voltage adjustments

sorry guys i have a question to ask . basically how would you design a circuit that increases the voltage output of another circuit under 2.5volts but on the other hand increases the voltage output if the output of the previous circuit is above 2.5volts?
thanks

I'm slightly confused by your question....What are you trying to achieve?

sorry for the poor explanation. actually i making an ultrasonic sensor that would help the visually impaired through sounds transmitted through earphones. the output i get form my sensors would be would be amplified using an op amp before i feed the circuit to a VCO to get my frequency. but the ulltrasonic sensor would give me a low voltage for detecting a near object and thus the frequency given by the VCO would be low. but ill would like a high frequency for detecting a a near object. lets say the input voltage to the VCO ranges for 1-5volts but ill would like to get a high freq for a low voltage input and a low freq for a high voltage input. any suggestions??

I'm still confused by your description, but it sounds like you want a simple inverting opamp with a gain of 1.