JPS6051803B2 - variable impedance circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable impedance circuit applied to variable frequency oscillators, FM modulation circuits, and the like.

The applicant of the present application previously proposed a variable reactance circuit in Japanese Patent Application No. 49-76779 (Japanese Unexamined Patent Publication No. 49-76779), in which a capacitor 3 is connected between the input and output terminals of an amplifier 1 having a mutual conductance band and a load resistance 2, as shown in FIG. It was proposed in 1984-6444). When the values of the load resistor 2 and capacitor 3 are R and Cf, respectively, the equivalent impedance Z seen from the input side is expressed by the following equation. −− +RL JωCfj(wo)Cf Zi■ 1+gmRL61+gmRL :− j−・ − ・・・・・・(1) 胛 ωRLCf
That is, the equivalent impedance Z0 becomes a pure reactance component, and its value is proportional to 1/gm, so it can be varied by controlling gm.

However, as shown in Fig. 2, Co
If there exists a distributed capacitance 4 such that

That is, in general, as shown in FIG. 3, when a feedback impedance 5 of Z is connected between the input and output terminals of the amplifier 1, and the output terminal is AC grounded via an impedance of 4, the equivalent impedance ZI becomes Z, ■L”1 ・・・・・・(2
) 1+gm4, and if (Z, >4), the above equation becomes zi=-...(3) 1+gm4.

Here, since generally (gm4>1), Zi: - ・
1=-・Z, Yo... (4)gm4gm.

Therefore, in the case shown in Fig. 2, (Zf■11jωCf, Y
o■IIRL+jωCo), so from equation (4), Z,: -.

As is clear from equation (5), the equivalent impedance Zi is no longer a pure reactance component, but includes an effective resistance component (1 Gm-COIC,). In this way, the variable reactance circuit shown in FIG. 1 changes not only the reactance component but also the resistance component, which causes undesirable effects when applied to an FM modulator or the like. In view of this, the present invention aims to realize a variable impedance circuit in which only the reactance component changes, and also to realize a variable resistance circuit by selecting the vector value of the impedance connected to the output terminal of the amplifier 1. The second goal is to provide.

FIG. 4 shows the principle connection of an example of the present invention, in which a coil 7 with an inductance L is connected in parallel with the load resistor 2.

In this case, (Zf=11jωC,, YO=11RL+
j(ωCO−11ωL), so by substituting this into equation (4), we can satisfy the condition (5=±), that is, (ω2LC0= C, ω2LC, 1) in equation (6). If the inductance L of the coil 7 is selected, equation (6) becomes, equation (7) is the same as equation (1), and the equivalent impedance Z, becomes the pure reactance component (
capacity ratio).

FIG. 5 shows an embodiment in which the present invention is applied to an FM modulation circuit.

In FIG. 5, 8A and 8B are transistors constituting a differential amplifier, and a parallel circuit of a load resistor 2 and a coil 7 is connected to the collector of the transistor 8A, that is, the output terminal of the differential amplifier, and between the collector and base of the transistor 8A. Capacitor 3 is connected to . A common connection point between the emitters of transistors 8A and 8B is connected to the collector of transistor 9, the emitter of transistor 9 is grounded via a resistor 10, and the base of transistor 9 is connected to a modulation signal source 11 having a DC component superimposed thereon. be done. In addition, 12 is an oscillator 13
A ceramic resonator 14 is connected in the feedback path between the emitter and base of the transistor.

Further, the bases of transistors 8A and 12 are connected to the emitter of transistor 16 and resistor 1 via bias resistor 15.
7, and a transistor 8B is connected to this connection point.
The base of is connected via a bias resistor 18. In such a configuration, the equivalent impedance Zj viewed from the base side of the transistor 8A is expressed by equation (7).

Here, if the current flowing through the transistor 9 is I, the mutual conductance is given by the characteristics of the differential amplifier.

Substituting equation (8) into equation (7) yields. This (9)
As is clear from the equation, this can be considered equivalent to connecting the variable capacitor 19 in parallel with the ceramic resonator 14 of the oscillator 13. Since the value of the variable capacitor 19 changes in proportion to the current 1 from the modulation signal source 11, F
N4 modulation can be performed. Furthermore, if a differential amplifier is used as the amplifier 1, stable operation can be expected and DC coupling is possible. Note that in the configuration shown in FIG. 4 or FIG. 5, the capacitor 3 is used as the feedback impedance 5, but the present invention is not limited to this, and a coil may also be used.

