Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

[SOLVED] Basic question about poles and nodes.

Status
Not open for further replies.
P

palmeiras

Full Member level 6
Joined
Feb 22, 2010
Messages
375
Helped
61
Reputation
122
Reaction score
50
Trophy points
1,308
Location
South America
Activity points
4,199
Hi guys,

In analog design, we learn that we can associate poles and nodes through the following rule:
"The pole frequency is given by the product of the inverse of R and C, between the node and the AC ground". For instance, for the output node of the circuit given bellow, the pole frequency (wp2) is 1/RD*CL.
Regarding the node "vin", Razavi book says that the is wp1 = 1/Rs*CL. It considers that vin is a AC ground. However, Vin is where the input signal is applied, and then, I cannot consider it as AC ground.
Why does Razavi considers Vin as a AC ground? And then apply the rule?
Thanks,
Poles_Source_amplifier.jpg
 

Vin isn't ground, but low impedance voltage source. The effect is the same when calculating RC time constants.
 
Rs is intended to mean the source impedance of your non ideal voltage source. When a cap is added the Ideal source is a 0 Ohm impedance. The real one is Rs .You add more R to Rs to make any filter time constant you want since load impedance is relatively high. >>10Mohm in parallel with Ciss which increases inversely with RdsOn.
 
Hi FvM

Thanks for your reply. I did not get it. Why is the effect the same? There is AC signal there. Could you, please, explain it?
 

There is AC signal there.
Click to expand...
Not when you determine the node impedance against ground. According to superposition principle, other sources are set to zero.
 
I do estimate the poles/zeros based on the rule above (1/Req*Ceq), because I would like to estimate the voltage gain transfer function (Av = Vout/Vin) as a function of frequency for my circuit:

Av = (1+s/wz1)(1+s/wz2)… / (1+s/wp1)(1+s/wp2)
where wz1 and wp1 is the zero and pole frequencies.

The transfer function depends on where the input signal is applied. Thus, when you consider that no Vin is applied (superposition principle), I don’t know where the signal is applied. For instance, I don’t know if it is a source common, gate common or drain common amplifier.
Thus, I still did not fully understand the reason why I can use the superposition principle in this case. Could you, please, clarify it?
 

Hi,

It's a common source. Analog wiki have tutorials on transistor configurations, and other topics.
 

You do know your freq is in Rad/sec, not cycles per sec?
 

The consideration was specifically that the pole frequency is the same if Vin node is grounded or connected to a low impedance voltage source. That's no statement about the whole transfer function.

Nevertheless it's a kind of intuitive explanation. If you can't "see" it, calculate the exact transfer function.
 
Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top