<PAGE>   1
                                                                   EXHIBIT 10.13


                          LOAN MODIFICATION AGREEMENT

        This Loan Modification Agreement is entered into as of August 14, 1996,
by and between Celeritek, Inc. ("Borrower") whose address is 3236 Scott
Boulevard, Santa Clara, CA 95054, and Silicon Valley Bank ("Lender") whose
address is 3003 Tasman Drive, Santa Clara, CA 95054.

1.      DESCRIPTION OF EXISTING INDEBTEDNESS: Among other indebtedness which may
be owing by Borrower to Lender, Borrower is indebted to Lender pursuant to,
among other documents, a Promissory Note, dated September 11, 1992, in the
original principal amount of Five Million and 00/100 Dollars ($5,000,000.00)
(the "Note"). The Note has been amended pursuant to Loan Modification Agreements
dated August 15, 1993, August 15, 1994, pursuant to which, among other things,
the principal amount of the Note was increased to Six Million and 00/100 Dollars
($6,000,000.00) and August 15, 1995. The Note, together with other promissory
notes from Borrower to Lender, is governed by the terms of a Business Loan
Agreement, dated September 11, 1992, as such agreement may be amended from time
to time, between Borrower and Lender (the "Loan Agreement"). Defined terms used
but not otherwise defined herein shall have the same meanings as in the Loan
Agreement.

Hereinafter, all indebtedness owing by Borrower to Lender shall be referred to
as the "Indebtedness".

2.      DESCRIPTION OF COLLATERAL AND GUARANTIES. Repayment of the Indebtedness
is secured by a Commercial Security Agreement, dated September 11, 1992 (the
"Security Agreement"). Concurrently herewith, Lender shall agree to release its
security interest in the Collateral described in the Security Agreement.

Hereinafter, the above-described security documents and guaranties, together
with all other documents securing repayment of the Indebtedness shall be
referred to as the "Security Documents". Hereinafter, the Security Documents,
together with all other documents evidencing or securing the Indebtedness shall
be referred to as the "Existing Loan Documents".

3.      DESCRIPTION OF CHANGE IN TERMS.

        A.      Modification(s) to Note.

                1.      Payable in one payment of all outstanding principal plus
                        all accrued unpaid interest on August 13, 1997. In
                        addition, Borrower will pay regular monthly payments of
                        all accrued unpaid interest due as of each payment date
                        beginning September 13, 1996, and all subsequent
                        interest payments are due on the same day of each month
                        thereafter.

                2.      The interest rate to be applied to the unpaid principal
                        balance of the Note is hereby decreased, effective as of
                        this date to a rate equal to the Lender's current Index
                        (as defined therein).

                3.      The principal amount of the Note is hereby decreased to
                        Four Million and 00/100 Dollars ($4,000,000.00).

        B.      Modification(s) to Loan Agreement.

                1.      The paragraph entitled "Borrowing Base Formula" is
                        hereby deleted in its entirety.

                2.      The paragraph entitled "Accounts Receivable and Accounts
                        Payable" is hereby deleted in its entirety.


                                       1
<PAGE>   2
                3.      The paragraph entitled "Financial Covenants" is hereby
                        amended in its entirety, to read as follows:

                        Borrower shall maintain, on a quarterly basis, beginning
                        with the quarter ending June 30, 1996, a minimum quick
                        ratio of 2.00 to 1.00; a minimum tangible net worth of
                        $25,000,000.00; and a maximum total debt to tangible net
                        worth ratio of 0.60 to 1.00.

                4.      The paragraph entitled "Financial Statements" is hereby
                        amended in its entirety, to read as follows:

                        Furnish Lender with, as soon as available, but in no
                        event later than five (5) days after filing with the
                        Securities Exchange Commission (SEC), Borrower's forms
                        10K and 10Q together with a Compliance Certificate. All
                        financial reports required to be provided under this
                        Agreement shall be prepared in accordance with generally
                        accepted accounting principles, applied on a consistent
                        basis, and certified by Borrower as being true and
                        correct.

                5.      The paragraph entitled "Foreign Exchange Sublimit" is
                        hereby amended to allow Borrower to utilize up to
                        $4,000,000.00 for spot and future foreign exchange
                        contracts.

                6.      The paragraph entitled "Letter of Credit Sublimit" is
                        hereby amended to reduce the aggregate face amount of
                        standby and commercial letters of credit to an amount
                        not to exceed $4,000,000.00.

        C.      Release of Security Agreement.

                1.      As an accommodation to Borrower and for good and
                        valuable consideration, Lender, with this Loan
                        Modification Agreement, has agreed to release its
                        security interest granted under the Security Agreement
                        and the related UCC financing statements provided that
                        no Event of Default has occurred and is continuing.

4.      CONSISTENT CHANGES.  The Existing Loan Documents are hereby amended
wherever necessary to reflect the changes described above.

5.      PAYMENT OF LOAN FEE.  Borrower shall pay to Lender a fee in the amount
of Ten Thousand and 00/100 Dollars ($10,000.00) (the "Loan Fee") plus all
out-of-pocket expenses.

6.      NO DEFENSES OF BORROWER.  Borrower (and each guarantor and pledgor
signing below) agrees that it has no defenses against the obligations to pay
any amounts under the Indebtedness.

7.      CONTINUING VALIDITY.  Borrower (and each guarantor and pledgor signing
below) understands and agrees that in modifying the existing Indebtedness,
Lender is relying upon Borrower's representations, warranties, and agreements,
as set forth in the Existing Loan Documents. Except as expressly modified
pursuant to this Loan Modification Agreement, the terms of the Existing Loan
Documents remain unchanged and in full force and effect. Lender's agreement to
modifications to the existing Indebtedness pursuant to this Loan Modification
Agreement in no way shall obligate Lender to make any future modifications to
the Indebtedness. Nothing in this Loan Modification Agreement shall constitute
a satisfaction of the Indebtedness. It is the intention of Lender and Borrower
to retain as liable parties all makers and endorsers of Existing Loan
Documents, unless the party is expressly released by Lender in writing. No
maker, endorser, or guarantor will be released by virtue of this Loan
Modification Agreement. The terms of this 


                                       2
<PAGE>   3
paragraph apply not only to this Loan Modification Agreement, but also to all
subsequent loan modification agreements.

8.      CONDITIONS.     The effectiveness of this Loan Modification Agreement
is conditioned upon Borrower's payment of the Loan Fee.

        This Loan Modification Agreement is executed as of the date first
written above.

                                       .
BORROWER                                  LENDER

CELERITEK, INC.                           SILICON VALLEY BANK



By:  /s/ MARGARET SMITH                   By:  /s/ MARY J. TOOMEY
   ----------------------------------        ---------------------------------
Name:  Margaret Smith                     Name:  Mary J. Toomey
   ----------------------------------        ---------------------------------
Title: CFO, Asst. Secretary               Title: Vice President
   ----------------------------------        ---------------------------------




                                       3