Document:

<PAGE>

                                                                    EXHIBIT 10.5

                           STOCK PURCHASE AGREEMENT

     This Stock Purchase Agreement (this "Agreement") is made as of June 27,
2000 by and between Mitchell E. Hersh, an individual ("Purchaser"), and Ampulla,
LLC, a Texas limited liability company (the "Seller").

     WHEREAS, the Seller owns all of the outstanding voting capital stock (the
"Shares") of Prentiss Properties Limited, Inc., a Delaware corporation (the
"Company").

     WHEREAS, the Seller desires to sell to Purchaser, and the Purchaser desires
to purchase from the Seller, all of the Shares on the terms and subject to the
conditions contained herein.

     WHEREAS, the execution of this Agreement is a condition to and an
inducement for Mack-Cali Realty Corporation and Mack-Cali Realty, L.P. to enter
into that certain Agreement and Plan of Merger, dated as of the date hereof, by
and among Mack-Cali Realty Corporation, Mack-Cali Realty, L.P., Prentiss
Properties Trust and Prentiss Properties Acquisition Partners, L.P. (the "Merger
Agreement").

     NOW THEREFORE, in consideration of the premises and mutual covenants and
agreements set forth herein, the parties hereto agree as follows:

     1.   Sale and Purchase of the Shares.  At the Closing (as hereinafter
          -------------------------------
defined) and upon the terms and subject to the conditions of this Agreement, the
Seller shall sell, deliver and transfer to the Purchaser, and the Purchaser
shall purchase and acquire from the Seller, all right, title and interest in and
to the Shares.  As consideration for the purchase of the Shares, Purchaser shall
pay to the Seller the sum of One Hundred and Twenty Five Thousand Dollars
($125,000) (the "Purchase Price").  The Purchase Price shall be paid at Closing
by wire transfer of immediately available funds.

     2.   Closing Deliveries.  Subject to the conditions contained in this
          ------------------
Agreement, the deliveries contemplated by Section 1 hereof shall occur at such
                                          ---------
time and date as the Merger (as defined in the Merger Agreement) is effected
(the "Closing").  To effect the transfer of the Shares, at the Closing (a) the
Seller will deliver to the Purchaser all original stock certificates evidencing
the Shares, duly endorsed or accompanied by appropriate stock powers, and (b)
the Purchaser will deliver to the Seller the Purchase Price in the manner
contemplated in Section 1 hereof.  The deliveries set forth above shall be
                ---------
considered to have taken place simultaneously, and no delivery shall be deemed
to have been completed until all other deliveries have been completed.

     3.   Representations and Warranties of the Seller. The Seller represents
          --------------------------------------------
and warrants to the Purchaser as follows:

          3.1. Ownership.  The Seller is the record and beneficial owner of the
               ---------
Shares, free and clear of all liens, security interests, pledges, claims,
liabilities and restrictions of any nature whatsoever.

                                       1
<PAGE>

          3.2. Title to Shares.  The Purchaser shall acquire good and marketable
               ---------------
title to such Shares being purchased by such Purchaser from the Seller free and
clear of any liens, security interests, encumbrances and restrictions of any
nature whatsoever, except for restrictions imposed by applicable federal and
state securities laws.

          3.3. Organization and Authority. The Seller is a limited liability
               --------------------------
company duly organized, validly existing and in good standing under the laws of
the State of Texas and has all requisite limited liability company power and
authority to enter into and perform this Agreement and to carry out the
transactions contemplated by this Agreement.  The execution, delivery and
performance by the Seller of this Agreement and the consummation of the
transactions contemplated hereby by the Seller have been duly authorized by all
necessary company action on the part of the Seller. This Agreement has been duly
executed and delivered by the Seller and is a valid and binding obligation of
the Seller enforceable against the Seller in accordance with its terms, except
as such enforceability may be limited by bankruptcy, insolvency, reorganization,
moratorium and similar laws relating to or affecting creditors' rights generally
and by general equitable principles (regardless of whether such enforceability
is considered in a proceeding in law or in equity).

