JPS6262085B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a balance circuit that prevents shock noise from being generated by a variable impedance circuit even when a large-level burst input signal arrives. In tape recorders or FM radio receivers, in order to reduce noise in high frequencies, when recording or at a broadcasting station, the signal level of high frequencies is increased by encoding, and conversely, when playing back, the signal level of the high frequencies is Lowering the frequency signal level. It is necessary to control the signal level not uniformly, but in inverse proportion to the level of the high frequency. Therefore, as shown in FIG. 1, after the audio signal obtained from the signal source 1 is amplified by the amplifier circuit 2,
A part of the audio signal is applied to the variable filter 3 to extract only the signal to be encoded or decoded, and after amplifying the extracted signal in the amplifier circuit 4, it is input to the summing amplifier circuit 5, and then to the summing amplifier circuit 5 separately. It is superimposed on the applied audio signal and encoded or decoded the audio signal, and the passing frequency of the variable filter 3 can be changed by changing the variable impedance with the bias voltage from the bias circuit 6. Since this bias voltage is proportional to the audio signal, the magnitude of the feedback signal ultimately changes depending on the audio signal. Therefore, depending on the level of the audio signal, a high frequency signal is encoded or decoded. By the way, FIG. 2 shows a conventional example of the variable impedance 7, in which diode-connected transistors 8, 8..., a fixed bias transistor 9, and a diode-connected collector-emitter path of the transistor 9 are connected. The collector current Ic is changed by the bias voltage from the bias circuit 6 applied to the base of the transistor 11, and the register at point D is changed. However, a current Ic corresponding to the level of voltage Vin from signal source 1 flows to the collector of transistor 11, but at this time transistors 8, 8...
There is a difference between the emitter current of and the collector current of transistors 9, 10, . . . by I _B =I _C /2β+1.
This is because the collector currents of the transistors 9, 10, . . . are reversed by the current mirror circuit 12 and flow into the transistors 8, 8, . Therefore, the base voltage V _B0 of transistor 9 and transistor 8...
The following differential voltage ΔV is generated in the bias voltage V _{B1 of} . 〓ΔV=V _B1 −V _B0 =n・KT/qlnI/I−I _B However

[Equation] Furthermore, there is a relationship of I _C =I ₀ +KVin, and if a large burst signal arrives from a state where Vin is zero, this will generate a large differential voltage ΔV, which will be heard as shock noise. The present invention provides a balance circuit that cancels the differential voltage generated depending on the magnitude of the input signal, and will be explained below with reference to the drawings. In FIG. 3, 15 is a variable filter circuit consisting of a capacitor 19 and a variable impedance circuit 20 connected via a switch 18 to an amplifier circuit 17 for amplifying the audio signal of the signal source 16, and the variable impedance circuit 20 has a collector. Transistor 2 connected as a diode with its base short-circuited
1, 21, a transistor 22 to which a reference bias voltage is applied, a diode-connected transistor 23 coupled to the collector-emitter path of the transistor 22, a biasing transistor 24, and a resistor constituting a current mirror circuit 25. 26, 26 and transistors 27, 28, 29
It becomes more. 30 is an amplifier circuit having transistors 31, 32 and 33; 34, 35 are currents I ₁ flowing through the transistors 21, 21...
The base of transistor 34 is connected to the base of transistor ₂₉ , and the base of transistor 35 is connected to the base of transistor 24. The collector current of transistor 35 is the collector current of transistor 24.
The resistance values of resistors 36 and 37 are selected so that 1/2I ₀ is 1/2 of I ₀ . (By making the resistance values of resistors 36 and 37 equal, transistor 24 and transistor 3
(The emitter area ratio with 5 may be 2:1).
38 takes out a part of the output of the amplifier circuit 30,
This is a bias voltage circuit that obtains a bias voltage according to the output. Next, the operation of the present invention will be explained. First, the audio signal obtained from the signal source 16 is sent to the amplifier circuits 17 and 3.
9, a part of the audio signal amplified by the amplifier circuit 17 is applied to the variable filter circuit 15 via the switch 18, and only the signal in the high frequency range determined by the variable filter circuit 15 is output. After being passed through and amplified by an amplifier circuit 30, the signal is input to the amplifier circuit 39 to enhance or lower the high frequency range of the audio signal. The signal input to the amplifier circuit 39 is controlled by the variable filter circuit 15 . That is, the signal amplified by the amplifier circuit 30 is supplied to a bias voltage circuit 38, which detects and smooths the signal, generates a bias voltage according to the output of the amplifier circuit 30, and applies it to the variable impedance circuit 20. Then, the impedance of the variable impedance circuit 20 changes, and the passing frequency of the variable filter circuit 15 changes, so that the signal input to the amplifier circuit 39 also changes depending on the audio signal. By the way, as mentioned above, there is a difference between the current I ₁ flowing through the transistors 21 _, 21, . . . of the variable impedance circuit 20 and the current I ₂ flowing through the transistors ₂₃ , 23, . D
A differential voltage ΔV is generated between the voltage V _B1 of the transistor 22 and the base voltage V _B2 of the transistor 22. This differential voltage ΔV is an input signal applied to the variable impedance circuit 20.
Since it is proportional to Vin, sudden changes in the input signal Vin result in shock noise. However, in the present invention, as described above, the collector current of the transistor 35 is set to I〓, so that the transistor 34
The base current I _B2 of is I _B2 =1/β·I _C /2 (1). Therefore _, the current difference between the current I ₁ of the transistors 21, 21 . . . and the current _{I 2} of the transistors 22 _, 23 . This is compared to the conventional current difference I _B = I _C /2β+1. For example, if β = 100, the conventional I _B = I _C /201 becomes I _B2 - I _B1 = I _C /20100, which is almost ignored. It can be improved to the extent possible. FIG. 4 is a circuit diagram showing another embodiment of the present invention,
The base of the transistor 34 is connected to the current mirror circuit 25
It is the same as in FIG. 3 except that instead of being connected to the transistors 40 and 41, it is connected to another current mirror circuit 42 made up of transistors 40 and 41, and the current I _B2 flowing to the collector of the transistor 40 is caused to flow into the transistors 21... . As described above, the balance circuit of the present invention has the first,
Since the second compensation transistor is provided to prevent offset from occurring in the variable impedance circuit regardless of the level of the input signal, no shock noise will occur even if a large input burst signal arrives.

[Brief explanation of the drawing]

Fig. 1 is a block diagram for explaining the balance circuit of the present invention, Fig. 2 is a conventional balance circuit diagram,
3 and 4 are balance circuit diagrams of the present invention. 15 ... Variable filter circuit, 20... Variable impedance circuit, 24... Transistor, 3
4,35...transistor.

Claims (1)

[Claims] 1. A diode-connected first transistor whose base and collector are connected to a signal path, a second transistor differentially connected to the first transistor, and a current source transistor that draws the emitter sum current of the first and second transistors. and a variable impedance circuit comprising a current mirror circuit that inverts the collector current of the second transistor and supplies it to the collector of the first transistor, and is connected to the base of the current source transistor to change the impedance of the variable impedance circuit. a control signal generation circuit that generates a control signal to cause
a first compensation transistor through which a collector current of 2 flows; and a second compensation transistor whose collector emitter is connected in series with the collector emitter of the current source transistor and whose base is connected to the collector of the second transistor; A balance circuit that prevents the occurrence of shock noise by always making the base current of the first transistor equal to the base current of the second compensation transistor.