Abstract:
Provided is a voltage-controlled oscillator (VCO) using a current feedback network for use in a wireless communication terminal. The voltage-controlled oscillator has high input impedance and low output impedance, so that a degree of isolation from the external load is excellent, thereby preventing degradation of the Q-factor by the load in overall oscillation circuit. In the voltage-controlled oscillator of the present invention, an LC resonator is provided to generate positive feedback, and negative resistance may be obtained at a wider frequency range by tuning a varactor of the LC resonator. And a boosting inductor is inserted into the positive feedback loop to have a greater negative resistance, therefore it is possible to prevent a problem in which the oscillation does not occur due to the parasitic resistance components generated during circuit fabrication.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a voltage-controlled oscillator (VCO) and, more particularly, to a voltage-controlled oscillator using a current feedback network. 
   2. Discussion of Related Art 
   The voltage-controlled oscillator is a component used as a local oscillator in many wireless communication devices. As a conventional voltage-controlled oscillator, there are a Collpits oscillator that uses capacitor feedback, a Hartley oscillator that uses inductor feedback, and a Clapp oscillator that modifies the Collpits oscillator in terms of frequency stability. 
   Among these, the Hartley oscillator is in need of an inductor with a high Q-factor so that it is rarely used as a commercial product, while the Clapp oscillator and the Collpits oscillator are widely and commercially used due to good frequency stability and easy frequency tuning. 
   The Clapp oscillator and the Collpits oscillator generate negative resistance using the capacitor feedback. Therefore, due to a parasitic resistance component or a resistance component of the capacitor accompanied at the time of fabricating an oscillator circuit, the negative resistance may not be generated. Recently, in the commercial VCO products, a buffer is connected to an output of the Collpits oscillator or the Clapp oscillator to increase output power and improve a degree of isolation from the external load, thereby preventing degradation of the quality factor in the VCD circuit. However, when the buffer is connected to the output, the circuit becomes complicated so that a problem occurs that unpredictable parasitic components grow larger. Therefore, due to the unpredictable parasitic components, the oscillation may not be made at a designed frequency or the oscillation may not be made at all, which leads to a cumbersome task of tuning capacitors, resistors or inductors. It takes another time and money so that it is difficult to reduce the cost. Furthermore, since the tuning task cannot be performed in a semiconductor integrated circuit, yield is significantly reduced. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a voltage-controlled oscillator using a current feedback network insensitive to parasitic components, which are inevitably generated during circuit fabrication, and with the least phase noise caused by a load. 
   The present invention is also directed to a voltage-controlled oscillator using a current feedback network that acts as a buffer to the external load without employing an additionally separate buffer while problems that the oscillation is not made at a desired frequency or the oscillation is not made at all due to parasitic resistance components, which are generated during circuit fabrication, are addressed. 
   The present invention is also directed to a voltage-controlled oscillator using a current feedback network that suppresses the low-frequency noise such as 1/f noise to degrade the phase noise of a voltage-controlled oscillator. 
   According to a first aspect of the present invention, there is provided a voltage-controlled oscillator using a current feedback network, comprising: a first transistor; a second transistor having a base connected to an emitter of the first transistor and a collector connected to a base of the first transistor; a varactor connected to the emitter of the first transistor and a ground; an inductor connected to the emitter of the first transistor and the ground; and a resistor connected to the collector of the second transistor and a voltage source, wherein a collector of the first transistor and the voltage source are short-circuited, and wherein an emitter of the second transistor and the ground are short-circuited. 
   According to a second aspect of the present invention, there is provided a voltage-controlled oscillator using a current feedback network, comprising: a first transistor; a second transistor having a base connected to an emitter of the first transistor and a collector connected to a base of the first transistor; a capacitor connected to the emitter of the first transistor and a ground; a first resistor connected to the emitter of the first transistor and the ground; and a second resistor connected to the collector of the second transistor and a voltage source, wherein a collector of the first transistor and the voltage source are short-circuited, and wherein an emitter of the second transistor and the ground are short-circuited. 
   According to a third aspect of the present invention, there is provided a voltage-controlled oscillator using a current feedback network, comprising: a first transistor; a second transistor having a base connected to an emitter of the first transistor and a collector connected to a base of the first transistor; a first varactor connected to the emitter of the first transistor and a ground; a first resistor connected to the emitter of the first transistor and the ground; a second varactor connected to a collector of the first transistor and a voltage source; an inductor resistor connected to the collector of the first transistor and the voltage source; and a second resistor connected to the collector of the second transistor and the voltage source, wherein an emitter of the second transistor and the ground are short-circuited. 
