Phase information of a signal

[preethi19]

Hi when we take a signal say a simple sine wave, It has amplitude, frequency and phase. I can understand amplitude say representing the strength of the data. Frequency is how often ( the rate in which we get the data). But what does phase of a signal represent??? Say taking an opamp for eg. We have inverting and non inverting configs. For inverting output signal phase is 180 degree off phase. While for non-inverting output is in phase with a signal. So with a real life example what does this phase change in signal. For example in sound signal.

Becoz i was learning about op-amps and an ideal op-amp should have phase margin of 90 degree. And when PM is very low to a point wer it reaches 180 then the -ve feedback would change to +ve feedback changing output phase. So anyway we get amplified signal so what does phase hold????

 

[Vraj]

For an op-amp,

first thing that ideal op-amp has no phase margin of 90 degree. because ideal means all the ability must be covered in it. now, if i want to use it as non-inverting or inverting amp, it is difficult. it is not directly used . some converting circuits must be needed to achieve, 0 or 180 degree.

here is some properties of ideal op-amp.

Infinite open-loop gain G = vout / vin
Infinite input impedance Rin, and so zero input current
Zero input offset voltage
Infinite output voltage range
->Infinite bandwidth with zero phase shift and infinite slew rate
Zero output impedance Rout
Zero noise
Infinite common-mode rejection ratio (CMRR)
Infinite power supply rejection ratio.

phase difference is used in many sensors. like earthquake, etc.


for pm in op-amp, it can be used as sum and difference. like,

both signals are in 0 degree phase difference
1+1=2

but if the 2nd signal is in phase of 180, the equation will become,
1-1=0

 

[betwixt]

Note that phase is measured as the delay between the same point on one signal and another. It needs two signals to compare, a single signal has no usable phase information. In the case of op-amps, the two signals are the one being fed in to it (reference of 0 degrees) and the one leaving it (delayed by passage through the circuit).

For example, if you fed a sine wave to an amplifier and one channel of a dual race oscilloscope, and looked at the output waveform on the other oscilloscope channel, they would be slightly out of alignment because one had been delayed as it passed through the amplifier. The amount of delay is partially due to the passage of electrons through the amplifier and partly due to charging and discharging reactive components around it. Typically the delay would depend on the frequency of the signal and the properties of the reactive components because reactance is frequency dependant.

Brian.

 

[ted2]

As Betwix pointed out, the phase is always relative. For instance phase of output of an amplifier is expressed relative to the input.

To define a phase in degrees, one has to have a repetitive signal, which then has also a frequency of repetition. As our time goes only forward, the output is always behind (later) than the input, as no circuit can have a negative time delay.

Because the amplifier's delay time is usually roughly similar for frequencies in same magnitude (is not exactly constant - but mostly doesn't change abruptly) , the phase delay in degrees tend to increase pretty consistently with frequency -- as the cycle time is reduced with higher frequencies.

That causes the phase shift to increase, and if the amplifier feedback is on low frequencies properly negative, on higher frequencies it tends to turn, until it is even positive, leading to instability - if the gain is still sufficient on that high frequency to matter. (Usually the amplifier is designed so, that the gain rolls to below one before the phase has twisted 180 degrees)