 # LTspice Run And Probe A Circuit – AC Analysis

#### AC Analysis

When doing an AC sweep of a source, the sections highlighted Below in the source configuration window are relevant. The AC analysis allows you to plot magnitude and/or phase versus frequency for different inputs in your circuit.

Type of Sweep In the AC analysis menu you have the choice of three types of analysis:

• Linear,
• Octave and

These three choices describe the X-axis scaling which will be produced in probe. For example, if you choose decade then a sample of your X-axis might be 10Hz, 1kHz, 100kHz, 10MHz, etc…. Therefore if you want to see how your circuit reacts over a very large range of frequencies choose the decade option.

You now have to specify at how many points you want LTspice to calculate frequencies, and what the start and end frequency will be. That is, over what range of frequencies do you want to simulate your circuit.

• Number of points
• Start Frequency
• Stop frequency

In the above figure we are trying to apply Sine wave as the input. You can refer my post Transcient Analysis to learn how to apply the voltage. For AC analysis we provide sine wave as the input. To achieve it click on the sine wave (refer above figure) and enter the values of “AC Amplitude” and “Phase of the signal”. I f you are using sine wave then provide 0 as phase. In case if you want cosine wave as the input ,then provide 90 as phase.

Next step is the simulation of the circuit . For this first click on the simulate button and click on the edit simulation command, then a  pop up will appear. Click on the AC analysis and key in the following parameter.

• Type of sweep
• Number of points per sweep
• Start frequency
• Stop frequency

click OK click to enlarge

Now the next step is to run the circuit. You will receive the out put as shown below. From this figure we can also obtain cut off frequency by setting 3db point . You can set the 3Db point by right clicking on the trace label. Move the cross hair until you receive the 3Db point and click the left mouse button.

So we conclude this section and expect more on DC sweep in the next post.

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