JP3149604B2 - Variable impedance circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable impedance circuit used as a variable attenuator, a variable filter, and the like for electronic equipment such as an ultrasonic diagnostic apparatus.

[0002]

2. Description of the Related Art In general, an electronic volume is used as a variable attenuator in an electronic device. This electronic volume has an input signal I _SG as shown in the principle diagram in FIG.
By controlling the voltage applied to the transistors Q ₁ and Q ₂ when deriving the output current I ₂ , the current I ₂
Changing the ₁ and the ratio of I _2, is intended to vary the impedance of the circuit as a result.

[0003]

In the above-mentioned conventional electronic volume, since all circuit elements are configured by transistors, the actual circuit requires a large number of transistors and must be integrated into an IC. However, it is not easy, and there is a problem that an inexpensive electronic volume cannot be obtained.

The present invention has been made in view of the above problem, and has as its object to provide an inexpensive variable impedance circuit which can be realized even in a discrete manner.

[0005]

A variable impedance circuit according to the present invention comprises an amplifier including first and second transistors, a first capacitor connected between the input and output of the two transistors, and A second capacitor, which is a variable capacitance diode connected between the first capacitor and the ground, an impedance element connected between the input and the output of the amplifier, and a voltage applied to the second capacitor to change the electrostatic capacitance. And a capacity control circuit for changing the capacity.

In this variable impedance circuit, the impedance seen from the input side is Zi, the capacitance of the _first capacitor is C ₁ , the capacitance of the second capacitor is Cx, and the impedance connected between the input and output is Assuming that the impedance of the element is Z, it can be expressed as Zi = (1 + C ₁ / Cx) Z, and the capacitance Cx of the second capacitor is
By changing the impedance with the applied control voltage, the impedance Zi can be changed. Moreover, since it is composed of a transistor, a variable capacitance diode, a capacitor, and the like, it can also be realized in a discrete manner.

[0007]

The present invention will be described in more detail with reference to the following examples. FIG. 1 shows a principle diagram of a variable impedance circuit in which an impedance element 2 having an impedance Z is connected between an input terminal and an output terminal of an amplifier 1 having an attenuation factor K, and an impedance Zi viewed from the input side.
Can be expressed by Zi = Z / (1−K), and the impedance Zi can be changed by changing the attenuation rate K.

FIG. 2 is a connection diagram of a variable impedance circuit showing one embodiment of the present invention, and provides a novel circuit to which the principle of FIG. 1 is applied. In FIG. 2, a transistor (first) Q ₁ and a transistor (second) Q
₂ is emitter follower connected. In other words, the collector of the transistor Q ₁ and the transistor Q ₂ is common,
Is connected to the + V power supply, the emitter of the transistor Q ₁ is,
It is connected to the current source I ₁ and to the base of the transistor Q ₂ via the capacitor C ₁ . Then, the current source I ₂ is connected to the emitter of the transistor Q _2. And the base of the transistor Q ₁ (input end),
During the transistor Q ₂ of the emitter (the output end), the impedance element Z is connected. Also, the transistor Q
_{2 have} variable capacitance diodes V of opposite polarities
_One ends of C ₁ and VC ₂ are connected, and the variable capacitance diode V
The other end of C ₁ is connected to ground via a capacitor C ₂ , while being connected to a + Ec power supply via a resistor R ₁ . The other end of the variable capacitance diode VC ₂ is grounded connected via a capacitor C _3, through a resistor R _2, and is connected to -Ec power. Moreover, the base of the transistor Q ₂ is grounded connected via a resistor R _3. In the circuit of this embodiment, by changing the voltages of the + Ec and -Ec power supplies, the variable capacitance diode VC
₁ and the capacitance of VC ₂ are changed. In other words, this + E
The voltage applied from the c power supply and the −Ec power supply functions as an impedance control signal.

[0009] In the circuit of this embodiment, the attenuation factor K can be expressed by _{K ≒ C 1 / (C 1} + Cx). Therefore, the impedance Zi when viewed from the transistor Q ₁ input side, Zi = Z / (1- K) = become _{(1 + C 1 / Cx)} Z ... (1).

