JPH021449B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to a muting circuit, and more particularly to a muting circuit for eliminating switching noise generated when switching a band switching switch of an FM radio receiver. . (b) Prior art In an FM radio receiver, FM broadcast reception and
When a band switching switch is operated to switch between AM broadcast reception and AM broadcast reception, switching noise is generated in response to the operation. Therefore, in order to prevent the switching noise,
A measure has been taken to perform muting when operating the changeover switch. however,
Merely blocking the signal by muting cannot eliminate the noise caused by the bias change, so some kind of countermeasure is required. FIG. 1 shows a muting circuit in which such a countermeasure has been taken. If the switch 1 is in the state shown by the solid line (mutating off state), the first current source transistor 2 is turned on. , the input signal applied to the input terminal 3 is amplified by a first differential amplifier circuit 6 consisting of a first and a second transistor 4 and 5 whose emitters are commonly connected to the collector of the first current source transistor 2. , a current inversion circuit 9 consisting of a diode 7 and a transistor 8, and an output transistor 10 to an output terminal 11. At that time, the first
The base of the transistor 4 is connected to the bias power supply 12, and the base of the second transistor 5 is connected to the output terminal 11 for negative feedback, so that the first differential amplifier circuit 6 has a stable bias voltage. in a state. Meanwhile, at that time, a third transistor 13 whose base is connected to the bias power supply 12
and a fourth transistor 14 having an emitter commonly connected to the emitter of the third transistor 13 and a base connected to the output terminal 11, and a collector connected to the common emitter of the third and fourth transistors 13 and 14. Since the switch 1 is in the muting off state, the second differential amplifier circuit 16 for mutating, which is composed of the second current source transistor 15 having a current source transistor 15, is in an inoperative state. Now, for example, if you operate the band changeover switch, the switch 1 will change to the state indicated by the dotted line (mutating on state), the first current source transistor 2 will be turned off, and the second current source transistor 2 will be turned off.
Current source transistor 15 is turned on. As a result, the first differential amplifier circuit 6 becomes inactive, and the second differential amplifier circuit 16 becomes active.
The input signal applied to the input terminal 3 is blocked by the first differential amplifier circuit 6 and is no longer transmitted to the subsequent stage, resulting in a muting-on state. In that case,
The third transistor 13 of the second differential amplifier circuit 16
The base of the fourth transistor 14 is connected to the bias power supply 12 and a predetermined bias voltage is applied thereto, and the base of the fourth transistor 14 is connected to the output terminal 11 for negative feedback, so that the DC voltage at the output terminal 11 There is little variation and less switching noise. However, in the circuit shown in FIG. 1, in order to switch one of the first and second differential amplifier circuits 6 and 16 into an active state and the other into an inactive state when mutating is on or when muting is off,
When switching, the first and second differential amplifier circuits 6 and 1
However, the bias of 6 becomes unstable, and switching noise still occurs. (c) Purpose of the invention The present invention has been made in view of the above-mentioned points.
It is an object of the present invention to provide a muting circuit that can perform reliable muting without switching between two differential amplifier circuits, and can also reliably prevent switching noise from occurring. (D) Configuration of the Invention The muting circuit according to the present invention includes a first differential amplifier circuit that amplifies an input signal, a second differential amplifier circuit to which an output signal of the first differential amplifier circuit is applied, and a muting circuit that amplifies an input signal. In order to do this, a switch is inserted between the bias power supply and the base of one of the differential transistors constituting the second differential amplifier circuit. and a bias circuit that supplies a base bias voltage of the second differential amplifier circuit during muting. (E) Embodiment FIG. 2 is a circuit diagram showing an embodiment of the present invention.
