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well phase margin, pole and zero are related to system, and that's why when you want to design "construct some thing which doesn't exist" with
given i/p o/p relationship such performance parameters are specified.
assume : we have a system "sys"
we have time domain response, and frequency response.
we want to know at which frequency sys will have maximum gain("theoritically infinite gain") and minimum gain ("theoritically zero gain")
and in which frequncy range system will be stable, and unstable. (remember barkhausen criterian) and suppose sys working at given frequncy how much far it is from unstability.
Phase margin is given in terms of value Theta......This angle is a measure of the number of degrees by which the system under stability away from the instability.......start working on Polar plots which can give a graphical interpretation of these concepts
Hope this helps
After pole frequency transfer characterictics begins to drop 20dB per decade
something like Low pass filter.
After zero frequency transfer characterictics begins to rise every 20 dB per decade.
When you have more zero and poles you can draw aproximate transfer characterictics, on this way.
Phase margin is calculated for beta-A circuit and tells you how much will oscilate your closed loop system. For example if you have phase margin of 64 degrees you will have Butterworth response, wich is very fast, and have small overshoot of about 4 percent.
I am not sure what you exactly want but what I am thinking is that you need a physical explanation of the phase margin. So here is what I know:
When designing an amplifier you can never totaly isolate the input from the output thus a feedback path exists between the input and the output.
now imagine that the loop gain of this feedback path is larger than unity when the phase shift between the input and the output is 180 degrees. in such a case we have an oscillator. But we wanted to make an amplifier not an oscillator so to ensure our amplifier will not oscillate we must have the following:
- the gain is less than unity when the phase shift between the input and the output is 180 degree. The ratio of the gain in that case to unity is called the gain margin.
- when the gain is larger than unity we must have a phase shift between the input and the output less than 180 degree. thus the phase margin is defined as the (180 degrees) - (the phase shift between the input and the output when the gain is unity)
now imagine that the loop gain of this feedback path is larger than unity when the phase shift between the input and the output is 180 degrees. in such a case we have an oscillator.
adel, i thought berkhausens criterion states that the phase shift has to be 360 or 0 degs for a circuit to oscillate? 180 degrees means the output is out of phase with the input. this signal when fed back will reduce the amplitude of the input signal rather than setting up oscillations..
Be careful with the berkhausens criterion. It is from feedback control theory. It is frequently misused. In the full form, it states that if and only if there are poles in the right half plane (which is intentionally made so in oscillators) the frequency of oscillation will be the one where the phase shift through the entire feedback loop is 360 degrees or a multiple thereof.
In some complex feedback systems you can get the 360 degree phase shift without the right hand plane poles.
rkarthik1 talked about phase shift of the entire feedback loop
but
adel_48 talked about phase shift from input to output with negative feedback which is the same as rkarthink1.[/u]
ok i get the drift... the reason y we use the grounded centre taps in hartley and colpitts oscillators.. then i am assuming that bode plots are only valid for closed-loop systems with negative feedback.. have i got it right? in that case adel_48's explanation is perfect..
Hi,
Well, sorry for not clarifying this part. This was mainly because I usually use -ve gain amplifiers, which require a phase shift between the output and the input in the feedback loop to be less than 180 Deg in order not to oscillate. Thus you have a total phase shift between the input -> output -> input (i.e. the loop gain) less than 360 Deg.
Thus in such a case Barkausen condition is correct.
I didn't consider +ve feedback systems since these are normally used as oscillators, and there is no meaning of the phase margin in such a case. (though in the old times, I read that there were amplifiers with +ve feedback to increase gain).
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