Abstract:
The present invention uses an AC signal and an external DC control voltage to generate a plurality of levels of output DC voltages. The level of the output voltage is determined by the DC control voltage and has the opposite polarity. The invention is preferably implemented as a balanced circuit, which generates spurious signals at even harmonics of the AC frequency signal. The spurious signals can then be filtered out using a low-pass filter.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to microwave dc-dc converter circuits, and in particular, to an adjustable low-spurious signal dc-dc converter that can be fabricated as a monolithic integrated circuit. 
     BACKGROUND 
     Front-end amplifiers used in equipment such as cable television sets and other broadband RF receivers require power supplied from a source that has very low distortion, i.e., a very low incidence of spurious signals. Additionally, in many applications such as cellular/PCS telephones, it is desirable that a single power supply be used. 
     In amplifiers using depletion-mode devices such as Field Effect Transistors (FETs), there is a need for a bias voltage to turn the devices OFF (non-conducting) from a normally ON (conducting) state. Such depletion-mode devices are commonly used in high-frequency devices constructed from materials in Groups III-V (e.g., GaAs). Common examples of high-frequency semiconductor devices built using these and other semiconductor materials are MESFETs, BJTs, and others. 
     If a depletion-mode device requires a negative bias supply, but the main circuit power supply is positive, then the negative bias potential must in some manner be generated from the positive potential. Conventional systems use a charge pump or an oscillator-rectifier supply to provide such a negative bias potential. But these conventional devices are generally bulky or difficult to integrate in a monolithic integrated circuit chip. Moreover, rectification generally produces spurious signals in such conventional devices. 
     SUMMARY 
     In one aspect, a preferred embodiment of the present invention uses an AC signal and a DC control voltage to generate a plurality of levels of output DC voltages. The level of the voltage output is determined by the DC control voltage and has the opposite polarity of the DC control voltage. The present invention thus effectively converts the DC control voltage to an output voltage, i.e., it functions as a dc-to-dc converter. 
     In another aspect of the present invention, the dc-dc converter is preferably implemented as a balanced voltage source, which will only have spurious signals at even harmonics of the AC frequency signal. In a further aspect, the present invention comprises a circuit that does not load the output, thereby keeping the output voltage ripple to a minimum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description of the preferred embodiments, where like numerals designate like parts, in which, 
     FIG. 1 is a schematic of a preferred embodiment of a dc-dc converter circuit of the present invention; 
     FIG. 2 depicts simulated output voltages produced by a circuit configured according to the principles of the present invention at different control voltages; and 
     FIG. 3 is a schematic of another preferred embodiment of a dc-dc converter circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first preferred embodiment of a dc-dc converter circuit, which comprises two capacitors C 1 , and C 2 , each having a first terminal and a second terminal; and a plurality of very small diodes D 1 , D 2 , D 3  and D 4 , each having an anode and a cathode. An output voltage tapping point V OUT  is located at an output capacitor C OUT , which is connected to a ground at one end. 
     The capacitors C 1 , C 2  and diodes D 1 , D 2 , D 3  and D 4  are connected in the following way. The cathode of diode D 1  and the anode of diode D 3  are coupled to the second terminal of the capacitor C 1 . The cathode of the diode D 3  is coupled to the first terminal of the capacitor C 2 . The anode of the diode D 1  is coupled to the output tapping point V OUT . The cathode of diode D 2  and the anode of diode D 4  are coupled to the second terminal of the capacitor C 2 . The cathode of the diode D 4  is coupled to the first terminal of the capacitor C 1 . The anode of the diode D 2  is coupled to the output tapping point V OUT . 
     The capacitors C 1  and C 2  are coupled to a voltage source V S , which comprises a first terminal and a second terminal. The voltage source V S  is configured to provide a voltage of +1V at the first terminal and −1V at the second terminal during one half of a duty cycle, and a voltage of −1V at the first terminal and +1V at the second terminal during a second half of a duty cycle (i.e., the voltage between the first and second voltage source terminals varies between +2V during the first half cycle and −2V during the second half cycle). In a preferred embodiment, the first terminal of the voltage source V S  is coupled to the first terminal of the capacitor C 1 . The second terminal of the voltage source V S  is coupled to the first terminal of the capacitor C 2 . The control circuit  110  includes a voltage source V CONTROL  and a pair of inductors L 1  and L 2  to block any alternating voltage from affecting V CONTROL . 
     Advantageously, the voltage source V S  is a high frequency source, operating at 1-2 gigahertz. In a preferred embodiment, the voltage source V S  is a voltage-controlled oscillator or a local oscillator with a mixer circuit (not shown). The capacitors C 1 , and C 2  are small, preferably about 1 pF each. The diodes D 1 , D 2 , D 3  and D 4  are chosen to be of very small dimensions, preferably about 5 μm 2  in area. The small size of the diodes ensures that the capacitors C 1 , and C 2  are not shunted. C OUT  is also preferably about 1.0 pF. The AC blocking inductors L 1 , and L 2  are chosen to be large, preferably about 10-20 nH. The impedance should preferably be larger than the AC source impedance. The control voltage V CONTROL  is a DC feed voltage source preferably between about 0 to +3V. 