          3.4. No Conflicts. The execution, delivery and performance of this
               ------------
Agreement and the consummation of the transactions contemplated by this
Agreement do not and will not conflict with or result in any breach of any
terms, conditions or provisions of, or constitute a default, under (a) the
Seller's Articles of Organization or by-laws (each as amended to date and
presently in effect), (b) any agreement or other instrument to which the Seller
is a party or by which it or any of its properties is bound and which conflict,
breach or default would have a material adverse effect upon the Seller, its
assets, properties, business or condition (financial or otherwise) or (c) any
decree, judgment, order, statute, rule or regulation applicable to the Seller.

          3.5. No Consents. No consent, approval, order, authorization or waiver
               -----------
from, notice to or declaration, registration or filing with any governmental
authority or any other person is necessary in connection with the execution,
delivery and performance by the Seller of this Agreement or the consummation of
the transactions contemplated hereby.

     4.   Representations and Warranties of the Purchaser.  The Purchaser
          -----------------------------------------------
represents and warrants to the Seller as follows:

          4.1. Organization and Authority. This Agreement has been duly executed
               --------------------------
and delivered by the Purchaser and is a valid and binding obligation of the
Purchaser enforceable against the Purchaser in accordance with its terms, except
as such enforceability may be limited by bankruptcy, insolvency, reorganization,
moratorium and similar laws relating to or affecting creditors' rights generally
and by general equitable principles (regardless of whether such enforceability
is considered in a proceeding in law or in equity).

          4.2. No Conflicts. The execution, delivery and performance of this
               ------------
Agreement and the consummation of the transactions contemplated by this
Agreement do not and will not conflict with or result in any breach of any of
the terms, conditions or provisions of, or constitute

                                       2
<PAGE>

a default under (a) any agreement or other instrument to which the Purchaser is
party or by which it or any of its properties is bound and which conflict,
breach or default would have a material adverse effect upon the Purchaser, its
assets, properties, business or condition (financial or otherwise) or (b) any
decree, judgment, order, statute, rule or regulation applicable to the
Purchaser.

          4.3. No Consents. No consent, approval, order, authorization or waiver
               -----------
from, notice to or declaration, registration or filing with any governmental
authority or any other person is necessary in connection with the execution,
delivery and performance by the Purchaser of this Agreement or the consummation
of the transactions contemplated hereby.

          4.4. Access to Information.  The Purchaser has received or has had
               ---------------------
full access to all the information it considers necessary or appropriate to make
an informed decision with respect to the Shares to be purchased by such
Purchaser under this Agreement.  The Purchaser has had an opportunity to ask
questions and receive answers from the Company and the Seller regarding the
terms and conditions of the purchase of the Shares and to obtain additional
information (to the extent the Company or the Seller possessed such information
or could acquire it without unreasonable effort or expense) necessary to verify
any information furnished to such Purchaser or to which such Purchaser has had
access.

          4.5. Investment Representations.  The Purchaser understands that the
               --------------------------
Shares have not been registered under the Securities Act of 1933, as amended.
The Purchaser is acquiring the Shares for investment purposes only and is not
purchasing the Shares with a view to the sale or distribution of any part
thereof.  The Purchaser has made such investigation into the Company that
Purchaser considers necessary and appropriate to its purchase of the Shares, is
capable of evaluating the merits and risks of its purchase of the Shares, and is
relying solely upon such investigation and not upon any representation or
warranty made by Seller or the Company, other than the representations and
warranties specifically made in this Agreement.

     5.   Termination. This Agreement shall be terminated upon the occurrence of
          -----------
any of the following circumstances:

     (a)  By mutual written agreement of the Seller and the Purchaser; or

     (b)  Upon the termination of the Merger Agreement in accordance with its
          terms.

     6.   Miscellaneous.
          -------------

          6.1. Notices.  All notices, consents and other communications required
               -------
or permitted under this Agreement shall be in writing, and shall be deemed to
have been duly given (a) when delivered personally, (b) when received if sent by
first class certified mail, postage prepaid, return receipt requested, or (c)
when delivered by an internationally recognized overnight delivery service,
postage or delivery charges prepaid, to a party.  Notices may also be given by
facsimile and shall be effective on the date transmitted if confirmed within
twenty-four (24) hours thereafter by a signed original sent in the manner
provided in the preceding sentence.  Any party may change its address for
notices by giving notice of a new address to the other party in accordance with
this Section 6.1, except that any such change of address notice shall not be
     -----------

                                       3
<PAGE>

effective unless and until received.  Any notice shall be sent to the parties at
the addresses set forth below:

          If to the Seller:

          Ampulla, L.L.C.
          Michael V. Prentiss
          5006 Seneca
          Dallas, Texas 75209
          Fax Number: (214) 351-5006

          With a copy to:

          Akin, Gump, Strauss, Hauer & Feld, L.L.P.
          1700 Pacific Avenue, Suite 4100
          Dallas, Texas 75201
          Fax Number: (214) 969-4343
          Attn.: Michael E. Dillard, P.C.

