Class C amplifiers have a negative voltage at the base and zero volts at the emitter, reverse biasing the base-emitter junction. The transistor only conducts a small pulse through the collector output when the signal voltage reaches 4.7V. This amplifier is efficient but has limited usefulness. It is used in radio frequency applications where the collector resonant circuit responds to an impulse by ringing at its resonant frequency, like a swing being pushed. Sharp, short collector impulses occur at the resonant frequency of the tank circuit, which continues ringing and restoring the sine wave at the output.