     OPERATION 
     Referring still to FIG. 1, at each of its terminals, the voltage source V S  provides a signal between +1V and −1V. Any DC component in this signal is blocked by the DC blocking capacitors C DC1  and C DC2 . For the purposes of the present description, it will be assumed for clarity of explanation and without loss of generality that diodes D 1 , D 2 , D 3  and D 4  act as ideal rectifiers and therefore have no voltage drop across them when conducting. 
     During a first half of a first duty cycle, a voltage of +1V is applied at the first terminal of V S  and the capacitor C 1 , charges to +1V at its first terminal. At the same time, a voltage of −1V is applied to the first terminal of the capacitor C 2 , which voltage appears at the second terminal of the capacitor C 1 , due to the forward-biased diode D 3 . Diode D 4  is reverse-biased, and therefore acts as an open circuit. Thus, a total potential difference of 2V is formed across the first and the second terminals of the capacitor C 1 . 
     During a second half of the first duty cycle, a voltage of +IV is applied at the first terminal of V S , and a voltage of +1V is applied at the second terminal of the voltage source V S . D 3  is now reverse-biased, and therefore does not conduct any current. But because the voltage V S  of −1V appears at the first terminal of the capacitor C 1 , and since the voltage across a capacitor cannot change instantaneously, this produces a voltage of −3V (over the course of a few duty cycles) at the output tapping point V OUT , since the diode D 1  is forward-biased. 
     Additionally, during the second half of the first duty cycle, a voltage of+1V is applied at the second terminal of the voltage source V S , which appears at the first terminal of the capacitor C 2 . Similarly, a voltage of −1V appears at the second terminal of the capacitor C 2 , since the diode D 4  is forward-biased during this half-cycle. Thus, a potential difference of +2V appears at the capacitor C 2  during this half-cycle, in a manner similar to that across the capacitor C 1  during the first half of the duty cycle. Thus, it is easily seen that the operation of capacitors C 1 , and C 2 , are 180° out of phase with each other. 
     During the first half of a second duty cycle, a voltage of +1V is applied at the first terminal of the voltage source V S , which reverse-biases the diode D 4 . During this halfcycle, a voltage of −1V is applied at the second terminal of the voltage source V S , which causes a potential shift across C 2 , causing a voltage of −1V and −3V to appear (over the course of a few duty cycles) at the first and the second terminals of the capacitor C 2 , in a manner explained with reference to the capacitor C 1 , above. Because the source voltage V S  oscillates at a very high frequency such as 1-2Ghz, before the capacitors C 1 , or C 2  will have discharged, a next cycle starts, thereby ensuring a stable voltage at the output tapping point V OUT . 
     A DC feed signal is applied by the control voltage source V CONTROL , which is applied in such a way as to cancel the output voltage by discharging the capacitors via the diodes D 3  and D 4 . Thus, when V CONTROL  is +1V, the reverse bias across the diodes D 3  and D 4  is reduced by +1V, causing a potential at V OUT  to become −2V. Similarly, when V CONTROL  is adjusted to be +2V, the potential at V OUT  becomes −1V, and when V CONTROL  is increased to +3V, V OUT  becomes 0V. Any further increase in V CONTROL  is ineffective, since it reverse-biases the diodes D 3  and D 4 . By thus adjusting V CONTROL  to be between 0 and +3V, the output of the circuit at V OUT  can be configured to vary between 0 and −3V, thereby achieving a range of different output voltages at V OUT , as shown in FIG.  2 . 
     The pair of inductors L 1  and L 2  are preferably of high enough impedance to block any AC voltage from reaching the dc-feed V CONTROL . 
     In addition to producing a stable DC voltage at the output V OUT , the disclosed circuit generates an AC ripple voltage. Such AC ripple voltages are unwanted, as they introduce spurious frequencies into any subsequent stage to which the output voltage V OUT  is applied. 
     Since, as explained above, the two capacitors C 1 , and C 2  charge at a 180° phase difference, even harmonics are reinforced. This ripple can be eliminated easily by filtering with low pass filters at a next stage. 
     FIG. 3 shows a second preferred embodiment of a dc-dc converter circuit in accordance with the present invention. In the second embodiment, the inductors L 1  and L 2  are replaced by a pair of resistors R 1  and R 2 , each of which is preferably about 1 KΩ. The operation of the second embodiment is substantially identical to that of the first embodiment. Since resistors can be fabricated on smaller spaces compared with inductors, the chip space required by the second embodiment is less than that of the first embodiment. 
     In addition, instead of one voltage source Vs as in the first embodiment, in the second embodiment a pair of voltage sources Vs 1  and Vs 2  that are 180 degrees out of phase with each other is shown. When a pair of voltage sources is used, each voltage source is preferably connected to an input resistor, Rs 1  and Rs 2 . Rs 1  and Rs 2  are preferably each about 50 Ω. 
     It is further understood that the embodiments described herein are merely illustrative and not intended to limit the scope of the invention. One skilled in the art may make various changes, rearrangements and modifications without substantially departing from the principles of the invention, which is limited only in accordance with the claims. It should be easily understood that by changing the input voltage supplied by the voltage source V S  and the control voltage source V CONTROL , an entirely different range of output voltages can be achieved. Further, certain portions of the described circuitry can be modified to include discrete components, whereas the remaining portion can be etched into a monolithic integrated circuit. Additionally, the materials described herein may change. Accordingly, all such deviations and departures should be interpreted to be within the spirit and scope of the following claims.