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, coupling energy from the plate ciruit back into the grid, so that the stage would become an oscillator, creating whistling and shrieking sounds in the output. In the Neutrodyne circuit, a second feedback signal of opposite phase was coupled from the plate to the grid through a capacitor "C"N. By adjusting the capacitor to provide the correct amount of feedback, this second signal will cancel or "neutralize" the feedback through "C"gp, preventing oscillations. The feedback signal is taken from the opposite end of the grounded centertapped primary of the output interstage transformer "T2", so it's phase is 180° opposite to the plate signal. Neutrodyne receivers were used until about 1930.