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wjx197733

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what is the role of these two VCVS?
Feb 07^th, 2006, 5:22pm

Hi, all: I design a full differential opa, and I process the ac analysis.

Now, I have a simulation circuit that can do the ac analysis, but I do not know what the role of these two VCVS, i.e. E1 and E2.

at the same time, capacitance C1 and C3 were added to the input port of the opa, so I think they are short circuit for AC source, i.e. V1

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Cri Azzolini

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University of Parma, Italy

Re: what is the role of these two VCVS?
Reply #1 - Feb 8^th, 2006, 12:06am

In my opinion the VCVS sources E1 and E2 are used to provide a differential (symmetrical) AC stimulus to your opamp: please note that E2 has a reverse polarity with respect to E1 (the same effect could be obatined inverting the direction of E2 and setting its gain to -0.5). In this way the ota is stimulated with a balanced AC signal.

Capacitors C3 and C4 have two effects:
1) in DC analysis they connect resistors R1 and R2 in the feedback loop (probably in order to properly bias the input stage by means of the CMFB, inside the OTA, I suppose)
2) in AC analysis C3 and C4 break the opamp's feedback loop: resistors R1 and R2 are now connected to ground and, due to their value (1G), do not affect significantly the frequency response of the OTA. There are more solutions to break the feedback loop in order to calculate the open-loop transfer function. Sometimes it is said that breaking the loop is not the correct way to study a feedback stability: nevertheless, for designs including differential OTAs this technique is the most used (as far as I know).

Regards,
Cri

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Andrew Beckett

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Bracknell, UK

Re: what is the role of these two VCVS?
Reply #2 - Feb 8^th, 2006, 5:14am

Rather than using 1 Farad capacitors, you'd be better off using spectre's switch component (spt1switch in analogLib) which can be in different positions in different analyses. 1 Farad capacitors can cause trouble, even in a small signal analysis.

Anyway, a better solution still is to use spectre's "stb" (stability) analysis. For a differential circuit, you would use the cmdmprobe component from analogLib (two inputs, two outputs), and set the CMDM parameter to either 1 (to measure common mode stability) or -1 (to measure differential loop stability). Then in the stb analysis form, use the Select button next to the probe instance definition, and select the instance of cmdmprobe (it will automatically append /vinj to the instance name, because it actually needs to probe a source within the cmdmprobe schematic).

The stability analysis has the advantage of being able to keep the loop closed during simulation, which means that the DC is happy, and also the loading is handled correctly.

Regards,

Andrew.

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Croaker Senior Member Offline Posts: 235	Re: what is the role of these two VCVS? Reply #3 - Feb 8^th, 2006, 7:55am Cri's comments are right on. When setting the bias points in this manner the low frequency response will look screwed up if you don't use a big enough R and C.
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