JPS6360925B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable electronic impedance device used in a noise reduction device used for recording and reproducing audio signals on a magnetic tape.

As a conventional variable electronic impedance device, there is one disclosed in, for example, Japanese Patent Laid-Open No. 116052/1983.

Such conventionally known variable electronic impedance devices consist of a voltage-to-current converter and a variable gain current amplifier. Let R be the resistor connected to the emitters of the differential pair transistors that make up the voltage-current converter, I _bias be the bias current of the constant current source connected to the emitters of this differential pair transistor, and let the others make up the variable gain current amplifier. It is also known to those skilled in the art that if the control current of the variable constant current source connected to the emitters of the differential pair transistors is _Icpotrpl , the input impedance _Zio of such a variable electronic impedance device is expressed by the following relationship: .

Z _io =R·I _bias /I _cpotrpl (1) By the way, it is desirable that the input impedance Z _io of the variable electronic impedance device can be controlled in a wide range from a large impedance to a small impedance. In order to obtain a large input impedance Z _io , it is necessary to set the resistance R to a large value, and at the same time, it is necessary to set the bias current I _bias to a value sufficiently larger than the controllable minimum control current I _{cpotrpl MINI} determined by the parasitic leakage current. be.

Since the resistance R and the bias current I _bias are set to relatively large values as described above, in order to obtain a small input impedance _Zio , the control current must be sufficiently larger than the bias current I _bias , which is set to a relatively large value. I _cpotrpl must be applied to the variable current source of the variable gain current amplifier.

The variable current source as described above is generally constituted by a transistor, and if a control current exceeding the allowable current of the transistor flows, the transistor will be destroyed.

In some types of signal transmission systems, it is possible to control the amount of signal transmission over a wide range using such a variable electronic impedance device, and the input impedance of the variable electronic impedance device can be set to an extremely small value in response to a specific control signal. The inventor's studies regarding the signal transmission system have revealed that the amount of signal transmission should be reduced to a negligible amount.

The present invention has been made based on the above study results, and its purpose is to control the input impedance over a wide range, and to reduce the input impedance to an extremely small level in response to a specific control signal. The object of the present invention is to provide a variable electronic impedance device that can be reduced to a value of

Embodiments of the present invention will be described in detail below with reference to the drawings.

The drawing shows a circuit diagram of a variable electronic impedance device according to an embodiment of the invention.

Voltage-current converter 3 is PNP transistor Q ₁ , Q ₂
and resistors R ₁ and R ₂ . The base of transistor Q ₁ is supplied with a reference voltage V _REF ,
The base of the transistor Q ₂ is connected to the input terminal T _io , and the common connection point of the resistors R ₁ and R ₂ is connected to the bias current I _bias
It is connected to a constant current source 10 that supplies .

Therefore, the converted output current responsive to the AC input signal voltage V _io at the input terminal T _io flows to the collectors of the transistors Q ₁ and Q ₂ .

The variable gain current amplifier 4 includes diodes D ₁ , D ₂ ,
It is composed of NPN transistors Q ₃ and Q ₄ and PNP transistors Q ₅ and Q ₆ connected as a current mirror circuit. The common emitters of transistors Q ₃ and Q ₄ are connected to a variable constant current source 6 through which a control current I _cpotrpl flows.

The converted output current of the voltage-current converter 3 is converted into a voltage by the diodes D ₁ and D ₂ of the variable gain current amplifier 4, and this converted voltage is applied to each transistor.
Applied to the bases of Q ₃ and Q ₄ . transistor Q ₃
Changes in the collector current of the transistor _Q4 are fed back directly to the input terminal _Tio , and changes in the collector current of the transistor Q4 are fed back to the input terminal _Tio via the transistors _Q5 and _Q6 of the current mirror circuit.

In this case, the input impedance Z _io seen from the input terminal T _io can be damped according to the above equation (1).

Furthermore, in the variable electronic impedance device according to the invention, diodes D ₄ , D ₅ and a control transistor controlled by a specific control signal V _c are provided.
Q ₇ , Q ₈ , Q ₉ , Q ₁₀ , and Q ₁₁ are particularly arranged.

By applying a specific control voltage V _c to the control terminal T _c , the transistor Q ₇ becomes conductive,
The collector current I _c is supplied to the anodes of diodes D ₁ and D ₂ . At the same time, transistor Q ₈
also becomes conductive, and its collector current is supplied to transistor _Q9 . Since resistors R ₅ and R ₆ are set to the same resistance value, the transistor
The collector current of _Q7 and the collector current of transistor _Q8 are equal to each other. Furthermore, the resistances R ₇ , R ₈ ,
Since R ₉ are set to equal resistance values,
The collector currents of transistors Q ₁₀ and Q ₁₁ and the collector current of transistor Q ₉ are equal to each other.

Thus, when a specific control signal V _c is applied to the control terminal T _c , a specific control current I _c flows through the diodes D ₁ and D ₂ , and the AC resistance of the diodes D ₁ and D ₂ becomes an extremely small value. The input impedance of the variable electronic impedance device in the case is given as follows.

Z _io ′≒KT／qI _C・I _C ／I _cpotrpl ≒26mV／I _cpotrpl ……
...(2) where K is Boltzmann's constant, T is absolute temperature,
q is the electronic charge.

On the other hand, conventionally the resistance value R of resistors R ₁ and R ₂ is 5KΩ.
The bias current of the constant current source 10 is set to approximately
Since I _bias is set to about 100 μA, the numerator (R・I _bias ) of equation (1) is about 500 mV, and it is understood that equation (2) is a sufficiently smaller value than equation (1). can.

The present invention is not limited to the above embodiments, and various modified embodiments can be adopted.

For example, transistors Q ₅ , Q ₆ , Q ₇ , Q ₈ , Q ₈ ,
If the emitter area ratios of Q ₉ , Q ₁₀ , and Q ₁₁ are set with high precision, the resistors R ₃ , R ₄ , R ₅ , R ₆ , R ₇ , R ₈ , and R ₉ can be omitted.

Also, diode _D3 can be replaced with other constant voltage bias circuit.

[Brief explanation of drawings]

The drawing shows a circuit diagram of a variable electronic impedance device according to an embodiment of the invention. 3...Voltage-current converter, 4...Variable gain current amplifier, 10...Constant current source, 11...Variable constant current source, _Q1 , _Q2 ...Differential pair transistor, _R1 , _R2 ...
…resistance.

Claims (1)

[Claims] 1 Differential pair transistors Q ₁ , Q ₂ and emitter resistance
a voltage-current converter 3 configured by R ₁ and R ₂ and configured to convert an input signal voltage into a current signal change;
A variable electronic impedance device comprising a variable gain current amplifier 4 configured to feed back a current responsive to a current signal change of the voltage-current converter to the voltage-current converter, the cathode of which is connected to the differential pair transistor. A _first diode _D4 whose cathode is connected to the emitter of the other Q2 of the differential pair transistor, a second diode _D5 whose collector is connected to the emitter of the other _Q2 of the differential pair transistor, and a first control transistor of a first conductivity type connected to the anodes of two diodes D ₄ and D ₅ ;
a second control transistor Q ₁₀ of a second conductivity type, the collector of which is connected to the cathode of the first diode D ₄ ; and a second control transistor Q ₁₀ of a second conductivity type, the collector of which is connected to the cathode of the second diode D ₅ ; a third control transistor of type Q ₁₁ and a control signal V _C
The first, second and third control transistors
A variable electronic impedance device characterized by controlling Q ₇ , Q ₁₀ , and Q ₁₁ to a conductive state.