          If to the Purchaser:

          Mitchell E. Hersh
          Mack-Cali Realty Corporation
          11 Commerce Drive
          Cranford, New Jersey 07016
          Fax Number: (908) 272-6755

          With a copy to:

          Pryor, Cashman, Sherman & Flynn, LLP
          410 Park Avenue
          New York, New York 100222
          Fax Number: (212) 326-0806
          Attn.: Blake Hornick, Esq.

          6.2. Entire Agreement.  This Agreement constitutes the entire
               ----------------
agreement of the parties with respect to the subject matter hereof and
supersedes any and all prior written or oral communications and agreements, and
all contemporaneous oral communications among the parties concerning the subject
matter hereof.

          6.3. Amendments.  No provision of this Agreement may be amended,
               ----------
changed or modified in any manner, orally or otherwise, except by an instrument
in writing signed by the parties affected by such provision.

          6.4. Successors and Assigns.  This Agreement shall be binding upon and
               ----------------------
inure to the benefit of the parties and their respective beneficiaries, heirs,
executors, administrators,

                                       4
<PAGE>

successors and assigns. No party shall in any manner assign any of its rights or
obligations under this Agreement without the prior written consent of the other
party.

          6.5. GOVERNING LAW.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED
               -------------
IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS, WITHOUT GIVING EFFECT TO
PRINCIPLES OF CONFLICTS OF LAW OF ANY JURISDICTION.

          6.6. Counterparts.  This Agreement may be executed in two or more
               ------------
counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.

          6.7. Survival of Representations and Warranties.  All representations
               ------------------------------------------
and warranties contained herein shall survive the execution and delivery of this
Agreement and the Closing hereof, regardless of any investigation at any time
made by or on behalf of any party hereto.

          6.8. Further Assurances.  At the Closing and thereafter, from time to
               ------------------
time and without additional consideration, the Seller and the Purchaser, as the
case may be, shall execute and deliver, or cause to be executed and delivered,
such additional or further transfers, assignments, endorsements, consents and
other instruments as the Purchaser or the  Seller, as the case may be, may
reasonably request for the purpose of effectively carrying out the transactions
contemplated by this Agreement.

                            [SIGNATURE PAGE FOLLOWS]

                                       5
<PAGE>

     IN WITNESS WHEREOF, the parties have executed this Agreement as of the date
first above written.

                                             SELLER:

                                             AMPULLA, L.L.C.

                                             By: /s/ MICHAEL V. PRENTISS
                                                ---------------------------
                                                Michael V. Prentiss
                                                Sole Member

                                             PURCHASER:

                                               /s/ MITCHELL E. HERSH
                                             ------------------------------
                                             Mitchell E. Hersh

                                       6<PAGE>

                                                                   Exhibit 10.31

                 WAIVER AND FOURTH AMENDMENT TO LOAN AGREEMENT
                 ---------------------------------------------

          This WAIVER AND FOURTH AMENDMENT TO LOAN AGREEMENT ("Amendment") is
                                                               ---------
entered into as of June 30, 2000 (the "Amendment Date"), by and between RF
                                       --------------
MONOLITHICS, INC., a Delaware corporation ("Borrower") and BANK ONE, TEXAS,
                                            --------
NATIONAL ASSOCIATION ("Bank"), a national banking association, acting in its
                       ----
capacity as "Bank" under the Loan Documents and as collateral agent for itself
and Banc One Leasing Corporation ("BOLC").
                                   ----

                                   RECITALS:

          A.   Pursuant to that certain Letter Loan Agreement dated as of March
8, 1996, by and between Borrower and Bank (as amended by that certain First
Amendment to Loan Agreement dated as of December 31, 1997 between Borrower and
Bank, that certain Second Amendment to Loan Agreement dated as of July 15, 1999
between Borrower and Bank, and that certain Waiver and Third Amendment to Loan
Agreement dated as of February 29, 2000 among Borrower, BOLC, and Bank, and as
such agreement may be further amended, restated, or otherwise modified from time
to time, the "Loan Agreement"), Bank has agreed to provide to Borrower a secured
              --------------
credit facility as set forth therein.