17 is a first transistor 20 whose base is connected to an input terminal 19 via an input capacitor 18;
a diode-connected second transistor 21 whose emitter is commonly connected to the first transistor 20;
and the first and second transistors 20 and 21
a current source transistor 22 whose collector is connected to a common emitter of
A first differential amplifier circuit 26 has a base commonly connected to the base of the second transistor 21, and a first current inverting circuit 25 consisting of a diode 23 and a transistor 24 that supply the collector of the second transistor 21. A third transistor 27 to which the output signal of the first differential amplifier circuit 17 is applied, a fourth transistor 28 whose emitter is commonly connected to the third transistor 27, and a common transistor between the third and fourth transistors 27 and 28. A second current inversion circuit 32 includes a current source 29 connected to the emitter, and a second current inversion circuit 32 that inverts the collector current of the third transistor 27 and supplies it to the collector of the fourth transistor 28 and a diode 30 and a transistor 31. 33 is an output transistor whose base is connected to the collector of the fourth transistor 28; 34 is the output transistor 33;
An output terminal 35 is connected to the emitter of the current source transistor 22 and connected to the base of the fourth transistor 28 for negative feedback. A muting switch 37 for applying a voltage to the current source transistor 22 includes a fifth transistor 38 whose base is connected to the other fixed end of the mutating switch 35;
A third current inverting circuit 41 consisting of a diode 39 and a transistor 40 that inverts the collector current of the transistor 38, the output current of the third current inverting circuit 41 is applied to the base, and the emitter is connected to the base of the third transistor 27. A bias circuit consisting of a sixth transistor 42 and a sixth transistor 43,
The first transistor 20 via the bias resistor 44
This is a second bias power supply for supplying a bias voltage to the base of the third transistor 27 and also supplying a bias voltage to the base of the third transistor 27 via the sixth transistor 42. Next, the operation will be explained. Now, if the muting switch 35 is in the state shown by the solid line (mutating off state), the base of the current source transistor 22 is appropriately biased by the first bias power supply 36, and the base of the first transistor 20 is biased properly. Since it is appropriately biased by the second bias power supply 43, the first differential amplifier circuit 17 is in a normal operating state. Further, since the current source 29 operates normally and the base of the third transistor 27 is appropriately biased by the second transistor 21, the second differential amplifier circuit 26 is also in a normal operating state. Therefore, the input signal to be amplified applied to the input terminal 19 is differentially amplified by the first differential amplifier circuit 17 , and then is transmitted from the collector (base) of the second transistor 21 to the second differential amplifier circuit. 3rd of 26
The signal is applied to the base of the transistor 27, and after being differentially amplified by the second differential amplifier circuit 26 , it is led out to the output terminal 34 via the output transistor 33. The first differential amplifier circuit 17 operates as an amplifier circuit with a gain of "1" because the collector and base of the second transistor 21 are short-circuited to form a diode connection. The input signal is applied as it is from the collector (base) of the second transistor 21 to the second differential amplifier circuit 26 . Furthermore, as mentioned above, since the second transistor 21 is diode-connected, when the bias voltage of +V _B is supplied to the base of the first transistor 20 from the second bias power supply 43, the second transistor 21 follows it. The base bias voltage of the second transistor 21 also becomes +V _B ,
The voltage +V _B also becomes the base bias voltage of the third transistor 27 of the second differential amplifier circuit 26 .
In the second differential amplifier circuit 26 , the emitters of the third and fourth transistors 27 and 28 are connected to a current source 29.
However, since the base of the fourth transistor 28 is connected to the output terminal 34 and negative feedback is performed, it performs a stable amplification operation as a negative feedback amplifier circuit. Next, when the mutating switch 35 is operated to switch to the state indicated by the broken line (mutating on state), the current source transistor 22 is turned off and the operating current of the first differential amplifier circuit 17 is no longer supplied. . Therefore, the first and second transistors 20 and 21 are also turned off, and as a result, the input signal applied to the input terminal 19 is blocked by the first differential amplifier circuit 17 , and an output signal is generated at the output terminal 34. disappears, and miruteing is achieved. At the same time, the fifth transistor 38 of the bias circuit 37
A bias voltage from the first bias power supply 36 is supplied to the base of the fifth transistor 38, and the fifth transistor 38 is turned on. When the fifth transistor 38 is turned on, its collector current is transferred to the third current inversion circuit 41.
is inverted at and supplied to the base of the sixth transistor 42. Therefore, the sixth transistor 42 is turned on, and the bias voltage +V _B from the second bias power supply 43 is supplied to the base of the third transistor 27 via the collector-emitter of the sixth transistor 42 . Therefore, the base of the third transistor 27 is connected to the muting switch 3.
