JPS5921546Y2 - variable filter circuit - Google Patents

Description

[Detailed description of the invention] This invention relates to a variable filter circuit suitable for use in, for example, a timbre control circuit of an electronic musical instrument.
Related to filter circuits.

Conventionally, a secondary active low-pass filter circuit using a diode bridge as shown in FIG. 1 has been proposed (see, for example, Japanese Patent Laid-Open No. 15350/1983).

In this circuit, 10 is an input terminal for receiving an input signal ei, and 12 is a diode D1, D2, . D3. D
4 is a diode bridge circuit, and 14 is a filter output e.

G1 is an input buffer amplifier, G2 is an active element amplifier having a gain G2, C'1.
C″1.C2 represents a capacitor for forming a filter, and ICN represents a diode bias constant current determined according to a control signal.

Here, if the dynamic resistance of the diode is r, then this is It is expressed as

However, q is the amount of charge of the electron, k is Boltzmann's constant, and T is the absolute temperature.

If C1 and R are respectively defined as follows, then a circuit other than the circuit shown in FIG. 1 will be as shown in FIG. 2.

The circuit in Figure 2 is a typical second-order active low-pass circuit.
It is a filter circuit, and its transfer function Ds, cutoff frequency fc, selectivity Q, and passband gain are as follows.

Therefore, the above circuit can be applied to, for example, a tone control circuit of an electronic musical instrument.

That is, in this case, by controlling the constant current ICN in accordance with the control signal and changing the apparent resistance of the diode, it is possible to change the cutoff frequency fc and remotely control the timbre.

By the way, various problems arise when the above-mentioned circuit is concretely integrated into an IC, and the following problems are particularly noteworthy.

That is, if the active element amplifier G2 is constructed from a normal bipolar transistor type linear IC, a relatively large current is required because the input impedance is low.

Therefore, capacitor C'1. Since it is necessary to increase the capacitance of C"1, a polar capacitor such as an electrolytic capacitor is used as C"1.C"1.

This type of polarized capacitor requires care to ensure that the voltage applied polarity does not reverse from the predetermined polarity under any operating conditions, and if a reverse voltage is applied, it will easily break down.

The purpose of this invention is to provide a variable filter circuit suitable for IC implementation that can effectively prevent capacitor destruction due to reversal of voltage application polarity as described above.

This invention is based on the above-mentioned type of variable filter circuit.
The present invention attempts to achieve the above object by a simple means of simply connecting a diode between the signal input and output terminals of a diode bridge circuit, and the embodiment shown in the drawings will be described in detail below.

FIG. 3 shows a secondary active according to an embodiment of this invention.
This figure shows a low-pass filter circuit, and the same reference numerals as in FIG. 1 indicate the same parts.

In FIG. 3, 11 is a diode bridge circuit 1
A controlled constant current I supplied to 2.

A control current circuit for generating N, Q1□ is an emitter-follower transistor whose base receives the human input signal ei from the input terminal 10 and constitutes the input buffer amplifier G1;
D.

In accordance with the teachings of this invention, diodes for preventing capacitor destruction are connected between the signal input and output terminals a and l) of the bridge circuit 12 with the polarities shown in the figure, and C'1 and C''1 are connected to the output terminal of the active element amplifier G2. These are electrolytic capacitors, that is, polarized capacitors, connected between each bias terminal c and d of the bridge circuit 12, respectively, with the polarities shown.

The active element amplifier G2 consists of a differential amplifier circuit and an output circuit, and the differential amplifier circuit includes a transistor Q215.
Q22, Q239 Q245Q25, and the output circuit consists of Darlington connection transistors Q27,
Q28 and transistor Q29.

The bases of each of the transistors Q25 and Q29 are connected to a fixed bias source (8), so that a constant current flows therethrough.

Filter output e. is taken out from the output terminal 14 connected to the emitter side of the transistor 028.

Here, diode D is a feature of this invention.

The effects of this will be explained.

Now diode D. Assume that the is not connected to the position shown.

In this case, the current I.

If N is flowing, diode D3. Due to the voltage drop across D4, capacitor C'1. The polarity of the voltage applied to C''1 is as shown in the figure, and the capacitors C'1.C''1 are connected in accordance with this polarity.

However, if ICN becomes substantially zero due to some reason, such as the control input to the current control circuit 11 becoming substantially zero, the base bias of transistor Q21 is cut off, and the transistor Q2'8 becomes Emitter current increases.

Therefore, the emitter potential of transistor 028 increases, and the emitter capacitor C/ of transistor Q28 increases.
Current flows through the path of - diode D1 - emitter resistance of transistor Qll.

, which obviously means that the polarity of the voltage applied to the capacitor C'1 is opposite to that shown.

As a result, there is a possibility that the capacitor C'1 may be destroyed.

On the other hand, diode D as shown.

If ICN is connected to the diode D from the emitter of the transistor Qll of the input buffer amplifier G1, even if ICN becomes substantially zero.

A bias current I is applied to the base of transistor Q21 via
.

is supplied. Therefore, the emitter potential of the transistor Q28 does not rise, and the polarity of the voltage applied to the capacitor C'1 does not reverse, so that the capacitor C'1 is prevented from being destroyed.

As described above, the circuit shown in Figure 3 prevents the polarized capacitor for filter formation from being destroyed without damaging the circuit configuration suitable for IC implementation, and is extremely stable and reliable. is high.

Next, a specific example of the circuit shown in FIG. 3 will be explained with reference to FIG.

In FIG. 4, the same reference numerals as in FIG. 3 indicate the same parts.

