It is common in public-address systems to hear a loud tone when someone moves the microphone too close to a speaker. People call this feedback, and it is caused by sound from the speaker getting back into the microphone. When this happens, the sound is amplified again and again until the amplifier overloads. It is perhaps surprising that we hear a single tone rather than a broad range of frequencies, and it suggests that we can use feedback to make a sine-wave oscillator. We can distinguish between two kinds of feedback. The public-address oscillations are an example of positive feedback, where the output adds to the signal. In negative feedback, the output cancels part of the input, reducing the gain. We have had two examples of negative feedback so far, in the emitter resistor of the Driver Amplifier and in the source resistor of the Buffer Amplifier. In both cases, the output generates a voltage across a resistor that cancels part of the input. We will see two more examples of negative feedback when we consider the Automatic Gain Control and the Audio Amplifier. Positive feedback increases the gain of an amplifier. This was used in early radios in a circuit called a regenerative receiver, where positive feedback brought the receiver to the brink of oscillation. This gave a large gain and allowed a receiver to operate with only a single stage of amplification.