Don't understand the result of a very simple ngspice simulation of a TIA-circui - Page 1
Forum section: electronics / beginners

author: Bigman
author_rank: Regular Contributor
timestamp: July 13, 2016, 06:19:15 pm
content: Hello!I made a netlist of a very simple Transimpediance Amplifier (TIA).A current source is connect with the OpAmps inverting-input. His current flows over a "feedback" resistor to the OpAmp's output. The max. current is 1.5e-7 A (offset + amplitude of the sinus wave).With a feedback resistor of 500e3 Ohms, I would have expected to see at the OpAmp's output a maximum of: 500e3 Ohms * 1.5e-7 A = 0.075V.But the output of the ngspice simulation goes up to 2.2 kV. Since the output is in the kV area but Vdd is only 3.6V it is obvious that I must have somewhere a stupid mistake. But the longer I stare on the netlist, the more desperate I am getting.Perhaps more experienced "experts" can give me a hint.Code:R tia_out tia_in 500e3I tia_in 0 SIN(0.5e-7 0.1e-6 10)XU1 0 tia_in 3.6 0 tia_out amp_tlv276x_highVdd.tran 1e-5 1.include TLV276XA_2.4V.sub.end----if of interest, here also the model of the OpAmp:Code:* 276x 2.4 V operational amplifier "macromodel" subcircuit* updated using Model Editor release 9.1 on 05/15/00 at 14:40* Model Editor is an OrCAD product.** connections: non-inverting input* | inverting input* | | positive power supply* | | | negative power supply* | | | | output* | | | | |.subckt amp_tlv276x_highVdd 1 2 3 4 5*c1 11 12 457.48E-15c2 6 7 5.0000E-12css 10 99 1.1431E-12dc 5 53 dyde 54 5 dydlp 90 91 dxdln 92 90 dxdp 4 3 dxegnd 99 0 poly(2) (3,0) (4,0) 0 .5 .5fb 7 99 poly(5) vb vc ve vlp vln 0 176.02E6 -1E3 1E3 180E6 -180E6ga 6 0 11 12 16.272E-6gcm 0 6 10 99 6.8698E-9iss 10 4 dc 1.3371E-6hlim 90 0 vlim 1Kj1 11 2 10 jx1j2 12 1 10 jx2r2 6 9 100.00E3rd1 3 11 61.456E3rd2 3 12 61.456E3ro1 8 5 10ro2 7 99 10rp 3 4 150.51E3rss 10 99 149.58E6vb 9 0 dc 0vc 3 53 dc .78905ve 54 4 dc .78905vlim 7 8 dc 0vlp 91 0 dc 14.200vln 0 92 dc 14.200.model dx D(Is=800.00E-18).model dy D(Is=800.00E-18 Rs=1m Cjo=10p).model jx1 NJF(Is=500.00E-15 Beta=198.03E-6 Vto=-1).model jx2 NJF(Is=500.00E-15 Beta=198.03E-6 Vto=-1).ends

author: danadak
author_rank: Super Contributor
timestamp: July 13, 2016, 11:16:46 pm
content: Post the ngspice file to make it easy for forum to look at the problem.Regards, Dana.

author: Bigman
author_rank: Regular Contributor
timestamp: July 14, 2016, 05:38:32 am
content: Hi Dana,I am not certain what file you mean exactly.So I attached the netlist (Simple_Tia.net) and the subcircuit of the OpAmp (TLV276XA_2.4.sub).(Their content is exactly what I posted above).Pls. note: this forum doesn't allow to upload .net and .sub files. So changed from both their extention to .txtPls. let me know, in case you need something different.Cheers

author: LvW
author_rank: Frequent Contributor
timestamp: July 14, 2016, 02:22:01 pm
content: Quote from: Bigman on July 14, 2016, 05:38:32 amI have no experience with ngspice. But - performing a TRAN analysis (time domain) with real opamp models (and, of course, supply voltages ) I NEVER have observed that the output voltage of the amplifier was larger than than the supply voltages. This applies to PSpice, Superspice, Topspice, Microcap, TINA;...Suggestion: Reduce the input amplitude by a factor of 10 and/or by 100 and watch what happens.

author: Bigman
author_rank: Regular Contributor
timestamp: July 15, 2016, 02:00:57 pm
content: I believe to have found the mistake:it SEEMS, the very 1st line of netlist is taken is taken as a headline and is not included in the simulation. Since my 1st line described the feedback resistor, this resistor was not included in the simulation.