JPH0533066Y2 - - Google Patents

Description

[Detailed explanation of the invention] [Summary] Regarding a transistor switching type attenuation circuit, the purpose is to prevent signal distortion from occurring using a common bias source. In an attenuation circuit, a first resistor is inserted into a signal line divided by a signal line, and a second resistor and a switching transistor are connected in series between the output side of the first resistor and the ground. Insert a Zener diode between the ground,
Further, the Zener voltage generated in the diode is applied to the signal line.

[Industrial application field]

The present invention relates to a transistor switching type attenuation circuit.

Attenuation circuits (attenuator circuits) that vary the amount of attenuation by turning on and off transistors can be controlled by a microcomputer, so they can be easily used in audio equipment that is increasingly becoming digital. However, since signal distortion may occur when the control signal that drives the transistor becomes L (low) level, it is necessary to prevent this.

[Conventional technology]

FIG. 3 is an example of a conventional transistor switching type attenuation circuit. This attenuation circuit inserts a first resistor R 1 into a signal line l that is DC-separated by DC blocking capacitors C ₁ and C ₂ , and connects a second resistor R ₁ between the output side of the first resistor R ₁ and ground. A resistor _R2 and a switching transistor Q are connected in series, and the signal line 1 is biased to ground potential by a third resistor _R3 .

The transistor Q is of the npn type, and its collector is connected to the resistor _R2 side, and its emitter is connected to the ground side. T is a control terminal connected to the base of this transistor Q, and the control section is connected to this terminal T.
is set to H (high) level to turn on the transistor Q. When transistor Q turns on, resistance R ₂
Since the transistor side of the transistor is grounded, the input signal IN becomes a resistor due to the switching action of the transistor.
It is attenuated by the division ratio of R ₁ , R ₂ //R ₃ . On the other hand, if the transistor Q is off, the input signal IN is attenuated by the division ratio of the resistors R ₁ and R ₃ . However, if R ₃ is a large value, it hardly contributes to the damping operation.

To turn off the transistor Q, the terminal T may be opened or set to L level. However, if the terminal T is left open, the operation becomes unstable due to noise and the like, so the circuit configuration on the control section side for stabilizing this becomes complicated. Therefore,
Generally, a method is adopted in which a pull-up transistor (not shown) is turned on and the terminal T is grounded. _R5 shown in the figure is an example of some kind of resistance interposed between the terminal T and the ground at this time.

As mentioned above, the method of setting the terminal T to the L level is that when the signal line l becomes negative by approximately 0.6 V or more due to the signal N, the pn junction (equivalently represented as a diode) between the base and collector of the transistor Q is established. ) becomes conductive and the signal line l is clamped to a negative value lower than the ground potential by the base-collector voltage V _BC (~0.6V), which has the drawback that the negative side of the output OUT is distorted as shown by the solid line.

For this reason, conventionally a Zener diode with a Zener voltage V _Z was connected between the base of the transistor Q and the terminal T.
ZD is inserted to prevent distortion from occurring until the signal IN swings to the negative side by (V _Z + V _BC ). The broken line OUT' is the output waveform diagram at this time.

[The problem that the idea aims to solve]

However, the attenuation circuit shown in Figure 3 is one attenuation element, and in reality many of them are used to achieve finely variable attenuation characteristics, so it is not possible to connect individual Zener diodes to the base side of each transistor. This leads to an increase in the number of elements and costs.

The present invention enables multiple attenuation circuits to share one Zener diode for preventing signal distortion.

[Means to solve the problem]

Figure 1 is a diagram of the principle of the present invention, where C ₁ is a DC blocking capacitor on the input side, C ₂ is a DC blocking capacitor on the output side, l is the signal line between them, and R ₁ is the first capacitor inserted in the signal line l. resistance, R ₂ , Q, ZD are resistances
A second resistor, a switching transistor and a Zener diode are connected in series in this order between the output side of R ₁ and the ground, and R ₃ is a third resistor that applies the Zener voltage V _Z of the Zener diode ZD to the signal line l. The resistor _R4 is a fourth resistor through which a current for constantly generating the Zener voltage _VZ flows through the Zener diode ZD, and T is a control terminal connected to the base of the transistor Q on the control unit side.

[Effect]

In this invention, the signal line l is a Zener diode.
It is biased to the Zener voltage V _Z by ZD.
Therefore, the input signal IN is on the negative side with respect to V _Z (V _Z +
No signal distortion occurs until the signal swings by V _BC ). This is the same as in Figure 3. However, since the Zener diode ZD and resistors R ₃ and R ₄ of the present invention can be shared by multiple series circuits forming an attenuation circuit, finely variable attenuation characteristics can be realized with a small number of elements.

