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Schematic diagram of a triode vacuum tube RF amplifier stage with neutralization using the Neutrodyne circuit invented by Alan Hazeltine in 1922. RF amplifier stages like this were used in Tuned Radio Frequency (TRF) radio receivers during the 1920s. A major problem was that at radio frequencies the capacitance C_gp between plate and grid served as a feedback path so that the stage would become an oscillator, creating whistling and shrieking sounds in the output. In the Neutrodyne circuit, a feedback signal of opposite phase was coupled from the plate to the grid through a capacitor C_N. By adjusting it to feed back the correct amount, this second signal will cancel or "neutralize" the feedback through C_gp, preventing oscillations. In Hazeltine's original circuit the feedback signal was taken from the opposite end of the centertapped primary of the interstage coupling transformer T2. This is a modified form of the circuit in which the feedback is instead taken from the secondary of T2 The 180° phase inversion is created by the direction of the transformer windings. . Neutrodyne receivers were used until about 1930.