The maximum frequency at which amplification can be obtained from quantum well oscillators is discussed. Intrinsically, the frequency limit for having negative differential resistance (NDR) can be very high, of the order of the inverse of the electron transit time. Owing to the large capacitance of the well and barrier regions, the actual frequency limit at which amplification occurs may be lower than the intrinsic limit because of the capacitance charging time. We have estimated the frequency limit of NDR by considering the electron transit time and have calculated the maximum oscillation frequency from an equivalent circuit model. We have also obtained an expression for the high‐frequency power output as a function of frequency, based on a transmission line model.