          B.   Borrower and Bank have agreed to amend certain provisions of the
Loan Agreement.

          C.   Subject to satisfaction of the conditions set forth herein, Bank
is willing to amend the Loan Agreement.

          NOW, THEREFORE, in consideration of the mutual covenants and
agreements contained herein and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
hereby agree as follows:

                                   ARTICLE 1

                                  Definitions
                                  -----------

          Section 1.1  Definitions. Unless otherwise defined in this Amendment,
                       -----------
each capitalized term used in this Amendment has the meaning given to such term
in the Loan Agreement.

                                   ARTICLE 2

                          Waiver of Event of Default
                          --------------------------

          Section 2.1  Waiver of Events of Default. Pursuant to paragraph 9(e)
                       ---------------------------              --------------
of the Loan Agreement, Borrower was required to maintain a minimum EBITDA for
the three (3) month period ended May 31, 2000 of not less than $1,007,000 (the
"EBITDA Covenant"). For the three (3) month period ended May 31, 2000, Borrower
 ---------------
failed to maintain the EBITDA Covenant.

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 1
<PAGE>

Borrower's failure to maintain the EBITDA Covenant as of the three month period
ended May 31, 2000, constitutes an Event of Default under paragraph 11(b) of the
Loan Agreement (the "EBITDA Default"). Additionally, certain events of default
occurred under the Lease Agreements which could result in acceleration of the
payments due thereunder and as more specifically set forth in the waiver letter
agreement between Borrower and BOLC dated concurrently herewith (the "Lease
Defaults"). Each of the Lease Defaults constitutes an Event of Default under
paragraph 11(e) of the Loan Agreement. Effective as of the date of this
Amendment, and subject to the conditions precedent contained herein, Bank hereby
waives the Events of Default resulting from the EBITDA Default and the Lease
Defaults.

                                   ARTICLE 3

             Amendment to Loan Agreement and other Loan Documents
             ----------------------------------------------------

          Section 3.1  Amendment to Paragraph 1 of the Loan Agreement. Effective
as of the Amendment Date, paragraph 1 of the Loan Agreement is hereby amended
and restated in its entirety to read as follows:

               1.   Loan. Subject to the terms and conditions set
                    ----
          forth in this Loan Agreement and the other agreements,
          instruments, and documents at any time evidencing, securing,
          governing, guaranteeing, and/or pertaining to the
          Indebtedness, as hereinafter defined (collectively, together
          with this Loan Agreement, referred to hereinafter as the
          "Loan Documents"), Bank and Borrower hereby agree as
           --------------
          follows:

          Subject to the terms and conditions set forth herein, Bank
          agrees to lend to Borrower an amount equal to $6,897,878.91;
          provided, however, the total principal amount outstanding at
          any time shall not exceed the Borrowing Base (as such term
          is defined hereinbelow) (the "Loan"). If at any time the
                                        ----
          aggregate principal amount outstanding under the Loan shall
          exceed the Borrowing Base, unless Bank otherwise consents,
          Borrower agrees to immediately repay to Bank such excess
          amount, plus all accrued but unpaid interest thereon. All
          sums advanced hereunder, together with all accrued but
          unpaid interest thereon, shall be due and payable in full at
          11:00 a.m. (Dallas, Texas time) on December 31, 2000 (the
          "Termination Date"). The sums advanced under the Loan shall
           ----------------
          be used for working capital purposes. The term
          "Indebtedness", as used herein, shall mean the Loan and all
           ------------
          other indebtedness owing from time to time to Bank by
          Borrower.