The second differential amplifier circuit 26 is biased with the same bias voltage +V _B before and after switching 5.
The bias state does not change and no noise is generated. FIG. 3 shows another embodiment of the present invention,
A bias circuit 45 that supplies the base bias voltage of the third transistor 27 of the second differential amplifier circuit 26 when muting is on is connected at its base to the second bias power supply 43 and whose collector is commonly connected to the collector of the first transistor 20. a diode-connected eighth transistor 47 whose base is commonly connected to the base of the third transistor 27 and whose collector is commonly connected to the collector of the second transistor 21; A ninth transistor 48 serving as a current source is connected to the common emitter of the eighth transistors 46 and 47, and the emitter is connected to the ground, and the base is connected to the other fixed end of the muting switch 35. . In FIG. 3, circuit elements that are the same as those in FIG. 2 are given the same figure numbers and their explanations will be omitted. In FIG. 3, if the muting switch 35 is in the muting off state, the first and second differential amplifier circuits 1
7 and 26 operate normally, and an output signal is obtained at the output terminal 34. At that time, since no bias voltage is supplied to the base of the ninth transistor 48,
The ninth transistor 48 is off, and the seventh
and the eighth transistors 46 and 47 are also turned off,
The bias circuit 45 includes the first and second differential amplifier circuits 1
7 and 26 . When the mutating switch 35 is switched to the muting on state as shown by the broken line in the figure, the current source transistor 22 is turned off and the first
As the differential amplifier circuit 17 becomes inoperative,
The ninth transistor 48 is turned on, and accordingly, the seventh and eighth transistors 46 and 47 are also turned on. Therefore, the bias voltage +V _B is supplied to the base of the third transistor 27 of the second differential amplifier circuit 26 via the bias circuit 45 , and as in the case of FIG. 2, no change in the bias voltage occurs. No noise is generated. (F) Effects of the Invention As described above, the present invention has the advantage that accurate muting operations can be performed and switching noise can be completely prevented. Further, according to the present invention, since the bias voltage of the second differential amplifier circuit does not change even if the muting switch is changed, there is an advantage that no new noise is generated. Furthermore, according to the present invention, it is possible to increase the load impedance of the first differential amplifier circuit when muting is off to reduce loss, and to connect the bias power source from the base of the third transistor 27 when mutating is on. Since the observed impedance can be made small, the third transistor 2
This has the advantage that fluctuations in the base voltage of No. 7 can be minimized. In FIG. 3, since the circuit configuration of the first differential amplifier circuit and the circuit configuration of the bias circuit are the same, the base bias voltages of the second differential amplifier circuit are more equal.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional muting circuit, FIG. 2 is a circuit diagram showing an embodiment of the present invention,
and FIG. 3 are circuit diagrams showing another embodiment of the present invention. Explanation of main figure numbers, 17 ...First differential amplifier circuit,
20, 21, 27, 28...first to fourth transistors, 26 ...second differential amplifier circuit, 35...mutating switch, 36, 43...bias power supply, 37 , 45 ...bias circuit.

Claims (1)

[Claims] 1 a first differential amplifier circuit to which an input signal is applied;
When the first differential amplifier circuit is operating, the first
a second differential amplifier circuit that is supplied with a bias voltage from the differential amplifier circuit and is connected to amplify the output signal of the first differential amplifier circuit;
It consists of a switch means for stopping the operating current of the differential amplifier circuit, and a bias circuit for supplying a bias voltage to the second differential amplifier circuit when the first differential amplifier circuit is not operating, The circuit includes a transistor inserted between a bias power supply for biasing the first differential amplifier circuit and a base of one of a pair of transistors constituting the second differential amplifier circuit, and The switch means is operated to disable the first differential amplifier circuit to perform muting, and the switch means is operated to operate the bias circuit to supply a bias voltage to the second differential amplifier circuit. A mutating circuit characterized by: