Variable Electronic Impedance Circuit

The present invention relates to a variable impedance circuit which is utilized in a noise reduction device for use in the recording/playback of audio signals on a magnetic tape, etc.

In conventional variable electronic impedance circuits of the type described, for example, in Japanese Laid-open Patent Specification No. 116,052/1977, there often occurs the problem of abnormal radio frequency (RF) oscillation, particularly immediately after the closure or turn-on of a power supply switch, resulting in distortion in the output of the associated receiver circuit.

The cause of the above problem has been determined to be the fact that since such a variable electronic impedance circuit has a feedback path, positive feedback always takes place in the feedback path upon the closure of the power supply switch, resulting in the above-related abnormal oscillation of the radio frequency signal.

The present invention has been developed on the basis of the results of a study of the above problem, and is intended to provide a variable electronic impedance circuit which is free from an unstable operation immediately after the application of a supply voltage to the associated circuit.

FIG. 1 is a circuit diagram showing a variable electronic impedance circuit accoding to an embodiment of the present invention; and

FIG. 2 is a block diagram for explaining the operation of the circuit in FIG. 1.

Referring to FIG. 1 which shows a circuit diagram of a variable electronic impedance circuit according to one embodiment of the present invention, the circuit elements encompassed by broken lines IC are formed within a monolithic semiconductor integrated circuit. Numerals enclosed with circles indicate the external terminals of the integrated circuit.

A variable electronic impedance device EID is constructed of a voltage-current converter 3 having a high input impedance, a variable-gain current amplifier 4, a controllable constant current source 6, a constant current source 7 and a bias circuit BC.

The voltage-current converter 3 is constructed of P-N-P transistors Q.sub.1 and Q.sub.2, resistors R.sub.21 and R.sub.22, input N-P-N transistors Q.sub.14 and Q.sub.15, and constant current sources CS.sub.1 and CS.sub.2. The base of the transistor Q.sub.14 is grounded (in the AC sense) by a capacitance C.sub.2 for removing power supply ripple and is supplied with a reference voltage V.sub.REF from the bias circuit BC, so that it operates as a reference input terminal of the voltage-current converter 3. Since the base of transistor Q.sub.15 is fed with an A.C. input signal V.sub.in, it operates as the signal input terminal of the voltage-current converter 3. Converted output currents +I.sub.1 and -I.sub.1 are derived from the collectors of the respective transistors Q.sub.1 and Q.sub.2. Herein, the voltage-current conversion coefficient can be set by the resistances of the resistors R.sub.21 and R.sub.22 and the constant current value of the constant current source 7.

The variable-gain current amplifier 4 is constructed of diodes D.sub.11, D.sub.12, D.sub.21, D.sub.22 and D.sub.3, N-P-N transistors Q.sub.3 and Q.sub.4, and P-N-P transistors Q.sub.5 and Q.sub.6 which are connected as a current mirror circuit. The converted output currents +I.sub.1 and -I.sub.1 of the voltage-current converter 3 are converted by the diodes D.sub.11, D.sub.12 and D.sub.21, D.sub.22 of the variable-gain current amplifier 4 into voltages which are applied to the bases of the transistors Q.sub.3 and Q.sub.4, respectively. A change in the collector current of the transistor Q.sub.3 is directly coupled to the base of the transistor Q.sub.15, while a change in the collector current of the transistor Q.sub.4 is coupled to the base of the transistor Q.sub.15 through the transistors Q.sub.6 and Q.sub.5 of the current mirror circuit. Therefore, an amplified current I.sub.2 is fed back to the base of the transistor Q.sub.15 serving as the signal input terminal of the voltage-current converter 3. The current gain .beta. of the amplifier can be set by the constant current value of the variable constant current source 6.

Where a stable supply voltage V.sub.CC is steadily supplied, the reference voltage V.sub.REF determined by the resistance ratio between voltage dividing resistors R.sub.27 and R.sub.28 is continually applied to the base of the transistor Q.sub.14, and hence, the base of the transistor Q.sub.15 is maintained at a potential substantially equal to the reference voltage V.sub.REF by the feedback operation of the variable electronic impedance device. Accordingly, when the voltage dividing resistances R.sub.27 and R.sub.28 are equal to each other, the reference voltage V.sub.REF becomes V.sub.CC /2.

As noted above, letting .alpha. denote the conversion coefficient of the voltage-current converter 3, and .beta. the current gain of the variable-gain amplifier 4, an A.C. input impedance Z.sub.in, in the case where the A.C. signal V.sub.in has been applied to the signal input terminal of the voltage-current converter 3, can be evaluated as follows by referring to the block diagram of FIG. 2: ##EQU1##

Next, immediately after the supply voltage V.sub.CC has been applied to terminal No. 5 via a power supply switch (not shown), the reference voltage at terminal No. 7 does not rise to V.sub.CC /2 due to a time constant which is determined by the product of voltage dividing resistor R.sub.27 and the capacitor C.sub.2. Accordingly the transistor Q.sub.14 is turned "off", and a comparatively large current flows to the base of the transistor Q.sub.1 from constant current course CS.sub.1 which becomes operative quickly after the closure of the power supply switch, so that the transistor Q.sub.1 is turned "on".

