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Design Languages >> Verilog-AMS >> verilogA example code of opamp
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Message started by seanpai on Apr 17^th, 2012, 3:00pm

Title: verilogA example code of opamp
Post by seanpai on Apr 17^th, 2012, 3:00pm

Hi guys, I am using a verilog-A to build up an behavior model of opamp for LDO. My opamp is a differntail input folded cascode and single ended output amp, and I am going to limit the output range of it between vdd-2 Vov and 2Vov. I looked at the opamp example in the ahdlLib, which is in the virtuoso library, and can't figure out the code in "soft output limiting" part in the bottom. The code is showed as follows:
"if (V(vout) > (V(vspply_p) - vsoft))
I(cout, vref) <+ gm_nom*(V(vout, vspply_p)+vsoft);
else if (V(vout) < (V(vspply_n) + vsoft))
I(cout, vref) <+ gm_nom*(V(vout, vspply_n)-vsoft); "
In my understanding, the first and second line mean when Vout > Vdd-2 Vov, then I(out) will be gm*(V(vdd,vout)+2Vov) in my case. And the third and fourth line mean when Vout < vss(=0 in my case)+2Vov, then I(out) will be gm*(V(vout,0)+2Vov) in my case. Am I correct? Moreover, I don't know how the "V(vout, vspply_p)+vsoft" and "V(vout, vspply_n)-vsoft" are derived? Finally, why I(out) here become a Vout related value? It should be gm*vin_val by definition, right? Can anyone help me figure out these problems please? Thank you so much.

Title: Re: verilogA example code of opamp
Post by boe on Apr 17^th, 2012, 3:11pm

seanpai,

the model reduces gm if you get closer than vsoft to either rail.

- B O E

Title: Re: verilogA example code of opamp
Post by seanpai on Apr 17^th, 2012, 3:51pm

boe wrote on Apr 17^th, 2012, 3:11pm:

seanpai,

the model reduces gm if you get closer than vsoft to either rail.

- B O E

Hello B.O.E, Thank you for the reply. But I am still confused in the equation "I(cout,vref) <+ gm_nom*(V(vout,vspply_p+vsoft))". Why gm_nom would time "(V(vout,vspply_p+vsoft))" here? :-X

Title: Re: verilogA example code of opamp
Post by boe on Apr 18^th, 2012, 1:50pm

seanpai wrote on Apr 17^th, 2012, 3:51pm:

...
Hello B.O.E, Thank you for the reply. But I am still confused in the equation "I(cout,vref) <+ gm_nom*(V(vout,vspply_p+vsoft))". Why gm_nom would time "(V(vout,vspply_p+vsoft))" here? :-X

Note that V(vout,vspply_p)+vsoft = 0 is equivalent to V(out) = V(supply_p)-vsoft.

- B O E

Title: Re: verilogA example code of opamp
Post by seanpai on Apr 18^th, 2012, 2:11pm

But why I(out) comes from gm_nom * "(V(vout,vspply_p+vsoft))" not gm_nom * "vin_val" here? And if V(vout,vspply_p)+vsoft = 0, we'll get a zero I(out), right? Always appreciate for your response. Thanks.

Title: Re: verilogA example code of opamp
Post by boe on Apr 19^th, 2012, 1:44pm

Seanpai,

Note that all contribution statements to one branch current are added.

- B O E