What Is a Phase Shift on an Amp?

Reactive Components

• The electronic components in amplifier circuits contribute significantly to phase shift. Reactive components, such as capacitors and inductors, respond differently to voltage and current signals. In a capacitor, for example, the current peaks before the voltage by 90 degrees of a complete 360-degree wave cycle. An inductor has the opposite behavior: Voltage peaks before current. Several components in a row will advance or delay phase in multiples of 90 degrees, so a circuit may have a total phase shift of 720 degrees.

Active Components

• Depending on the amplifier circuit design, active components such as transistors may also shift a signal̵7;s phase. Here, the transistor may either follow the incoming current or voltage exactly or it may output a mirror image of the input. If it creates a mirror image, then the output signal will be at a minimum when the input is at a maximum. This is a 180-degree phase shift.

Propagation Delay

• All electronic circuitry, including the wiring, produces a small phase shift simply due to the time it takes for electrical signals to move from the input to the output. This is known as propagation delay, and it creates a phase shift that depends on the signal̵7;s frequency. The delay through 5 feet of a typical cable is about 6 nanoseconds. Dividing 6 nanoseconds by the waveform period of a 10kHz audio signal, .0001 seconds, gives .00006; multiplying that by 360 degrees yields a phase shift of about .02 degrees. For audio, this phase shift is negligible. For radio-frequency amplifiers, frequencies can be more than 100 MHz; the degrees of phase shift become large enough to be a problem.

Consequences

• For audio amplifiers, phase shift is inaudible under most circumstances. You hear only the output signal, not the input, so a difference in phase is irrelevant. If you hear both the source and output, such as when you amplify an acoustic piano or singer, phase shift may result in destructive interference. Here, the out-of-phase signals may cancel each other out slightly, though the louder amplified signal will prevail over the weaker original signal.

Oscillation

• Oscillators are electronic circuits that produce a continuous signal. Some oscillators work by exploiting an amplifier's phase shift. The oscillator circuit connects the amplifier's output back into its input to form a feedback loop. As the signal moves from input to output and back in, the amplifier shifts the phase by 180 degrees. After two times through the amplifier, the signal forms a complete 360-degree cycle, so the circuit generates a continuous, oscillating wave.