          As used in this Loan Agreement, the term "Borrowing Base"
                                                    --------------
          shall have the meaning set forth hereinbelow:

               An amount equal to eighty percent (80.0%) of
               Borrower's Eligible Accounts, plus twenty-five
                                             ----
               percent (25.0%) of Borrower's Eligible Inventory;
               provided, however, the outstanding amount advanced
               with respect to Eligible Inventory at any time
               shall not exceed $1,000,000, plus one hundred
                                            ----
               percent (100%) of the balance of cash and ninety
               percent (90.0%) of the value of U.S. Treasury
               securities, respectively, held in the securities
               account described in paragraph 3(d) hereof.
                                    --------------

          As used herein, the term "Eligible Accounts" shall mean at
                                    -----------------
          any time, an amount equal to the aggregate net invoice or
          ledger amount owing on all

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 2
<PAGE>

          trade accounts receivable of Borrower for goods sold or
          leased or services rendered in the ordinary course of
          business, in which Bank has a perfected, first priority
          lien, after deducting (without duplication): (i) each such
          account that is unpaid ninety (90) days or more after the
          original invoice date thereof, (ii) the amount of all
          discounts, allowances, rebates, credits, and adjustments to
          such accounts, (iii) the amount of all contra accounts, set-
          offs, defenses, or counterclaims asserted by or available to
          the account debtors, (iv) all accounts with respect to which
          goods are placed on consignment or subject to a guaranteed
          sale or other terms by reason of which payment by the
          account debtor may be conditional, (v) all accounts with
          respect to which Borrower has furnished a payment and/or
          performance bond and that portion of any accounts for or
          representing retainage, if any, until all prerequisites to
          the immediate payment of retainage have been satisfied, (vi)
          all accounts owing by account debtors for which there has
          been instituted a proceeding in bankruptcy or reorganization
          under the United States Bankruptcy Code or other law,
          whether state or federal, now or hereafter existing for
          relief of debtors, (vii) all accounts owing by any Affiliate
          (as such term is defined hereinbelow) of Borrower, (viii)
          all accounts in which the account debtor is the United
          States or any department, agency, or instrumentality of the
          United States, except to the extent an acknowledgment of
          assignment to Bank of such account in compliance with the
          Federal Assignment of Claims Act and other applicable laws
          has been received by Bank, (ix) all accounts due Borrower by
          any account debtor whose principal place of business is
          located outside the United States and its territories;
          provided that if any such account is supported by either a
          letter of credit or an agreement of indemnity or insurance,
          each in form and substance satisfactory to Bank, Bank may,
          in its discretion, include such accounts, or any portion
          thereof, in "Eligible Accounts", (x) all accounts subject to
          any provision prohibiting assignment or requiring notice of
          or consent to such assignment, (xi) that portion of all
          account balances owing by any single account debtor which
          exceeds twenty-five percent (25.0%) of the aggregate of all
          accounts which are owing to Borrower by all account debtors,
          and (xii) any other accounts deemed unacceptable by Bank in
          its discretion exercising its reasonable credit judgment;
          provided, however, if more than ten percent (10.0%) of the
          then balance owing by any single account debtor does not
          qualify as an Eligible Account under the foregoing
          provisions, then the aggregate amount of all accounts owing
          by such account debtor shall be excluded from Eligible
          Accounts.

          As used herein, the term "Eligible Inventory" shall mean as
                                    ------------------
          of any date, the aggregate value of all inventory of raw
          materials (excluding specifically work in progress and
          packaging materials, supplies, finished goods, and any
          advertising costs capitalized into inventory) then owned by
          Borrower and held for use in the ordinary course of
          Borrower's business, in which Bank has a first priority
          lien, excluding (i) inventory which is damaged, defective,
          obsolete, or otherwise unusable in the ordinary course of
          Borrower's business, (ii) inventory which is not in the
          possession of Borrower or which has been rejected by
          Borrower for any reason, and (iii) inventory subject to any
          consignment arrangement between Borrower and any other
          person or entity. For purposes of this definition, Eligible
          Inventory shall be valued at the lower of cost (excluding
          the cost of labor) or market value.

