SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549


                                 SCHEDULE 13G
                                (Rule 13d-102)

                 INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT
              TO RULES 13d-1(b), (c) AND (d) AND AMENDMENTS THERETO FILED
                              PURSUANT TO 13d-2


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                                NOVEMBER 3, 2006

                      CHECK THE APPROPRIATE BOX TO DESIGNATE THE RULE
                        PURSUANT TO WHICH THIS SCHEDULE IS FILED:
                               [   ] Rule 13d-1(b)
                               [ x ] Rule 13d-1(c)
                               [   ] Rule 13d-1(d)

                                 
                             
                       MONOLITHIC POWER SYSTEMS INC.
-----------------------------------------------------------------------------
                             (Name of Issuer)



                                  COMMON STOCK
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                         (Title of Class of Securities)


                                   609839105
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                               (CUSIP Number)



<PAGE>

CUSIP Number  609839105
1) NAME OF REPORTING PERSON
   S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

           SELIGMAN TECH SPECTRUM (MASTER) FUND
           98-0361992

2) CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

                                                            (a) /   /
                                                            (b) / x /

3) SEC USE ONLY



4) CITIZENSHIP OR PLACE OF ORGANIZATION

           CAYMAN ISLANDS

                   (5) SOLE VOTING POWER                -0-
NUMBER OF SHARES
BENEFICIALLY OWNED (6) SHARED VOTING POWER       1,578,305*
BY EACH REPORTING
PERSON WITH        (7) SOLE DISPOSITIVE POWER           -0-

                   (8) SHARED DISPOSITIVE POWER  1,578,305*


9) AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

                                                  1,578,305*

10) CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  /  /



11) PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

                                                       5.23%

12) TYPE OF REPORTING PERSON

        CO

        * J. & W. Seligman & Co. Incorporated (JWS) is the investment adviser
 for the reporting person, and the shares reported herein will be included in 
 the Schedule 13G filed by JWS with respect to the issuer to the extent
 such shares are still deemed to be beneficially owned by JWS as of the
 reporting date. 
 

            --------------------------------------------------




<PAGE>


Item 1(a) Name of Issuer:

          MONOLITHIC POWER SYSTEM INC.

Item 1(b) Address of Issuer's Principal Executive Offices:

          983 UNIVERSITY AVENUE
          BLDG A
          LOS GATOS,  CA 95032          

Item 2(a) Name of Person Filing:

          SELIGMAN TECH SPECTRUM (MASTER) FUND 
          
Item 2(b) Address or Principal Business Office or, if none, Residence:

          C/o J. & W. SELIGMAN & CO. INCORPORATED
          100 PARK AVENUE
          NEW YORK, NEW YORK 10017

Item 2(c) Citizenship:

          CAYMAN ISLANDS
          
Item 2(d) Title of Class of Securities:

          COMMON STOCK

Item 2(e) CUSIP Number:

          609839105

Item 3    If this statement is filed pursuant to Rules 13d-1(b), or 13d-2(b),
          check whether the person is filing as a:

          
 

Item 4    Ownership.

            Incorporated by reference to Items (5)-(9) and (11) of the cover
            page.    


Item 5    Ownership of Five Percent or Less of a Class.

          NOT APPLICABLE

Item 6    Ownership of More than Five Percent on Behalf of Another Person.

          NOT APPLICABLE

Item 7    Identification and Classification of the Subsidiary which acquired
          the security being reported on by the Parent Holding Company.

          NOT APPLICABLE

Item 8    Identification and Classification of Members of the Group.

          NOT APPLICABLE

<PAGE>

Item 9    Notice of Dissolution of Group.

          NOT APPLICABLE

Item 10   Certification

          By signing below I certify that, to the best of my
          knowledge and belief, the securities referred to above
          were not acquired and are not held for the purpose of
            or with the effect of changing or influencing the control of
          the issuer of such securities and were not acquired and are not
          held in connection with or as a participant in any transaction
          having such purpose or effect. 



                             SIGNATURE

     After reasonable inquiry and to the best of my knowledge and belief,
 I certify that the information set forth in this statement is true,
complete and correct.

DATE:  January 16, 2007

SELIGMAN TECH SPECTRUM (MASTER) FUND

BY  /s/ LAWRENCE P. VOGEL
   -------------------------------------
   Name: LAWRENCE P. VOGEL
   Title: AUTHORIZED PERSON

 



<PAGE>

EXHIBIT

EXHIBIT 1

Seligman Tech Spectrum (Master) Fund
(the "Company")


CERTIFICATE

        The undersigned certifies that he is a Managing Director
 of J. & W. Seligman & Co. Incorporated, the Company's investment
 manager (the "Manager"), and that he is authorized to execute this
 certificate on behalf of the Company and further certifies on behalf
 of the Company that the following is (in relevant part) a true and
 complete copy of resolutions duly adopted by the Board of Directors
 of the Company as of December 21, 2006, and such resolutions have not
 been rescinded or amended and remain in full force and effect on the
 date hereof:

        FURTHER RESOLVED, that each person named below be, and hereby
 is, designated as an authorized person (each an "Authorized Person")
 to act on behalf of the Company as from time to time authorized by 
 the Board of Directors (subject to the limitations provided herein or
 from time to time):

Joseph Cheung
Frank J. Nasta  
David F. Stein
Richard C. Dluzniewski
Richard M. Potocki
Edward G. Stromberg III
Thomas J. Figueira
Thomas G. Rose
Lawrence P. Vogel
Paul B. Goucher
Julie Rosenberg
Yvonne R. Walker
John Guancione
Rodney G.D. Smith
Brian T. Zino

.    .    .     

        FURTHER RESOLVED, that any Director of the Company and each
 Authorized Person be, and the same hereby are, authorized to perform
 on behalf of the Company any and all such acts as they or any of he
 or she may deem necessary or advisable in connection with the
 organization and operation of the Company (including providing
 instructions to the Manager, PFPC, Morgan Stanley & Co. Incorporated,
 SAI and any other service providers and executing contracts on behalf
 of the Company) or the offer and listing of the shares of the Company
 or in order to comply with all applicable laws or regulations and in
 connection therewith to execute and file all requisite papers and
 documents including but not limited to applications, reports,
 irrevocable consents and appointments of attorneys for service of 
 process, and the execution by such Director or Authorized Person of
 any such paper or document or the doing by him or her of any act in
 connection with the foregoing matters shall conclusively establish his
 or her authority therefor from the Company and the approval and 
 ratification of the Company of the papers and documents so executed and
 actions so taken.

        IN WITNESS WHEREOF, I have hereunto set my name as of this 11th
 day of January, 2007.

  /s/ Paul B Goucher
  Paul B Goucher
  Authorized Person