Disclosed is a system for generating a programmable exponential analog output signal, comprising a digital to analog conversion circuit for converting said digital signal into an analog output signal, the digital to analog conversion circuit having a substantially exponential transfer function defined by a programmable ratio of values of components. Preferably, the conversion circuit is implemented as a current mirror (100), with the exponential transfer function being defined by the mirror ratio. Thus, each transistor of the current mirror (100) defines a step of the digital to analog conversion circuit, and the ratio between adjacent steps is substantially constant. The transistors may be substantially equally sized or binary weighted relative to each other, and can be switched from theinput to the output under the control of a tree based thermometer line decoder.