〔Example〕

FIG. 2 is a circuit diagram showing an embodiment of the present invention. This example has signal lines l ₁ and l ₂ that branch the same input from an operational amplifier into two outputs 1 and 2, and first resistors R ₁₁ and R ₁₂ are inserted respectively.
Such a configuration is used when the same audio signal is separately provided to front speakers and rear speakers. One signal line l ₁ is DC isolated from the outside by DC blocking capacitors C ₁ and C ₂ , and the other signal line l ₂ is DC isolated from the outside by DC blocking capacitors C ₁ and C ₃ .

Three sets each of a second resistor R _2i and a switching transistor Q _i are connected to the signal lines l ₁ and l ₂ (i=1 to 6). The transistor Qi has a built-in base resistor R _5i , and the non-base side of each base resistor R _5i is connected to the control terminal T _i of the control section.

One Zener diode ZD is commonly connected between the emitters of all transistors Q ₁ to Q ₆ and ground. Capacitor C ₄ connected in parallel with this is used for noise and switching transistor
This is to prevent abnormal noises that occur when turning on and off, and when separation deteriorates.

R ₃₁ and R ₃₂ are the third resistors that apply the Zener voltage V _Z to the signal lines l ₁ and l _2, respectively, and R ₄ is the fourth resistor that flows the current for stable Zener voltage generation from the power supply V _CC to the Zener diode ZD. These resistors can be omitted in the following cases. In other words, in the set of transistors Q ₁ to _{Q 3} , when Q ₁ is turned on, Q ₂ and Q ₃ are turned off, and when Q ₁ is turned off,
When Q ₂ and Q ₃ are turned on and no drive is performed other than this combination, the base resistance of transistors Q ₂ and Q ₃
The values of R ₅₂ and R ₅₃ are the base resistance of transistor Q ₁ R ₅₁
If you set it to twice that, the Zener diode
The same amount of base current always flows through ZD.
Do the same thing for the set of transistors Q ₄ to _{Q 6} , and if the Zener diode ZD can generate a stable Zener voltage V _Z with these base currents, the fourth resistor R ₄ can be omitted. do not have.

Note that since the output resistance of the operational amplifier is very small, the potential of l ₁ is almost constant regardless of whether transistors Q ₄ to Q ₆ are on or off, and the potential of l ₂ is constant regardless of whether transistors Q ₁ to Q ₃ are on or off. It remains almost constant regardless of whether it is off or not. Therefore, the attenuation amounts of outputs 1 and 2 can be independently controlled to arbitrary values by the outputs of the control terminals T1 to T6.

In addition, in this case, one of the transistors Q ₁ to _{Q 3} and one of the transistors Q ₄ to _{Q 6} are always on, so even if the third resistors R ₃₁ and R ₃₂ are omitted, the Zener voltage V _Z cannot be used as a signal. It can be applied to lines l ₁ and l ₂ .

If resistors R ₃₁ , R ₃₂ , and R ₄ are used, the above-mentioned restrictions are eliminated, and transistors Q ₁ to _{Q 6} can be turned on/off at will.
It can be turned off and finely variable damping characteristics can be achieved. In the illustrated example, a driver transistor Q ₀ on the control unit side that drives the transistor Q ₃ is representatively shown. The collector of this transistor _Q0 is connected to the power supply +B with a pull-up resistor _R0 ,
At the same time, it is also connected to the control terminal _T3 . This power supply +B is the base-emitter voltage V _BE of the transistor _Q3 and the Zener voltage of the Zener diode ZD.
Set to a value higher than the sum of V _{and Z.}

[Effect of idea]

As described above, according to the present invention, in a transistor switching type attenuation circuit, the Zener diode for preventing signal distortion can be shared by multiple series circuits, so the advantage is that finely variable attenuation characteristics can be realized with a small number of elements. There is.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle of the present invention, FIG. 2 is a circuit diagram showing an embodiment of the present invention, and FIG. 3 is an explanatory diagram of a conventional transistor switching type attenuation circuit. In the figure, C ₁ and C ₂ are DC blocking capacitors, l is a signal line, Q is a transistor, ZD is a Zener diode, and R ₁ to R ₄ are first to fourth resistors.

Claims (1)

[Claims for Utility Model Registration] A signal line divided into a DC blocking capacitor _C1 on the input side and a DC blocking capacitor _C2 on the output side.
A first resistor _R1 is inserted into the circuit, and a plurality of series circuits each having a second resistor _R2 and a switching transistor Q are commonly connected to the output side and the ground side of the first resistor. An attenuation circuit characterized in that a Zener diode ZD is inserted between a common connection point of the transistors and ground, and a Zener voltage generated in the diode is applied to the signal line.