JPS5951173B2 - variable impedance circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable impedance circuit suitable for application to a gain control circuit, a phase control circuit, a voltage (current) controlled vibrator, a voltage (current) controlled seven-channel wave generator, etc. The purpose of this invention is to propose a simple method that has a wide impedance variable range, a wide dynamic range, and does not change the absolute value of the signal amplitude when applied to a phase control circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings.

First, an embodiment of the present invention will be described with reference to FIG.

t is a first power supply terminal to which a positive series voltage is supplied.

The second power supply terminal is here grounded.

A series circuit between the collector and emitter of the transistor 1 and the first diode 2 is connected between the terminal t and ground.

The collector of transistor 1 is connected to power supply terminal t, and θ
Its emitter is connected to the base of a diode-configured transistor 2 whose collector and base are directly connected, and the emitter of the transistor 2 is grounded.

A series circuit of second and third diodes 3 and 4 is connected between the base of transistor 1 and ground.

The diodes 3 and 4 are diode-configured transistors in which the collector and base are directly connected, respectively.The base of the transistor 3 is connected to the base of the transistor 1, the emitter of the transistor 3 is connected to the base of the transistor 4, and The emitter of is grounded.

These transistors 1 to 4 are NPN type transistors in this example.

The base of the transistor 1 is connected to a power supply terminal t through a current source circuit 8.

A midpoint between the transistor 1 and the first diode 2 is connected to an input signal source 5 via an impedance circuit 6 .

The other end of the signal source 5 is grounded. An output terminal 7 from which an output signal is obtained is led out from the midpoint of connection between the transistor 1 and the first diode 2.

In this example, the above-described current source circuit 8 is used as the impedance control means 35, and by varying the direct current flowing through it, the impedance between the output terminal 7 and the ground can be varied.

In the circuit shown in Fig. 1, when the impedance circuit 6 is a resistor, it can constitute a gain control circuit as a whole, and when it is constituted by a capacitor, it can be a high-pass filter whose cutoff frequency can be varied. When configured with an inductor, a low-pass P-wave device whose cutoff frequency can be varied can be configured.

Next, let's analyze the operation of the variable impedance circuit shown in FIG. 1 using mathematical formulas.

The base-emitter voltage VBE of a transistor is generally expressed by the following equation.

22, q is the electron charge, k is Boltzmann's constant, T is the absolute temperature, ■5 is the collector saturation current, and ■ is the collector current.

In addition, the base-emitter voltage VB of transistors 1 to 4
If EI to VBE4, the following equation holds true since the base potential of transistor 1 and the base potential of transistor 3 are equal.

VBEI + VBE2= VBE3+ VBE4
(2) Next, the voltage at the output terminal 7, that is, the collector voltage (2) of the transistor 2, is calculated as shown in the following equation.

In the second equation, is represents the input signal current, and 2 represents the DC current of the current source circuit 8.

In the above equation (3), VC2 when 18=0 is ■.

Then, equation (3) can be expressed as the following equation.

This V.

is called the reference potential point. In this way, VC2 in equation (3) and this reference potential ■.

Let v' be the difference between .

Also, this reference potential V.

Letting the difference between and VC2 be V//, this is expressed as the following equation.

If the relationship between equations (5) and (6) is expressed graphically, the second
The curves are as shown in curves (9), (10) and 01) in the figure.

In this case (9). (10) and 0.11 respectively show the curves when Ix is changed from small to large.

The horizontal axis is 18, and the vertical axis is V' and V//.

What can be seen from this graph is that these curves are point symmetrical, and that by changing (1)8, the slope changes, that is, the input impedance of this variable impedance circuit changes.

Further, a long straight line portion indicates a wide dynamic range.

Here, if the input impedance is r, it is expressed as the following equation.

On the other hand, the impedance of a diode or the like is generally expressed as shown in the following equation.

It can be seen that the impedance of this diode also changes depending on the signal current i5.

If we set the condition iζ<Ix in equation (7), then (7
) is approximated as follows.

Therefore, it can be seen from this equation (9) that the range of linearity with respect to the input signal is wide and the dynamic range is wide.

Next, another embodiment of the present invention will be described with reference to FIG.

In this embodiment, the impedance control means 35 does not vary the current of the current source circuit 8 as shown in FIG.
This is done using the following configuration.

That is, a resistor 1 is connected between the collector and base of the transistor 4.
2 is connected, and a series circuit of a resistor 13 and a variable DC power supply 14 is connected between the base of the transistor 4 and the ground.
By varying the voltage of this DC power supply 14, the impedance is varied.

Next, still another example of the present invention will be described with reference to FIG.

In this example, a resistor 15 is connected between the base of transistor 1 and the base of transistor 3, and a variable current source circuit 16 is connected between the base of transistor 1 and ground to constitute impedance control means 35. It is.

Next, still another embodiment of the present invention will be described with reference to FIG.

In the embodiment shown in FIG. 1, transistors 1 to 4 are all NPN type transistors, but in this example, only transistors 2 and 4 are PNP type transistors.