   According to a fourth aspect of the present invention, there is provided a voltage-controlled oscillator using a current feedback network, comprising: a first transistor; a second transistor having a base connected to an emitter of the first transistor and a collector connected to a base of the first transistor; a first varactor connected to the emitter of the first transistor and a ground; a resistor connected to the emitter of the first transistor and the ground; a second varactor connected to the collector of the second transistor and a voltage source; and an inductor resistor connected to the collector of the second transistor and the voltage source, wherein a collector of the first transistor and the voltage source are short-circuited, and wherein an emitter of the second transistor and the ground are short-circuited. 
   According to a fifth aspect of the present invention, there is provided a voltage-controlled oscillator using a current feedback network, comprising: an LC tank connected to a voltage source; a first transistor having a collector connected to the LC tank; a second transistor having a base connected to an emitter of the first transistor and a collector connected to a base of the first transistor and the LC tank; a first capacitor connected to the emitter of the first transistor and a ground; a first resistor connected to the emitter of the first transistor and the ground; a first inductor connected to the emitter of the second transistor and the ground; a third transistor having a collector connected to the LC tank; a fourth transistor having a base connected to an emitter of the third transistor and a collector connected to a base of the third transistor and the LC tank; a second capacitor connected to the emitter of the third transistor and a ground; a second resistor connected to the emitter of the third transistor and the ground; a second inductor connected to the emitter of the fourth transistor and the ground; and varactors connected to the collectors of the first transistor and the second transistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a diagram showing a voltage-controlled oscillator using a current feedback network according to a first embodiment of the present invention; 
       FIG. 2  is a diagram showing a voltage-controlled oscillator using a current feedback network according to a second embodiment of the present invention; 
       FIG. 3  is a diagram showing a voltage-controlled oscillator using a current feedback network according to a third embodiment of the present invention; 
       FIG. 4  is a diagram showing a fully differential voltage-controlled oscillator using a current feedback network according to a fourth embodiment of the present invention; 
       FIG. 5  is a Bode diagram for a voltage-controlled oscillator using a current feedback network according to a first embodiment of the present invention; 
       FIG. 6  is a diagram showing a change over time in a base voltage VB and an emitter voltage VE of a first transistor and an emitter voltage VLB of a second transistor shown in  FIG. 1 ; 
       FIG. 7  is a diagram showing a simulation result of a transient response of an output voltage Vout (LB) with a boosting inductor and an output voltage Vout (no LB) without a boosting inductor shown in  FIG. 1 ; and 
       FIG. 8  is a diagram showing a measured output frequency spectrum of a voltage-controlled oscillator using a current feedback network according to a first embodiment of the present invention. 
       FIG. 9  is a diagram showing a measured output frequency spectrum of a fully differential voltage-controlled oscillator using a current feedback network according to a fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will now be described with reference to the accompanying drawings. A variety of modification may be made herein, but it should be not construed that the present invention is limited hereto. The preferred embodiments of the present invention will be provided for thorough understanding of the present invention to those skilled in the art. 
     FIG. 1  is a diagram showing a voltage-controlled oscillator using a current feedback network according to a first embodiment of the present invention. 
   In  FIG. 1 , the voltage-controlled oscillator using the current feedback network is comprised of a first transistor  103  and a second transistor  104  connected to each other to form a feedback loop, an LC tank  106 , a capacitor  109  and a first resistor  110  connected to an emitter of the first transistor  103 , a first inductor  105  connected to an emitter of the second transistor  104 , a second inductor  101  connected to a collector of the first transistor  103 , a second resistor  111  connected to a collector of the second transistor  104 , and a voltage source  100  connected to the second inductor  101  and the second resistor  111 . Further, the LC tank  106  is comprised of a third inductor  107  and a varactor  108 . 
