How can we know that a zero exists at the RHP or LHP without much calculation?

[surianova]

Hi

In a circuit, for example opamp, how we can know a zero is exist at the RHP or LHP without much calculation. I mean something very fast, from the cuicuit, you know where is the zero at RHP or LHP.

Surianova

 

[ezt]

Re: zero

Hi.
I think there is no such a very easy way that could help you always because sometimes there exists a complex zero and I think there is no realization in real world for this concept. However there are some techniques. when you have any feedforward path in your circuit, it may place a zero in your transfer function. Or any mirrors (I mean current mirrors) will place a zero twice its pole (for more information about the latter you can refer to "Design of Analog CMOS Integrated Circuits" by B. Razavi. -> frequency response) There are other techniques like : place small signal model for every device in the circuit and ampedances of passive devices like inductors or capacitors. then you can think how will output goes to zero; sometime by shortcuttng an impedance, sometime by opening circuit. and in this way you can find some zeroes just by inspection.
I don't know if I was clear?! However the best way is to plot the bode diagram and when you have a positive slope or a flat line after a negative slope, it means nearby exists a zero.

Regards,
EZT

 

[layes2]

Re: zero

a resistor cause a zaro
acap a pole