Then, the emitter of transistor 2 is connected to the emitter of transistor 1, the collector and base of transistor 2 are directly connected to form a diode configuration, and the collector and base are grounded, and the emitter of transistor 4 is connected to the emitter of transistor 3, The collector and base of the transistor 4 are directly connected to form a diode configuration, and the collector and base are grounded.

The impedance control means 35 is the same as in FIG.

Next, still another embodiment of the present invention will be described with reference to FIG.

In this example, an output circuit constituted by a differential amplification circuit 17 is provided on the output side of the variable impedance circuit of the present invention.

This differential amplification circuit 17 is composed of NPN transistors 18 and 19 and a constant current source circuit 20. The base of transistor 18 is connected to the emitter of transistor 1, and the base of transistor 19 is connected to the emitter of transistor 3. has been done.

Note that the output is obtained from the collectors of transistors 18 and 19.

Next, an example in which the variable impedance circuit of the present invention is applied to a phase control circuit will be described with reference to FIG.

That is, when applied to a color correction circuit of a color television receiver, for example, a local subcarrier oscillator 5 is provided, one end of which is grounded, and the other end is connected to the output terminal 7, that is, the emitter of the transistor 1, through a capacitor 6. At the same time, the other end of the local subcarrier oscillator 5 is connected to the base of the transistor 19 of the differential amplification circuit 17 through the resistor 22, and the base of the transistor 19 is connected to the emitter of the transistor 3 through the resistor 21.

In this case, the resistors 21 and 22 have approximately equal values.

An equivalent circuit of FIG. 7 is shown in FIG.

In this case, 29 represents a variable impedance circuit other than the differential amplification circuit 17 in FIG. 7, and the same parts are given the same reference numerals as in FIG. 7 for the rest.

Next, the operation of the phase control circuit shown in FIGS. 7 and 8 will be explained using mathematical expressions.

The signal voltage e1 of the signal source, that is, the local subcarrier oscillator 5, and the output signal voltage obtained at the output terminal 7 are e.

When the capacitance of the capacitor 6 is C1, the resistance value of the variable impedance circuit 29 is R1, and the angular frequency is ω, the output signal voltage is e.

is expressed as the following equation.

Also, the output signal voltage e.

Letting the phase with respect to the input signal voltage e1 be θ, this is expressed as the following equation.

θ=2tan−1ωcR−・・・・(111In this case θ
can be varied within a range of 180°.

The group delay time (envelope delay time) dB/dω is as shown in the following equation.

The variable impedance circuit of the present invention can also be combined with a capacitor or inductor to form an oscillator as shown in FIG. 9, as described above.

That is, a load resistor 24 is connected between the collector of the transistor 19 of the differential amplifier circuit 17 and the power supply terminal t, and the output from the collector of the transistor 19 is supplied to the base of the NPN transistor 25.

The collector of the transistor 25 is connected to the power supply terminal t, and its emitter is grounded through a series circuit of a crystal oscillator 26 and a resistor 27, and further through a capacitor 28 for 90° phase shift.

It is connected to the output terminal 7, ie, the emitter of the transistor 1, through this series circuit and a capacitor 6, and also to the base of the transistor 19 through a resistor 22.

In the oscillator shown in FIG. 9, the oscillation frequency can be varied by varying the current of the current source circuit 8.

The relationship between frequency and phase of this oscillator is shown in FIG.

Incidentally, instead of the crystal resonator 26, a series resonant circuit consisting of a series circuit of a coil and a capacitor may be used.

Further, the capacitor 6 is a coupling capacitor.

This oscillator is a phase controlled current controlled oscillator.

According to the variable impedance circuit of the present invention described above, it is possible to obtain a variable impedance circuit that has a simple configuration, has a wide impedance variable range, a wide dynamic range, and does not change the absolute value of the signal amplitude when applied to a phase control circuit. .

[Brief explanation of drawings]

Fig. 1 is a circuit wiring diagram showing one embodiment of the present invention, Fig. 2 is a characteristic curve diagram, Figs. 3 to 9 are circuit wiring diagrams showing other embodiments of the invention, and Fig. 10 is a characteristic curve diagram. It is a diagram. t is the first power supply terminal, 1 is a transistor, 2.3 and 4
are first, second and third diodes, 6 is an impedance circuit, 7 is an output terminal, 8 is a current source circuit, and 35 is an impedance control means.

Claims (1)

[Claims] 1 A series circuit consisting of a collector-emitter of a transistor and a first diode is connected between the first and second power supply terminals, and a series circuit of a first diode and a second diode is connected between the base of the transistor and the second power supply terminal. an impedance control circuit that connects a series circuit of diodes, connects the base of the transistor to the first power supply terminal through a current source circuit, and changes the current flowing through the transistor to vary the emitter input impedance of the transistor. is provided in the base circuit of the transistor, an impedance is provided at the midpoint of the connection between the transistor and the first diode, and an input signal is supplied through the circuit; A variable impedance circuit characterized by extracting an output signal.