   A part of the emitter current of the first transistor  103  is sampled by a first resistor  110  and fed back into a base input port of the second transistor  104  through a parallel RLC resonant circuit  106 ,  109  and  110 . This configuration in which the current is fed back is referred to as current-current negative feedback. Here, a reference numeral  102  indicates a loop along which the current is fed back. In general, the feedback loop is a negative feedback network if the feedback network is composed of only resistors, but it is possible for the loop to be a positive feedback network if a phase-shifting element such as a capacitor or inductor is included in the feedback network. In the voltage-controlled oscillator using the current feedback network, the first transistor  103 , the second transistor  104 , the capacitor  109 , and the varactor  108  forms the positive feedback loop to generate a negative resistance. The second transistor  104  amplifies the voltage locally generated by the capacitor  109  and the varactor  108 , so that a larger voltage is fed back and the negative resistance becomes larger. In addition, a frequency range where the negative resistance is generated may be tuned by adjusting a value of the varactor  108 . Also, in the voltage-controlled oscillator using the current feedback network, the first resistor  110  samples the low-frequency noise such as 1/f noise, and the sampled low-frequency noise is feedback to the base of the first transistor  103  through the second transistor  104 . The low-frequency noise source at the base of the first transistor  103  is cancelled by the feedback low-frequency noise, and then the phase noise of a voltage-controlled oscillator using current feedback network is improved. The emitter of the first transistor  103  may be connected only to the LC tank  106 , or only to the capacitor  109  and the first resistor  110 , or only to the capacitor  109 , the first resistor  110  and the varactor  108 . The second inductor  101  connected to the collector of the first transistor  103  is an RF choke inductor. When it is not necessary to use the RF choke, a resistor may take the place of the RF choke, or the inductor may be omitted. In other words, the voltage source and the collector of the first transistor  103  are short-circuited. A voltage of the first inductor  105  is in phase with the voltage positively fed back by the capacitor  109  and the varactor  108 , thereby obtaining a fast transient response and a large output. The first inductor  105  is also referred to as a boosting inductor. However, the first inductor  105  may be omitted. In other words, the ground and the emitter of the second transistor  104  are short-circuited. When an output is made at the emitter of the first transistor  103 , there appear high input impedance and low output impedance. Therefore, the voltage-controlled oscillator using the current feedback network may serve as a buffer to make smaller the effect of the load onto the inside of the oscillator. Further, when the larger output power is required, the output may be made at the collector or the base of the first transistor  103 . 
     FIG. 2  is a diagram showing a voltage-controlled oscillator using a current feedback network according to a second embodiment of the present invention. 
   In  FIG. 2 , the voltage-controlled oscillator using the current feedback network is comprised of a first transistor  103  and a second transistor  104  connected to each other to form a feedback loop, a first varactor  109  and a first resistor  110  connected to an emitter of the first transistor  103 , a first inductor  105  connected to an emitter of the second transistor  104 , an LC tank  106  connected to a collector of the first transistor  103 , a second resistor  111  connected to a collector of the second transistor  104 , and a voltage source  100  connected of the LC tank  106  and the second resistor  111 . Further, the LC tank  106  is comprised of a second inductor  107  and a second varactor  108 . 
   The voltage-controlled oscillator in  FIG. 2  is different from that of  FIG. 1  just in that the LC tank is arranged to the collector of the first transistor, and the emitter of the first transistor is connected to the first varactor, while the working principles are substantially the same. In  FIG. 2 , the first inductor  105  leads to a fast transient response and a larger output. However, the first inductor  105  may be omitted. In other words, the ground and the emitter of the second transistor  104  are short-circuited. 
     FIG. 3  is a voltage-controlled oscillator using a current feedback network according to a third embodiment of the present invention. 
   In  FIG. 3 , the voltage-controlled oscillator using the current feedback network is comprised of a first transistor  103  and a second transistor  104  connected to each other to form a feedback loop, a first varactor  109  and a first resistor  110  connected to an emitter of the first transistor  103 , a first inductor  105  connected to an emitter of the second transistor  104 , a second inductor  101  connected to a collector of the first transistor  103 , an LC tank  106  connected to a collector of the second transistor  104 , and a voltage source  100  connected of the LC tank  106  and the second inductor  101 . Further, the LC tank  106  is comprised of a third inductor  107  and a second varactor  108 . 
   The voltage-controlled oscillator in  FIG. 3  is different from that of  FIG. 1  just in that the LC tank is arranged to the collector of the second transistor, and the emitter of the first transistor is connected to the first varactor, while the working principles are substantially the same. In  FIG. 3 , the second inductor  101  connected to the collector of the first transistor  103  is an RF choke inductor. When it is not necessary to use the RF choke, a resistor may take the place of the RF choke, or the inductor may be omitted. In other words, the voltage source and the collector of the first transistor  103  are short-circuited. In addition, the first inductor  105  leads to a fast transient response and a larger output. However, the first inductor  105  may be omitted. In other words, the ground and the emitter of the second transistor  104  are short-circuited. 
