Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage controlled oscillator having a wide frequency variable range and suitable for making a circuit monolithic. [Prior Art] In recent years, PLL frequency synthesizers have been used as local oscillators in wireless devices. This is because the frequency can be switched only by simple digital control. A voltage-controlled oscillator (VCO) is an indispensable component of this frequency synthesizer.
There is. The VCO oscillates at a frequency proportional to a DC voltage applied from the outside. The configuration of a conventional VCO is shown in FIG. The collector of the transistor 11 is grounded through the inductor 12-varactor diode 13, and grounded through the capacitors 14-15, the base is grounded, and the emitter is connected to the connection point of the capacitors 14,15. The control terminal 16 is derived from the connection point between the inductor 12 and the varactor diode 13 and
The connection points 14 and 15 are connected to the output terminal 19 through the capacitor 17 and the buffer amplifier 18. Although a transistor is used as an oscillation active element, a similar configuration can be used for an FET. The oscillation conditions of this circuit are as follows. And impedance Z C viewed right from A-A, the impedance Z L viewed from the left A-A are respectively expressed as follows. The current amplification factor of g m is the transistor 11 in the above equation, C 1, C 2 is the capacitance of the capacitor 14, 15, L is the inductance of the inductor 12, R is the loss resistance of the inductor 12 and the varactor diode 13. C 12 is represented by the following equation. This circuit Oscillates when the condition is satisfied. Its oscillation frequency ω 0
Is Becomes Here, CV is the capacitance of the varactor diode 13. Varactor diode 13 capacitance
C V is given assuming that the applied voltage is V C Here, C VO is the capacitance when V C = 0, φ is the diffusion potential, and m is 1/3 to 1/2. Therefore, when V C is increased, C V decreases as shown in FIG. 13, and the oscillation frequency increases as shown in FIG. As an example, C 1 = C 2 = 5p
F, L = 10 nH, and the varactor diode 13 has V C =
Assuming a change of 2 to 10 pF at 1 to 5 V, the oscillation frequency is 1.6 GHz
Changes at ~ 2.1GHz. As described above, the conventional VCO changes the oscillation frequency by changing the voltage applied to the varactor diode 13. Such a conventional VCO has the following disadvantages. The frequency variable range is determined by the change range of the capacitance of the varactor diode 13. The change amount of the capacitance in the practical applied voltage range (1 to 5 V) is about five times, and therefore the oscillation frequency is changed over a wide band. Can not do. C V does not change linearly with V C as shown in equation (2-5). Further, the oscillation frequency is not linear with respect to CV as shown in the equation (2-5). For this reason, the relationship between V C and the oscillation frequency is as shown in FIG. 14. When V C is small, the change amount of the oscillation frequency is large, and when V C is large, the change amount of the oscillation frequency is small. Thus, VC
The sensitivity of O (frequency change / V C ) greatly changes depending on V C. When a frequency synthesizer is configured using such a VCO, there is a large difference between the noise characteristic and the pull-in characteristic at the upper and lower limits of the oscillation frequency, and uniform characteristics cannot be created. The use of the inductor 12 in the circuit requires a coil or a distributed constant line, and it is difficult to reduce the size by monolithicization. "Means for Solving the Problems" In the present invention, (i) a fixed capacitance is converted into a variable inductance by using a variable gyrator circuit composed of an amplifier circuit capable of changing a gain and an inverting amplifier circuit; ) Both amplifiers use a FET amplifier circuit (Claim 1) or a differential amplifier circuit (Claim 2), respectively.
The most main feature is to change the frequency by controlling the gate voltage of the amplifier circuit or the current of the constant current power supply circuit of the differential amplifier circuit to change the respective gains. The difference from the conventional VCO is that a varactor diode is not used as a means for controlling the oscillation frequency. FIG. 1 shows a basic configuration of the present invention. In the present invention, a gyrator circuit 21 is used. The gyrator circuit 21 generally has two ports. When an element having an impedance Z is connected to one port, the input impedance seen from the other port is a value K / proportional to the reciprocal of Z.
This is a circuit that is converted to Z. Here, K is a real value. However, this gyrator circuit 21 is provided with another terminal 22 so that another value of K can be externally controlled, and is a variable gyrator circuit. A load capacitor 25 is connected to one port 23, 24 of the gyrator circuit 21, and a negative resistor 29 is connected to the other port 26, 27 via a capacitor 28. Impedance Z G viewed from the left A-A in this configuration is represented by Z G = jωC L K (4-1 ), the capacitance C L of the load capacitor 25 is converted into an inductive impedance, the inductance L, L = the KC L (4-2). When the resistance value of the negative resistance 29 and -R a, -R a to the circuit and the oscillation state is set to balance the loss resistance component viewed from the left A-A. The oscillation angular frequency ω 0 is It is represented by K so can be varied by a control voltage V C of are terminals 22 a constant determined from the internal circuit of the gyrator circuit 21 can control the omega 0. FIG. 2 shows another example of the basic configuration. The difference from the circuit of FIG. 1 is that the capacitor 28 and the negative conductance 31 are connected to the terminal 26.
And the terminal 27 are connected in parallel. Negative conductance G a is set to balance the loss conductance when viewed left from A-A to the circuit and the oscillation state. At this time, the oscillation frequency is expressed in the same manner as in equation (4-3). By configuring the oscillation circuit as described above, a voltage-controlled oscillator having the terminal 22 as a frequency control terminal can be obtained as in the circuit shown in FIG. As shown in FIG. 3, the variable gyrator 21 can be configured by connecting an amplifier 32 with a variable gain terminal and an inverting amplifier 33 with a variable gain terminal in a ring shape. 4 and 5 show equivalent circuits of the amplifiers 32 and 33, respectively.
