How to determine if a node is high or low impedance ?

[tghani]

Node impedance

Hi,

Can someone explain to me how do you determine a node is high impedance or low impedance when you look at the circuit (e.g. a CMOS opamp)? Thank you.

 

[willyboy19]

Re: Node impedance

To really understand the difference of high/low impedance and appreciate its importance, you need to be familiar with some basic circuit configuration, such as diode-connected transistors, cascode transistors and some simple feedback configuration. It is not an easy problem, but it can not be too hard either.

 

[cookey]

Re: Node impedance

Normally gate and drain are high impedance node, while source is typically low impedance. But it is not always true. If the transistor is diode connected, the impedance of drain and gate will be reduced.

For a two or three stage opamp, the highest impedance node is normally the output of gain stage. You could find that the compensation capacitor is located here in most of the cases.

 

[enchanter]

Re: Node impedance

Cookey, thanks or you explain. But when normally gate and drain are hi-z and source is not?

 

[khaled2k]

Source is usually a low impedance node unless an ideal current source or a current mirror is used to bias the source. Gate is a high impedance node in common source configuration.
Drain is usually a high impedance node except if connected to diode-connected MOS load.