In this case, when the inductance of the coil is Lf, the equivalent impedance Zi becomes as described above, (LdL=ω2L,C0), that is, (ω2LC0=1)
If the value of the coil 7 is selected so that the equivalent impedance Zi can be made into a pure reactance component (inductive).

FIG. 6 shows the principle connection when the present invention is applied to a variable resistance circuit, in which the output end of the amplifier 1 is grounded via a capacitor 20 in an alternating current manner.

A constant current source 21 in parallel with the capacitor 20 is a direct current source for the amplifier 1. In the configuration shown in FIG. 6, the equivalent impedance Zi becomes \11 from equation (4), and by changing Gm, a variable resistance circuit can be realized.

Furthermore, the value of the resistance becomes independent of the input frequency. FIG. 7 shows an example of a circuit based on this principle, in which 22 represents an input signal source and 23 represents an output terminal.

Similar to the configuration shown in FIG. 5 described above, by changing the control current 1 using the control voltage source 24 connected to the base of the transistor 9, the mutual conductance Gm of the differential amplifier consisting of the transistors 8A and 8B is changed. The input signal source 22 is connected to the base of the transistor 8A via a resistor 25 and a capacitor 26, and the base is led out as an output terminal 23. A capacitor 20 is inserted between the collector of the transistor 8A and the power supply terminal, and a constant current source made of a transistor 27 is connected in parallel with the capacitor 20. With the circuit configuration shown in Figure 7,
The equivalent impedance Z, seen from the base side of the transistor 8A, is a pure resistance component, and its value is equal to the control current 1.
This equivalent impedance Zi
If the variable resistor 28 is replaced, the circuit of FIG.
It can be expressed as the equivalent circuit shown in the figure.

In addition, in the circuit configuration of FIG. 7, capacitors 3 and 2
0 as L and L respectively. If we replace it with a coil of inductance, the equivalent impedance at this time will be:
Since it is a coil, a separate constant current source 21 is not required.

FIG. 9 shows another configuration example when the present invention is applied to a variable resistance circuit, in which the output terminal of the amplifier 1 is grounded in an alternating current manner via a parallel circuit of capacitors 29 and 30. .

In this case, from equation (4), the equivalent impedance Zi is
The value of capacitor 29 is C. and the value of the coil 30 is L. Here, if the value of the inductance L is selected so that (ω2LC0>1), the equivalent impedance Z becomes a positive resistance, and if the value is chosen so that (ω2LC0<1), the equivalent impedance Zi becomes a negative resistance. Become. Of course, the equivalent impedance Zi is proportional to (11 gm). Corresponding to the principle connection shown in FIG. 9, a specific connection configuration shown in FIG. 10 can also be realized.

The equivalent circuit corresponding to FIG. 10 is shown in FIG. 11. In FIG. 11, the variable resistor 31 corresponds to the equivalent impedance Z. Note that in the configuration of FIG. 10, the capacitor 3 can be replaced with a coil, and in that case, if the value of the coil is L, then the following equation is obtained.

Therefore, when (ω2LC0>1), it becomes a positive resistance circuit, and when (ω2LC0>1), it becomes a negative resistance circuit.

[Brief explanation of drawings]

Figure 1 is a basic connection diagram of an example of a variable reactance circuit.
Figures 2 and 3 are connection diagrams used for the explanation, Figures 4 and 5 are theoretical and concrete connection diagrams of one example of the present invention, and Figures 6 to 8 are other examples of the present invention. The principle connection diagram, concrete connection diagram, and equivalent circuit diagram of FIGS. 9 to 11 are the principle connection diagram, concrete connection diagram, and equivalent circuit diagram of still other examples of the present invention. 1 is amplifier, 2 is load resistance, 4 is distributed capacitance, 8A, 8B
11 is a modulation signal source, 13 is an oscillator, 22 is an input signal source, and 23 is an output terminal.

Claims (1)

[Claims] 1 Connecting a reactance element between the input and output terminals of an amplifier having mutual conductance gm, connecting the output terminal of the amplifier to alternating current ground via an impedance Z_0 including the reactance element, and selecting a vector value of the impedance Z_0. By making the equivalent impedance of the amplifier viewed from the input side of the amplifier substantially only a pure reactance component or only a pure resistance component, and by changing the mutual conductance gm, the value of the equivalent impedance is controlled. variable impedance circuit.