When such gyrator connecting the load impedance Z L to the terminal 23 and the terminal 24 of the circuit, the impedance Z G as viewed from the terminal 26 and the terminal 27 It is expressed as Here, g m1 and g m2 are the current amplification factors of the amplifiers 32 and 33, and the above equation is effective when the output current of the amplifiers 32 and 33 does not flow into the input terminals of the other amplifiers. That is, the condition is satisfied under the following conditional expression. | Z L |, | Z G |, << | Z i |, | Z O | (4-5) Z i is the input impedance of the amplifiers 32 and 33, and Z O is the amplifier 32 and
33 is the output impedance. Thus to set the circuit, by connecting the 1 / j [omega] C L by a capacitance C L as Z L, the impedance seen from the terminal 26 and the terminal 27 becomes inductive. The inductance L at that time is It is. In order to make K variable, the current amplification factors g m1 and g m2 of the amplifiers 32 and 33 may be made variable. An amplifier or an inverting amplifier having a variable gain terminal as described above can be realized by an FET or a bipolar transistor. FIG. 6 shows an example of an inverting amplifier using an FET. The gate of the FET 34 is connected to the terminal 23 and to the gate voltage terminal 36 through the high-frequency blocking inductor 35, the source is grounded, the drain is connected to the terminal 26, and the drain is connected to the drain voltage terminal 38 through the high-frequency blocking inductor 37. You. As shown in FIG. 7 in this inverting amplifier, it can be varied current amplification factor g m zero by the gate bias V g the gate voltage terminal 36. Therefore, if the gate voltage terminal 36 is used as the gain variable terminal 22 and the oscillation circuits shown in FIGS. 1 and 2 are configured, a VCO can be configured. Another method of realizing an amplifier with a variable gain terminal is to use a differential amplifier. FIG. 8 shows an example in which a bipolar transistor is used. Transistors 41 and 42
Are connected to an inverting output terminal 43 and an in-phase output terminal 44, and are connected to a power supply terminal 47 through load resistors 45 and 46.Each emitter is connected to the collector of a transistor 48 for a constant current circuit. Base is an input terminal
49, and the base of the transistor 42 is connected to a reference voltage source 51.
And the base of the transistor 48 is connected to the variable gain terminal 52.
, And grounded through the emitter resistor 53. This differential amplifier becomes an in-phase amplifier if the terminal 49 is an input and the terminal 44 is an output.
Is an output of an inverting amplifier. Gain g m of the amplifier changes as shown in FIG. 9 with respect to the base voltage V B of the transistor 48. g m has a region which can be linearly changed with respect to V B, in this region can be expressed as g m = αV B. Here, α is a real constant. FIG. 10 shows a variable gyrator circuit using this amplifier. Therefore, if a capacitor 25 is connected to the load terminals 23 and 24 as shown in the figure, the impedance seen from the input is converted into an inductive impedance. However (4-5) R 0 from the conditions of expression, R 0 "1 / (jωC L), is set to be Z G. Inductance L seen from the time the terminal 26 and 27, than if g m1 = g m2 = g m (4-6) equation, Becomes VCO using the variable gyrator circuit shown in Fig. 10
Is shown in FIG. The collector of the transistor 54 is connected to the terminal 26, the capacitor 55 is connected between the collector and the emitter of the transistor 54, the capacitor 56 is connected between the emitter and the base, the base is grounded, and the emitter is connected through the capacitor 57 and the buffer amplifier 58. Connected to output terminal 59. The impedance between the base and the emitter and between the collector and the emitter of the transistor 54 is
If sufficiently smaller than the impedance, the negative resistance R a of the transistor 54, g m0 is represented by the current amplification factor of the transistor 54, and C 1 and C 2 are represented by the capacitance of the capacitors 55 and 56. C 1 and C 2 are set so that the loss of R a and variable gyrator circuit 21 are balanced. This is the oscillation condition. The oscillation angular frequency ω 0 is ω 0 = 1 / (LC C ) 1/2 (4-9) L: inductance C C = C 1 C 2 / (C 1 + C 2 ) of the impedance as viewed from the variable gyrator circuit 21 side. ) Is expressed by equation (4-7), so that the oscillation angular frequency ω 0 is Becomes Therefore, ω 0 can vary linearly with V B. Further, since the gain variable range of the amplifier with a variable gain terminal used in the variable gyrator is wide, a VCO that can change the frequency over a wide band can be realized. "Effects of the Invention" As described above, the VCO of the present invention can oscillate in a wide band and can change the output frequency linearly with respect to the frequency control voltage. Further, the inductance element used in the VCO of the present invention is realized by a gyrator circuit using an active element. Therefore, it is suitable for making the circuit monolithic, and it is easy to reduce the size of the circuit.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a basic circuit of the present invention, FIG. 2 is a diagram showing another example, FIG. 3 is a diagram showing an example of a variable gyrator circuit, and FIG. 5 is an equivalent circuit diagram of the amplifier 32 in FIG.
FIG. 6 is an equivalent circuit diagram of the amplifier 33 in FIG. 3, FIG. 6 is a circuit diagram showing an inverting amplifier with a variable gain terminal using an FET, and FIG.
FIG. 8 is a gain-gate voltage characteristic diagram, FIG. 8 is a circuit diagram showing an amplifier with a variable gain terminal using a differential amplifier, FIG. 9 is a gain-base voltage characteristic diagram thereof, and FIG. FIG. 11 is a circuit diagram showing an embodiment of the present invention, FIG. 12 is a circuit diagram showing a conventional voltage controlled oscillator, and FIG. 13 is a junction capacitance-control voltage of a varactor diode. FIG. 14 is a characteristic diagram of the oscillator of FIG. 12 versus the control voltage.