switch. The base voltage V2 of the transistor 58 is maintained from the time the switch is switched until the time T1 elapses.
is the perspective voltage V of the transistor 59.

A lower state occurs during which transistor 58 is briefly turned off while transistor 59 is conductive. Therefore, a constant current flows to the resistor 64 through the transistor 59 and the transistor 80 of the differential amplifier composed of transistors 80 and 81, and the transistors 65 and 67 become conductive. That is, the voltage VA at point A becomes lower than the voltage VB at point B, and the transistors 10, 11,
14 and 15 become conductive, and a circuit state is established in which the AM signal is passed to the power supply line.

After a certain period of time T, a circuit state is established in which the transistors 65 and 67 are gradually disconnected, but the state of the differential switch circuit is maintained for an additional time T2 by the capacitor 84, similar to when switching from AM to FM. After that, the differential switch circuit is in a state of passing the AM signal to the output terminals 17 and 18.

This operation also suppresses the occurrence of shock noise when switching from AM to FM. Figures 2A and 2b are diagrams showing changes in voltage at the time of band switching in the switching circuit of the present invention that has been explained above. shows the changes in voltages VA and VB.

Note that a voltage waveform V67 in FIG. 2b shows a change in the base voltage of the transistor 67. As explained above, according to the circuit of the present invention, the power supply terminals of the AM circuit block and the FM circuit block are shared, and especially when both circuit blocks are integrated into an IC, external terminals used for other functions can be used. The number of terminals can be increased, and the occurrence of shock noise during switching can be effectively suppressed.

Note that although the above explanation has been made using FMAM band switching as an example, the present invention is not limited to this example. In addition, the above explanation applies to the capacitor 84 and the same 54.
Although the shock noise was removed based on the delay in the charging and discharging time due to the combination with the capacitor 54, it is of course also possible in principle to use the charging and discharging time of the capacitor 54 alone.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an AMFM switching circuit of the present invention, and FIGS. 2A and 2B are diagrams showing potential waveforms of important parts when switching the AMFM. 1...FM intermediate frequency signal input terminal, 2...
...AM signal application terminal, 3...FM intermediate wave amplification circuit section, 4...FM detection circuit section, 5...
... Signal changeover switch, 6, 8 ... Demodulator, 7 ... Subcarrier input terminal, 9 to 16 ...
...Differential connection type switch circuit section, 17, 18...
・Output terminal, 19...AM high frequency amplifier, 20
...AM mixing amplifier, 21 ...AM local oscillator, 22 ...AM intermediate frequency filter, 2
3...AM intermediate frequency amplifier, 24...
AM detector, 25... AMAGC signal application terminal, 26... oscillation coil, 27... constant current source circuit section, 28... power supply changeover switch,
29...Power terminal, 30, 36, 55, 73,
79...Bias determining transistor, 31,
32, 37, 38, 39, 56, 57, 74, 75...
...Resistance for bias determination, 33, 34, 58, 59
, 62, 63, 80, 81...Transistor for differential amplifier configuration, 35, 69...Fixed base bias application terminal, 40...Diode for bias determination, 41, 60, 70, 76... Constant current source transistor, 42, 61, 71, 77...
Constant current source resistor, 43, 45, 47, 50, 64, 66
, 68, 72, 78, 83... Collector resistance,
44, 46, 65, 67, 82...Switching transistor, 481-48n, 51,-51n.
...Switching transistor, 49
, 52... Switch circuit section, 53... Resistor for determining charging time constant, 54, 84... Capacitor for charging/discharging.

Claims (1)

[Claims] 1. A first switch means for selectively switching the supply of signal outputs of the first signal system and the second signal system to the output circuit section, and in conjunction with this, a power supply path to the second signal system. a second switch means for opening and closing; a first switch circuit section that operates simultaneously with the opening and closing of the second switch means; and a charging/discharging circuit; a second switch circuit section that operates with a predetermined time delay from the opening/closing point of the switch; and a second switch circuit section that operates with the output of the first switch circuit section and operates the bias supply point of the first and second signal systems. Two bias potential setting circuit sections that set the potential and the non-operating potential to mutually opposite potentials, and the output of the second switch circuit section operate with the output of the second switch circuit section, and the signal passing through the output circuit section is controlled by the first switch. 1. A band switching circuit comprising: a signal passage control circuit section that cuts off signal passage for a predetermined period of time from the point in time when the means is switched.