          Section 3.2  Amendment to Paragraph 2 of the Loan Agreement. Effective
                       ----------------------------------------------
as of the Amendment Date, paragraph 2 of the Loan Agreement is hereby amended
                          -----------
and restated in its entirety to read as follows:

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 3
<PAGE>

               2.   Promissory Note. The Loan shall be evidenced by a
                    ---------------
          promissory note (together with all renewals, extensions, or
          other modifications thereto, the "Note") duly executed by
                                            ----
          Borrower in the stated principal amount of $6,897,878.91 and
          otherwise in form and substance acceptable to Bank. Interest
          on the Note shall accrue at the rate set forth therein. The
          principal of and interest on the Note shall be due and
          payable in accordance with the terms and conditions set
          forth in the Note and in this Loan Agreement.

          Section 3.3  Amendment to Paragraph 9 of the Loan Agreement. Effective
                       ----------------------------------------------
as of the Amendment Date, paragraph 9 of the Loan Agreement is hereby amended by
                          -----------
adding thereto, immediately following the last sentence thereof, the following
sentence which shall read in its entirety as follows:

          Notwithstanding any other provision of this Loan Agreement
          to the contrary, Borrower shall certify its compliance with
          each of the covenants set forth in this paragraph 9 for the
                                                  -----------
          period ending August 31, 2000 by reference to the quarterly
          financial statements of Borrower prepared for such period
          end as required by paragraph 10(b), such certification to be
                             ---------------
          received by Bank not later than October 15, 2000.

          Section 3.4  Amendment Loan Documents. Effective as of the Amendment
                       ------------------------
Date, each and every reference in the Loan Documents to the "Revolving Line of
Credit", as defined in the Loan Agreement prior to the effectiveness of this
Amendment, is hereby amended to refer to the "Loan", as defined in the Loan
Agreement, as amended by this Amendment.

                                   ARTICLE 4

                                  Conditions
                                  ----------

          Section 4.1  Items to be Delivered By Borrower. The effectiveness of
                       ---------------------------------
this Amendment is subject to Borrower's delivery to Bank of each of the
following items prior to or simultaneously with execution and delivery of this
Amendment:

               (a)  Amendment Documents. Each other agreement, certificate,
                    -------------------
          document, or instrument required by Bank to be executed or delivered
          by Borrower (including, without limitation, a Pledge Agreement in form
          and substance satisfactory to Bank) or any other party in connection
          with this Amendment or the transactions contemplated in connection
          herewith (the "Amendment Documents"), duly executed or delivered by
                         -------------------
          the parties thereto;

               (b)  Amendment Fee. Payment to Bank of a fee of $7,000 in respect
                    -------------
          of the amendments contained herein; and

               (c)  Fees and Expenses. Payment or reimbursement to Bank for all
                    -----------------
          expenses, costs, and fees (including, without limitation, fees and
          costs of Bank's legal counsel) incurred by, or due to, Bank in
          connection with negotiating and documenting this Amendment, the other
          Amendment Documents, and any other Loan Documents;

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 4
<PAGE>

          Section 4.2  Other Conditions. This Amendment shall become effective
                       ----------------
upon the following conditions precedent being performed to Bank's satisfaction.
Borrower's failure to comply with the following conditions shall be an Event of
Default under the Loan Agreement:

               (a)  Continued Effect of Representations and Warranties. All
                    --------------------------------------------------
          representations and warranties contained in the Loan Documents (as
          amended hereby) shall be true, correct, and complete in all material
          respects (as determined by Bank in its sole discretion) except as
          disclosed otherwise to Bank in writing and as acceptable to Bank or
          representations specifically relating to a prior date or no longer
          relevant due to the occurrence of an event or circumstances
          specifically permitted hereunder or by any other Loan Document;

               (b)  Absence of Default. No Default or Event of Default shall
                    ------------------
          have occurred and be continuing (after giving effect to this
          Amendment);

               (c)  Corporate Proceedings. All corporate proceedings taken in
                    ---------------------
          connection with the transactions contemplated by this Amendment and
          all other agreements, documents, and instruments executed and/or
          delivered pursuant hereto, and all legal matters incident thereto,
          shall be satisfactory to Bank and its legal counsel;

               (d)  Additional Information. Bank shall have received such
                    ----------------------
          additional agreements, certificates, documents, instruments, and
          information as Bank or its respective legal counsel may request to
          effect the transactions contemplated hereby.

