Differential amplifier AC analysis

[tyanata]

ac analysis of differential amplifier

Hi,
Can any give me a article about the simulatin of diferentialal op. amplifiers. The circuit is a two stage amplifier: the first stage is telescopic amplifier with PMOS
input transistors, the second stages are inverters with activ load (PMOS tranzistor).

I tryed to make AC simulation but the output voltages of the inverters went to VDD.

There is a cooommon mode feedback circuit but obviously it doesn't work properly.

 

[Vamsi Mocherla]

differential amplifier ac analysis

Actually the common mode feedback circuit is not for ac analysis. The common mode circuit is used when we have large signal swings. ac analysis is not a large swing analysis. When you do a differential amplifier analysis, you test the main path removing the CMFB circuit. You can substitute, the node where the CMFB circuit corrects with an ideal voltage source. Then you can test the CMFB circuit.

The reason why your outputs are seeing VDD could be because of the absence of bias voltages at the inputs. Try and check if you have given the correct bias voltages at the input. Ideally it could be (VDD-VSS)/2.

 

[mady79]

differential amplifier analysis

Use an ideal VCVS connecting input terminals to VOUTP ,VCM and output terminals to gate of the cmfb tail transistor .Give CMFB gain in VCVS .
See the AC gain at VOUTP now .or Give initial conditions for VOUTP,VOUTM .

Hope this helps .

 

[chinito]

ac analysis of a differential amplifier

it would help if post your circuit here. When you say output goes to VDD, are you talking about excursion or the bias point.

 

[Lantis]

ac analysis of diffrential amplifier

probobly you need to draw the whole circuit in, so somebody can help.

 

[2steps]

dc and ac analysis of differential amplifier

AC simulation is used for small signal analysis, this point is very important

 

[rainman.cn]

ac simulation differential

when test, verilog-a will be usefull to describe the cmfb ckt...

Added after 4 minutes:

nathan's attchment is very good

MODERATOR ACTION:
**broken link removed**

 

[analoger]

differential amplifier a.c analysis

Make sure that your cmfb circuit is not connected in positive feedback

 

[mosti]

differential amplifier ac and dc analysis

Before doing ac-analysis, make sure the DC Operation Point is working correct. Try to write down the small signal equivalent circuit. From this you could calculate a lot of nice parameters like A0. You could compare this to your simulation. For hand calculation you the gm and gds values of your transistors, you will obtain them from your DC-OP analysis. Try it, it will help you.