# LT spice frequency analysis

Discussion in 'General Electronics Chat' started by shafee sk, May 5, 2012.

1. ### shafee sk Thread Starter New Member

May 5, 2012
3
0
Hi,

In LTspice, for any AC analysis circuit, can I be able to plot 20 log (Vo / Vi) directly. If so how??

thanks & Regards,
shafee.

2. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Have you looked in the Help file?
Open LTSpice Help, click the Contents tab, and expand the "Waveform Viewer" topic.
Waveform Arithmetic (this contains exactly what you wish to know) and User-Defined Functions are the 3rd and 4th in the topic list, but I strongly suggest that the entire section is well worth the time spent reading through it. You will save yourself a considerable amount of time by doing so. Ask me how I know this.

May 5, 2012
3
0

4. ### shafee sk Thread Starter New Member

May 5, 2012
3
0
ofcourse please tell me how did you come to know this??

5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
I've been using LTSpice for a few years now. When I first started using it, it took me about three months to discover you could display the average and RMS of a waveform interval by just ctrl+left-clicking on the signal name on the plot. Reading the HELP file would have saved me a lot of time trying to figure that (and many other things) out.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
In an AC analysis, the network is first linearized at the DC operating point. This means that there will be no distortion, no matter what amplitude you apply to it. If you have a circuit such as a transistor amplifier with a small signal gain of 100, and set the AC amplitude to 1mV, the simulated output will be 100mV. If you now change the AC amplitude to 1000V, the simulated output will be 100,000V.
What this means is that if you apply 1V AC, and then plot the output, you will get an output of 100V. 20 log(100) is 40dB, which is the gain of the circuit.
In other words, apply 1V AC, plot Bode plot (voltage vs frequency) of the output , and you will see the gain.

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