     FIG. 4  is a fully differential voltage-controlled oscillator using a current feedback network according to a fourth embodiment of the present invention. 
   An LC tank  200  has a first and second inductors L 1  and L 2  connected to a voltage source, respectively, and a capacitor C connected to the first and second inductors L 1  and L 2 . A first transistor  103  and a second transistor  104  are connected to each other to form a feedback loop. Collectors of the first transistor  103  and second transistor  104  are connected to the LC tank  200 , respectively. A base of the first transistor  103  is connected to the collector of the second transistor  104 , and a base of the second transistor  104  is connected to an emitter of the first transistor  103 . A capacitor  109  and a resistor  110  are connected to the emitter of the first transistor  103  and a ground. An inductor  105  is connected to an emitter of the second transistor  104  and the ground. Varactor  201  having two varactor diodes D 1  and D 2  is connected to the collector of the first transistor  103  and the collector of the second transistor  104 . 
   The same feedback loop  102  in the fully differential voltage-controlled oscillator is differentially connected through the LC-tank  200 . 
   The fully differential voltage-controlled oscillator in  FIG. 4  is different from that of  FIG. 3  just in that the same feedback loop  102  is connected through the LC tank  200 , and each collector of the first transistor  103  and the second transistor  104  is connected to the varactor  201  and the capacitor of the LC tank  200 , while the working principles are substantially the same. In  FIG. 4 , the inductor  105  leads to a fast transient response and a larger output. However, the inductor  105  may be omitted. In other words, the ground and the emitter of the second transistor  104  are short 
     FIG. 5  is a Bode diagram for a voltage-controlled oscillator using a current feedback network according to a first embodiment of the present invention. In  FIG. 5 , it may be understood that the oscillation is made at a frequency of 1.8 GHz where the magnitude is greater than 0 dB and its phase crosses −180°. 
     FIG. 6  is a diagram showing a change over time in a base voltage VB and an emitter voltage VE of the first transistor and an emitter voltage VLB of the second transistor shown in  FIG. 1 . In  FIG. 6 , it may be understood that the respective voltages are in phase. 
     FIG. 7  is a diagram showing a simulation result of a transient response of an output voltage Vout (LB) with a boosting inductor and an output voltage Vout (no LB) without a boosting inductor. In  FIG. 7 , it may be appreciated that the fast response and larger voltage waveform may be obtained with the boosting inductor, 
     FIG. 8  is a diagram showing a measured output frequency spectrum of a voltage-controlled oscillator using a current feedback network according to a first embodiment of the present invention. In  FIG. 8 , it may be appreciated that the oscillation frequency is 1867 MHz. 
     FIG. 9  is a diagram showing a measured output frequency spectrum of a fully differential voltage-controlled oscillator using a current feedback network according to a fourth embodiment of the present invention. In  FIG. 9 , it may be appreciated that the oscillation frequency is 5490 MHz. 
   According to a voltage-controlled oscillator using a current feedback network of the present invention, an amplifier with a boosting inductor is included in a positive feedback loop, so that the voltage-controlled oscillator becomes insensitive to the parasitic components generated during circuit fabrication. 
   Further, according to a voltage-controlled oscillator using a current feedback network of the present invention, a low-frequency noise source of the first transistor is suppressed by the feedback loop, thereby reducing the phase noise of the voltage-controlled oscillator using a current feedback network. 
   Further, according to a voltage-controlled oscillator using a current feedback network of the present invention, a negative resistance may be obtained at a desired frequency by a varactor having a feedback loop, thereby enabling frequency tuning after circuit fabrication. 
   Further, according to a voltage-controlled oscillator using a current feedback network of the present invention, the current feedback network is used so that a degree of isolation from the external load is higher and the impact of the load onto the oscillator core may be buffered. 
   While the preferred embodiments have been described, these embodiments are for illustration only, and not for limitation of the present invention. Those skilled in the art will appreciate that a variety of modifications, changes, and adjustments may be made without departing from the spirit of the present invention. Therefore, the scope of the present invention is only limited by the attached claims, and should be construed to include afore-mentioned modifications, changes and adjustments.