An input clamping circuit of a photo detector preamplifier is activated when an input transistor is turned off by an input overload, and the drain voltage of the input transistor is pulled toward ground by a current source. Even with extreme overloads, the operating conditions (Vgs and Id) of the input transistor remain within normal range. During normal operation, the clamping circuit is biased completely off, and has essentially no effect on circuit performance. Since the input FET itself, rather than a separate device, detects the onset of an overload, significantly improved clamping performance is realized without adding additional circuit complexity. The input transistor can be a FET. The preamplifier can be a cascode preamplifier. The clamping circuit can include a clamping FET or other clamping transistor gated by the input transistor drain. In embodiments, the clamping circuit increases current requirements of the preamplifier by no more than 25%.