However, Cx is a + Ec voltage and a -Ec voltage.
Controlled variable capacitance diode VC ₁, VC_TwoParallel combination of
It is formed capacity. Change the control voltage of + Ec, -Ec
The variable capacitance diode VC ₁, VC_TwoParallel
The combined capacitance Cx changes and the impedance Zi changes.
Can be

The impedance Z of the impedance element 2 is represented by a capacitance C as shown in FIG. 3A, an inductance L as shown in FIG. 4A, and an impedance L as shown in FIG. Assuming that the resistance is R as shown, each circuit has a capacitance C ′ = C / (1-
K), the inductance L ′ = 1 (1) shown in FIG.
−K) and the resistance R = 1 / (1−K) shown in FIG. 5B, and the capacitance of the variable capacitance diodes VC ₁ and VC ₂ is changed by changing the attenuation rate K, respectively.
That is, a variable impedance circuit can be realized by changing the control voltages + Ec and -Ec.

In the above embodiment, _two variable capacitance diodes VC ₁ and VC ₂ connected in opposite polarities are used, but any one of them may be used. Or it may be a capacitor C ₁ to the variable capacitance diode. In the above embodiment, a bipolar transistor is used. However, the present invention is not limited to this, and an FET (electric field transistor) may be used.

Next, a case where the variable impedance circuit of this embodiment is adopted in an ultrasonic cut-off device will be described. Generally, as shown in FIG. 6, an ultrasonic cutoff device includes an oscillation circuit 61 that generates a pulse, a buffer amplifier 62, an ultrasonic transducer 63, and a preamplifier 64 that amplifies a received signal.
A delay circuit 65, a filter circuit 66 and the like are provided. In this ultrasonic cut-off device, as the filter circuit 66, for example, a T-type circuit including coils L ₁ and L ₂ and a capacitor C is used, and variable impedance elements VR ₁ and VR ₂ for adjusting characteristics are used. The variable impedance circuit shown in FIG. 2 is used as the variable impedance element. As the delay circuit 65, for example, the circuit shown in FIG. 8 is used, and the circuit shown in FIG. 2 is also used for the variable impedance elements VR ₁ and VR ₂ for adjusting the characteristics of this circuit. The use of such a variable impedance circuit can contribute to a reduction in the overall cost of the ultrasonic cutoff device.

[0014]

According to the present invention, a variable impedance (L, C, R) circuit can be realized with a simple circuit, a small number of components, and even with a discrete circuit. In addition, since a variable capacitance diode is used and the attenuation rate is changed according to the applied control voltage, it is possible to make a variable attenuator, bandpass filter, and variable delay line that are inexpensive but have a wide dynamic range with a high bandwidth. it can.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of a variable impedance circuit.

FIG. 2 is a connection diagram of a variable impedance circuit showing one embodiment of the present invention.

FIG. 3 is an equivalent circuit in a case where a capacitance is used as the impedance of the embodiment.

FIG. 4 is an equivalent circuit when an inductance is used as the impedance of the embodiment.

FIG. 5 is an equivalent circuit using a resistor as an impedance of the embodiment.

FIG. 6 is a block diagram showing a partial configuration of an ultrasonic wave blocking device in which the variable impedance circuit of the embodiment is implemented.

FIG. 7 is a circuit diagram showing a specific example of a filter circuit of the ultrasonic wave blocking device.

FIG. 8 is a circuit diagram showing a specific example of a delay circuit of the ultrasonic cutoff device.

FIG. 9 is a connection diagram showing a conventional variable impedance circuit.

[Explanation of symbols]

Q ₁ , Q ₂ transistor VC ₁ , VC ₂ variable capacitance diode 2 impedance element C ₁ , C ₂ , C ₃ capacitor + Ec, -Ec control power supply

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 11/48 H03H 7/25 H03H 11/46

Claims (1)

(57) [Claims] 1. An amplifier including first and second transistors, a first capacitor connected between the input and output of both transistors, and a variable capacitance diode connected between the first capacitor and ground. A second capacitor;
A variable impedance circuit comprising: an impedance element connected between an input and an output of the amplifier; and a capacitance control circuit that changes a voltage applied to the second capacitor to change a capacitance. .