Classify OTA by High Impedance Nodes

[crystalballs]

Hello all,

I read form Chap6 of the book "design of analog integrated circuits and systems" by Laker/Sansen:

In an OTA... A node is counted only if it has a high impedance...
...the single transitor amplifier is a two-node circuit.
...The input node is connected to the input source Vin over a source resistance Rs...
The differential amplifier...is still a two node circuit...
...The input terminals do not count as nodes because they are connected directly to an input voltage source...

WHY, about input impedance, is not the same for a single-transistor and differential amplifiers?!
WHY the input signal should be connected through an Rs for the first case and not for differential amplifier?

Attached is the page of that book.

TIA!
CBs

 

[sutapanaki]

I don't think there is a special meaning in this. What the authors want to emphasize is that if we consider the inputs connected directly to voltage source with Rs=0 then we don't count the input node as a limiting factor in the frequency response. If Rs is not 0 but finite then of course it will have its effect on the frequency response. Both the single transistor amplifier and the diff amp can be considered as being connected to a input voltage source with some Rs or without. Depends on the assumptions.

 

[LvW]

Laker & Sansen have decided - for comparison purposes - to classify different circuit alternatives by defining "nodes". The reasoning is given on page 476 (second paragraph): Only high impedance "nodes" can create poles.
Thus, they count the (theoretical !!!) number of poles of a circuitry by simply counting the high impedance nodes. That´s all.