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cjupiter
Joined: 10 May 2004
Posts: 45
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 20 Oct 2004 5:43 how to plot open-loop gain vs frequency |
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| Im not sure how to plot the open-loop gain of my opamp in spice. I want it so i have the gain in decibels on y-axis and log frequency on the x-axis. Do i have to configure my circuit in order to achieve correct plot?
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Fom
Joined: 10 Mar 2004
Posts: 759
Helped: 56
Location: Taiwan
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| 20 Oct 2004 5:50 Re: how to plot open-loop gain vs frequency |
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| If you have no feedback from output to input in your simulation circuit no need to change anything.
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sunking
Joined: 25 May 2004
Posts: 914
Helped: 46
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| 20 Oct 2004 7:20 Re: how to plot open-loop gain vs frequency |
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Add a Big RI(1G or so) between output and input, then simulate.
plot with db
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Hughes
Joined: 10 Jun 2003
Posts: 712
Helped: 84
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| 20 Oct 2004 9:29 Re: how to plot open-loop gain vs frequency |
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| cjupiter wrote: |
| Im not sure how to plot the open-loop gain of my opamp in spice. I want it so i have the gain in decibels on y-axis and log frequency on the x-axis. Do i have to configure my circuit in order to achieve correct plot? |
I'm not very sure about what's the matter with you. If the problem is about the simulation tool, providing the tool name your are using may be helpful. In hspice, you may get the gain vs. frequency curve by ac analysis and a probe statement like this:
.probe ac gain=vdb(out)
If the problem is about circuit configuration, a feedback network may or may not be needed to set the opamp's operating point. In hspice, you may set the AC resistance different from DC value for a resistor. Thus, a feedback network can be constructed to set the DC operting point of the op amp while the opamp is almost 'OPEN-CIRCIRT' in AC analysis.
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cjupiter
Joined: 10 May 2004
Posts: 45
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| 20 Oct 2004 12:00 Re: how to plot open-loop gain vs frequency |
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Thanks for all the feedback everyone. I have plotted the gain by doing a frequency sweep and plotted Vout in db. I have a result but im pretty sure it is wrong. My response is in the negative decibel range instead of the positive range. I have a flat response across - 60dB and then it drops off steeply after 1MHz, becoming more negative. Is there a common reason for this?
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Hughes
Joined: 10 Jun 2003
Posts: 712
Helped: 84
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| 20 Oct 2004 13:51 Re: how to plot open-loop gain vs frequency |
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Maybe your op amp didn't work with proper DC operting point. You may check the operating of each active devices. If this is the case, you may try to use the attached configuration to do a frequency sweep. The inductor and capacitor should have a large enough value.
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hanjiemy
Joined: 11 Jun 2004
Posts: 54
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| 21 Oct 2004 6:12 how to plot open-loop gain vs frequency |
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I think that u have sth. wrong with your circuit.
Every problem must have a more simple than it , need be solved.
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devrimaksin
Joined: 13 Oct 2004
Posts: 93
Helped: 9
Location: Dallas, Texas, USA
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| 21 Oct 2004 19:45 how to plot open-loop gain vs frequency |
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Instead of an inductor put a resistor
this way you can simulate
DC gain also
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hawkin
Joined: 05 Jul 2004
Posts: 10
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| 24 Oct 2004 12:33 how to plot open-loop gain vs frequency |
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which tool do you use?
when i use spectre,it's easily to plot the bode curves in the direct plot fuction.
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anadigi
Joined: 01 Oct 2004
Posts: 31
Helped: 1
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| 24 Oct 2004 13:53 Re: how to plot open-loop gain vs frequency |
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| devrimaksin wrote: |
Instead of an inductor put a resistor
this way you can simulate
DC gain also |
Inductor (very high value) will be the better choice..
If u use resistor, u need to be careful..It may cause a false information of 1st pole in ur o/p response..
I hav also seen a T-network(RCR) been used in feedback..but never tried..
Regards
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VSWR
Joined: 07 Feb 2002
Posts: 640
Helped: 53
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| 25 Oct 2004 21:56 Re: how to plot open-loop gain vs frequency |
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Here is a good article:
http://www.spectrum-soft.com/news/winter2001/loopgain.shtm
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Ipanema
Joined: 27 Oct 2004
Posts: 41
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| 28 Oct 2004 4:20 Re: how to plot open-loop gain vs frequency |
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Pls check if any transistor is in cutoff with .op command. If your circuit is OK, then you can compare the AC analysis gain with that obtained from transient anlaysis for comparison in order to make sure that your AC analysis method is correct.
Thanks
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yakyao
Joined: 12 Oct 2004
Posts: 8
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| 28 Oct 2004 4:54 how to plot open-loop gain vs frequency |
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| maybe the DC bias is wrong
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cjupiter
Joined: 10 May 2004
Posts: 45
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| 28 Oct 2004 4:59 Re: how to plot open-loop gain vs frequency |
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| yakyao wrote: |
| maybe the DC bias is wrong |
So youre saying i should bias my input transistors first with a DC source before performing the AC frequency sweep?
Thanks
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Hughes
Joined: 10 Jun 2003
Posts: 712
Helped: 84
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| 05 Jan 2005 23:55 Re: how to plot open-loop gain vs frequency |
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| cjupiter wrote: |
| yakyao wrote: |
| maybe the DC bias is wrong |
So youre saying i should bias my input transistors first with a DC source before performing the AC frequency sweep?
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It may or may not be a DC source. If you use a transient function (such as SIN, PWL, PULSE), the value at time 0 is used for calculate the operating point (DC bias).
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