An input signal ei, such as a musical tone signal, is applied from an input terminal 10 to the base of the transistor Qll.

This transistor Qll is for configuring the amplifier G1 and operates as an emitter follower.

A protection diode Dp is connected between the base and emitter of the transistor Q1□.

A signal output taken out from the emitter side of the transistor Qll is applied to a signal input terminal a of a diode bridge circuit 12 including diodes D1 to D4, and taken out from a signal output terminal A.

The bridge circuit 12 has a pair of bias terminals c, d, and between these terminals c, d, a forward series diode pair D1-D2 . I).

D4 are connected in parallel.

Signal input terminal a is connected to diode D1. The signal output terminal is connected to the connection point of diode D3.D2. D4
connected to the connection point.

Constant current I supplied from the control current circuit 11.

N is connected from bias terminal C to diode pair UD2. D3-D
Since each of the diodes D1 to D4 is biased in the forward direction while the current is shunted and flows to the bias terminal d, a variable impedance determined by a dynamic resistance corresponding to the bias current value is generated between the input and output terminals a and t).

A diode D is connected between the input and output terminals a and l) according to the teachings of this invention.

is connected to this diode D.

This is to prevent a voltage with a polarity opposite to that shown in the figure from being applied to the capacitor C'1 when the current ICN becomes zero as described above.

Further, the variable resistor VR is a resistor for adjusting the balance of the bridge circuit 12.

Constant current I for diode bias.

The control current circuit 11 that generates N includes transistors Q14 and Qt that constitute a current mirror type constant current circuit.
s, Qt6. Ql□.

Q18 and Q19 are included, and the current 1. .

, the value of transistor Q1. A current limiting resistor Rc is installed at the base of
and the value of the control voltage -Vc applied to the control terminal 13 connected via the backflow prevention diode Dc.

A potential source -■ is connected to the base of the transistor Q]5 via a high resistance Ro of, for example, IOMΩ, and when the control current flowing through the control terminal 13 reaches zero or a value close to it, the current ICN In order to prevent the current from becoming zero, the minimum necessary control current is supplied.

One of the signal output terminals of the bridge circuit 12 is grounded via the filter-forming capacitor C2, and the other is connected to the base of the transistor Q2□ of the active element amplifier G2.

Amplifier G2 is a transistor Q2 that constitutes a differential amplifier.
□, Q2□, Q23.

Q24, Q25 and transistors Q27, Q2B, Q29 forming an output circuit. Due to the current mirror effect of Q13, a constant current flows.

The signal output taken out from the emitter side of the transistor 02B is connected to the filter forming electrolytic capacitor C'1. It is fed back to the bias terminals c and d of the bridge circuit 12 via C″1.

The filter output is taken from the input side of amplifier G2, ie from the base side of transistor Q21, and is taken from the buffer amplifier G2.
It is supplied to the base of transistor Q3□ of No. 3.

Buffer amplifier G3 includes transistors Q3□, Q3□, Q33 . Q34, Q3i
, and Darlington connection transistor Q37. which constitutes the output limiter follower stage. Q38, and the transistor Qas also includes the aforementioned transistors Q1□, Q1
A constant current flows due to the current mirror effect of No. 3.

An output terminal 14 is connected to the emitter side of the transistor Q38, and a filter output e is output from this terminal 14.

is taken out. In the above configuration, the capacitors C1□, C1□, and C13 are signal bypass capacitors to enable DC bias.

The circuit shown in FIG. 4 described above operates as a second-order active low-pass filter whose cutoff frequency changes according to the control voltage -Vc, and outputs a filter output e from the output terminal 14.

This is what can be obtained. In this case, diode D.

Since this is provided, there is an effect of preventing destruction of the capacitor C'1.

Also, the filter output e.

Since it is taken out from the input side of amplifier G2 via amplifier G3, the filter output e is caused by a change in the gain of amplifier G2.

Another advantage is that the amplitude level of the signal does not change.

Further, the circuit shown in FIG. 4 is suitable for being integrated into an IC, and when integrated into an IC, it is possible to achieve a distant reduction in size and cost by integrating the circuit into an IC.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional variable filter circuit, and Figure 2 is a circuit diagram showing a conventional variable filter circuit.
3 is an equivalent circuit diagram of the circuit shown in FIG. 1, FIG. 3 is a circuit diagram showing a variable filter circuit according to an embodiment of the invention, and FIG.
The figure is a wiring diagram showing a specific example of the circuit of FIG. 3. 11... Control current circuit, 12... Diode bridge circuit, C1, C'1. C″1.C2・
...Filter forming capacitor, R...
Filter forming resistor, G1°G2. G3...Amplifier, Do...Diode to prevent capacitor destruction.

Claims (1)

[Claims for Utility Model Registration] (a) A control current circuit that generates a constant current according to a control signal; (b) A control current circuit that generates a constant current in the forward direction that is supplied to a pair of bias terminals of diodes on each side of the bridge. (C) an input buffer that supplies an input signal to the signal input terminal of the bridge circuit; (d) a second transistor amplifier for an active element whose input terminal is connected to the signal output terminal of the bridge circuit; (e) an input of the second amplifier; (f) a first capacitor of the filter forming phase connected between the end and the reference potential point; and (f) a first capacitor connected between the output end of the second amplifier and each bias terminal of the bridge circuit. In a variable filter circuit comprising second and third polarized capacitors in a filter forming phase, the second polarized capacitor is connected between the signal input and output terminals of the bridge circuit when the constant current becomes substantially zero. and a variable filter circuit, further comprising a diode connected thereto to prevent the polarity of voltage applied to at least one of the third capacitors from being reversed.