On the other hand, the base of a transistor Q.sub.11 in the bias circuit BC is supplied with the supply voltage V.sub.CC through a starting resistor R.sub.34. Therefore, immediately after the supply voltage V.sub.CC has been applied, transistor Q.sub.11 is turned "on". Accordingly, the Zener voltage of a Zener diode ZD is divided by resistors R.sub.32 and R.sub.33, and the divided voltage is fed to the anode of a diode D.sub.1 which is particularly employed in accordance with the present invention. Diode D.sub.1 becomes conductive, the base potential of the transistor Q.sub.2 and the emitter potential of the transistor Q.sub.15 of the voltage-current converter 3 are controlled at a high level, and each of transistors Q.sub.2 and Q.sub.15 are turned "off".

In this manner, due to the provision of diode D.sub.1, transistors Q.sub.1 and Q.sub.2 of the voltage-current converter 3 become unbalanced immediately after the application of the supply voltage V.sub.CC. Transistor Q.sub.1 is turned "on" and the other transistor Q.sub.2 is turned "off", so that converted output currents responsive to the A.C. input signal V.sub.in cannot be derived from the collectors of the transistors Q.sub.1 and Q.sub.2. That is, diode D.sub.1 operates as a control circuit which substantially inhibits the operation of the voltage-current converter 3 for a predetermined period of time after the closure of the power supply switch. As a consequence, an amplified current responsive to the A.C. input signal is not fed from the variable-gain current amplifier 4 back to the signal input terminal of the voltage-current converter 3, so that the feedback operation of the variable electronic impedance device is prevented during this predetermined period of time.

Thus, according to the present invention, as illustrated in the block diagram of FIG. 2, immediately after the application of the supply voltage V.sub.CC, the bias circuit BC supplied a voltage of a comparatively high level to the control input terminal of the voltage-current converter 3 through the diode D.sub.1 and feeds a reference voltage V.sub.REF of a low level to the reference input terminal (+). Consequently, the voltage-current converter 3 is substantially incapable of responding to the A.C. input signal V.sub.in applied to its signal input terminal (-), and the feedback operation of the variable electronic impedance device is substantially prevented. Therefore, abnormal oscillation of a radio frequency signal immediately after the closure of the power supply switch can be prevented.

After the initial predetermined time following closure of the power supply switch has passed, the capacitor C.sub.2 becomes charged to a point that the base voltage of the transistor Q.sub.14 becomes sufficiently high-level so that the transistor Q.sub.14 becomes "conductive". As a result, the base voltage of the transistor Q.sub.1 finally reaches to a level of V.sub.REF -V.sub.BEQ14 =1/2V.sub.CC -V.sub.BEQ14 (where V.sub.BEQ14 denotes the base-emitter voltage of the transistor Q.sub.14).

By setting this level (1/2V.sub.CC -V.sub.BEQ14) to be higher than the voltage which is applied from the common junction of the dividing resistors R.sub.32 and R.sub.33 to the base of the transistor Q.sub.2 through the diode D.sub.1, the transistor Q.sub.2 becomes "conductive", after a predetermined period of time subsequent to the application of power supply. Thus, the large collector current of the transistor Q.sub.2 is converted to a large voltage by the diodes D.sub.21, D.sub.22 and D.sub.3. By the application of this large voltage to the base of the transistor Q.sub.4, the conductivity of the transistor Q.sub.4 is increased, while the conductivity of the transistor Q.sub.3 is decreased. Thus, the conductivities of the transistors Q.sub.5 and Q.sub.6 are also increased. Therefore, the current which is supplied from the collector of the transistor Q.sub.5 to the base of the transistor Q.sub.15 increases, so that the transistor Q.sub.15 becomes "conductive"

Since both of the transistors Q.sub.14 and Q.sub.15 are "conductive", the base voltage of the transistor Q.sub.15 becomes equal to the base voltage (1/2V.sub.CC) of the transistor Q.sub.14 due to the D.C. feedback operation from the output of the variable-gain current amplifier 4 to the signal input terminal (-) of the voltage-current converter 3. Thus, the emitter voltage of the transistor Q.sub.15 (i.e. the cathode voltage of the diode D.sub.1) reaches a level of 1/2V.sub.CC -V.sub.BEQ15 (V.sub.BEQ15 denotes the base-emitter voltage of the transistor Q.sub.15). Since the voltage at the common junction of the dividing resistors R.sub.32 and R.sub.33 is lower than the level of 1/2V.sub.CC -V.sub.BEQ15, the diode D.sub.1 becomes reversely biased and "non-conductive". Thus, this diode D.sub.1 does not affect the steady state operation of the variable electronic impedance circuit after the predetermined period of time subsequent to the application of power supply.

While we have shown and described an embodiment in accodance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.