This paper studies the effects produced by radiation single event transient (SET) injected in the transistors of a custom operational amplifier, in order to evaluate their sensitivity to the radiation transient faults. A BID (built-in detector), was included in the circuit in order to amplify and detect the SETs effect. The circuit was designed using a non-rad- hard AMS-CMOS 0.8 mum process. In this case, simulation results allow the identification of the operational amplifier most sensitive transistors and the operating conditions during which the worst effects in the operational amplifier response were produced.