A current to voltage converter is essential at the output of photo detector circuits to transform the generated photocurrent into a measurable voltage signal. This conversion allows for easier monitoring and analysis of the signal, as voltage can be more conveniently processed and interpreted in electronic systems. Without this conversion, the weak photocurrent produced by the detector would be difficult to measure accurately. The discussion emphasizes the importance of this conversion for effective signal processing in various applications. Overall, a current to voltage converter is crucial for optimizing the performance of photo detector circuits.