The board of supervisors of any county may delegate to the county administrative officer or the county executive officer in an equivalent position, the power to enter into and execute on behalf of the county any contracts, which do not involve the expenditure of more than five thousand dollars ($5,000), which relate to purposes previously approved and budgeted by the board of supervisors, subject to ratification of such approval and execution by the board of supervisors. This section shall not apply to:
(a) Matters within the purview of Section 20131.
(b) Contracts for the construction of public improvements.
(c) Contracts which must be awarded after public notice and competitive bidding.