We have calculated the frequency limit of negative differential resistance and a.c. amplification of quantum well oscillators. From the time development of resonant tunneling, negative resistance is expected to occur at very high frequencies. However, the large device impedance results in a resistive cut-off frequency that is several orders of magnitude below that predicted from transit time considerations. This cut-off frequency is estimated from an equivalent circuit model, and the power of an oscillator as a function of frequency and negative resistance is found from a transmission line model.
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