                                   ARTICLE 5

                        Representations and Warranties
                        ------------------------------

          Section 5.1  Representations and Warranties. Borrower hereby
                       ------------------------------
represents and warrants to Bank that, as of the date of and after giving effect
to this Amendment:

               (a)  the execution, delivery, and performance of this Amendment
          and any and all other Amendment Documents executed and/or delivered in
          connection herewith have been authorized by all requisite action on
          the part of Borrower and will not violate Borrower's certificate of
          incorporation, bylaws, or other similar charter documents;

               (b)  except as set forth in Exhibit A hereto, all representations
                                           ---------
          and warranties set forth in the Loan Agreement, the other Loan
          Documents, and the Lease Agreements are true and correct in all
          material respects as if made again on and as of such date (except as
          disclosed otherwise to Bank and BOLC in writing and as acceptable to
          Bank and BOLC or representations specifically relating to a prior date
          or no longer relevant due to the occurrence of an event or
          circumstances specifically permitted hereunder, by any other Loan
          Document, or the Lease Agreements);

               (c)  no Default or Event of Default has occurred and is
          continuing; and

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 5
<PAGE>

               (d)  the Loan Agreement and the other Loan Documents (as amended
          by this Amendment) are and remain legal, valid, binding, and
          enforceable obligations of each of the parties thereto.

                                   ARTICLE 6

                                 Miscellaneous
                                 -------------

          Section 6.1  The waiver specifically described in Section 2.1 of this
                                                            -----------
Amendment shall not constitute and shall not be deemed a waiver of any other
Default or Event of Default, whether arising as a result of the further
violation of the EBITDA Covenant, additional default under the Lease Agreements,
or otherwise, or a waiver of any rights or remedies arising as a result of such
other Defaults or Events of Default.

          Section 6.2  Governing Law. THIS AMENDMENT, AND ALL DOCUMENTS AND
                       -------------
INSTRUMENTS EXECUTED IN CONNECTION HEREWITH, SHALL BE GOVERNED BY AND CONSTRUED
ACCORDING TO THE LAWS OF THE STATE OF TEXAS.

          Section 6.3  Agreement Remains in Effect; No Waiver. Except as
                       --------------------------------------
expressly provided herein, all terms and provisions of the Loan Agreement, the
other Loan Documents, and the Lease Agreements shall remain unchanged and in
full force and effect and are hereby ratified and confirmed. No delay or
omission by Bank in exercising any power, right, or remedy shall impair such
power, right, or remedy or be construed as a waiver thereof or an acquiescence
therein, and no single or partial exercise of any such power, right, or remedy
shall preclude other or further exercise thereof or the exercise of any other
power, right, or remedy under the Loan Agreement, the other Loan Documents, the
Lease Agreements, or otherwise.

          Section 6.4  Survival of Representations and Warranties. All
                       ------------------------------------------
representations and warranties made in this Amendment or any other Loan Document
shall survive the execution and delivery of this Amendment and the other Loan
Documents, and no investigation by Bank or any closing shall affect the
representations and warranties or the right of Bank to rely upon them.

          Section 6.5  Reference to Loan Documents. Each of the Loan Documents,
                       ---------------------------
including, without limitation, the Loan Agreement, the Amendment Documents, and
any and all other agreements, documents, or instruments now or hereafter
executed and/or delivered pursuant to the terms hereof or pursuant to the terms
of the Loan Agreement as amended hereby, are hereby amended so that any
reference in such Loan Documents to the Loan Agreement shall mean a reference to
the Loan Agreement as amended hereby, and the term Loan Documents as defined in
the Loan Agreement and as used in any of the "Loan Documents" includes, without
limitation, the Amendment Documents.

          Section 6.6  Severability. Any provision of this Amendment held by a
                       ------------
court of competent jurisdiction to be invalid or unenforceable shall not impair
or invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 6
<PAGE>

          Section 6.7  Successors and Assigns. This Amendment is binding upon
                       ----------------------
and shall inure to the benefit of Borrower and Bank and their respective
successors in interest and assigns. Borrower may not assign any right, power,
duty, or obligation hereunder without the prior written consent of Bank.

          Section 6.8  Headings. The headings, captions, and arrangements used
                       --------
in this Amendment are for convenience only and shall not affect the
interpretation of this Amendment.

          Section 6.9  Expenses of Bank. As provided in the Loan Agreement,
                       ----------------
Borrower agrees to pay on demand all, third party out-of-pocket costs and
expenses incurred by Bank in connection with the preparation, negotiation, and
execution of this Amendment, the Amendment Documents, or any other Loan
Documents executed pursuant hereto and any and all amendments, modifications,
and supplements thereto (including, without limitation, fees and costs of Bank's
legal counsel) and all costs and expenses incurred by Bank in connection with
the enforcement or preservation of any rights under the Loan Agreement, as
amended hereby, or any other Loan Document, including, without limitation, the
costs and fees of Bank's legal counsel.

          Section 6.10 Counterparts. This Amendment may be executed
                       ------------
simultaneously in one or more multiple originals and on telecopy counterparts,
each of which shall be deemed an original, but all of which together shall
constitute one and the same agreement.

          Section 6.11 Waiver of Claims and Defenses. To induce Bank to enter
                       -----------------------------
into this Amendment, Borrower represents and warrants that as of the Amendment
Date there are no claims or offsets against or defenses or counterclaims to
Borrower's obligations under the Loan Documents or the Lease Agreements, and
Borrower waives any and all such claims, offsets, defenses, or counterclaims
whether known or unknown, arising prior to the Amendment Date. Additionally,
Borrower hereby releases Bank and each of its legal representatives, successors,
affiliates, parent, subsidiaries, predecessors, assigns, shareholders, partners,
trustees, beneficiaries, administrators, heirs, former and current officers,
directors, agents, attorneys, and employees, and its respective successors,
assigns, heirs, executors, and administrators (collectively, the "Creditor
                                                                  --------
Parties") from any and all claims, actions, suits, causes of action, accounts,
-------
judgments, agreements, promises, executions, debts, damages, demands, rights,
obligations, liabilities, and controversies now in existence concerning or in
connection with the Loan Agreement, this Amendment, or any other Loan Document
(collectively, the "Claims") of every nature and description, in law or in
                    ------
equity, whether known or unknown, foreseen or unforseen, and regardless of
whether Borrower hereafter discovers any facts which may give rise to any Claim.

          THIS AMENDMENT, TOGETHER WITH THE LOAN AGREEMENT AND THE
          OTHER LOAN DOCUMENTS, AS WRITTEN, REPRESENTS THE FINAL
          AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY
          EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL
          AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL
          AGREEMENTS BETWEEN THE PARTIES.

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 7
<PAGE>

          IN WITNESS WHEREOF, Borrower and Bank have caused this Amendment to be
executed and delivered by their duly authorized officers effective as of the
date first written above.

                                             BORROWER:

                                             RF MONOLITHICS, INC.

                                             By: _______________________________
                                             Name: _____________________________
                                             Title: ____________________________

                                             BANK:
                                             ----

                                             BANK ONE, TEXAS, NATIONAL
                                             ASSOCIATION

                                             By: _______________________________
                                             Name: _____________________________
                                             Title: ____________________________

CONSENT TO WAIVER AND AMENDMENT:

Each of the undersigned hereby consents and agrees to the foregoing agreement
and agrees that the Security Agreement to which it is a party, shall remain in
full force and effect.

BOLC:
----

BANC ONE LEASING CORPORATION

By: ______________________________
Name: ____________________________
Title: ___________________________

SECURED PARTY:
-------------

BANK ONE, TEXAS, NATIONAL ASSOCIATION

By: ______________________________
Name: ____________________________
Title: ___________________________

WAIVER AND FOURTH AMENDMENT TO LETTER LOAN AGREEMENT - Page 8
<PAGE>

                                   EXHIBIT A
                                   ---------

                 Exceptions to Representations and Warranties
                 --------------------------------------------

Section 5(a) of the Loan Agreement - Litigation:

          Borrower's lawsuit filed in the US District Court in Connecticut
          (Civil action no. 399cv00311) seeking to collect outstanding
          receivables from Akom Technologies, Inc. as more particularly
          described in note no. 7 of the "Notes to Condensed Financial
          Statements" which form a part of the Condensed Financial Information
          attached to Borrower's Form 10-Q dated February 29, 2000 filed with
          the United States Securities and Exchange Commission

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