Introduction to LTspice

ReadMeFirst

Lab Summary

In this lab, you will be using a SPICE simulator known as LTspice. SPICE (Simulation Program

with Integrated Circuit Emphasis) is a general-purpose, open source analog electronic circuit

simulator. It is a program used in integrated circuit and board-level design to check the integrity

of circuit designs and to predict circuit behavior.

The three different analyses that will be performed in this lab are DC Operating Point Analysis,

Transient Analysis and AC analysis.

Lab Preparation

Files:

LTspice_Shortcuts.pdf

Part 1 – Introduction

Open LTspice by clicking on the shortcut found on your desktop. Once the program is

open, create a new schematic by clicking on the “New Schematic” icon on the toolbar.

Once the schematic is created, click on “File” and “Save as” and save the new schematic as “DC

OP Analysis”. Start by adding a resistor to your new schematic. This can be done by either using

the shortcut “R” or by clicking on the resistor icon in the toolbar. Once the resistor is

selected you can rotate the component by pressing “Ctrl+R”. Left-click once to place the

resistor on the schematic. You can press “Esc” on your keyboard once you’re done placing the

resistor. You can move around your schematic by holding left-click and dragging the mouse

anywhere on your schematic. You can zoom in and out of your schematic using the mouse’s

scroll wheel or by using the icons on the toolbar. If you ever zoom out too much

you can click on the “Zoom Full Extents” icon (Spacebar or ) to zoom to fit your schematic.

The move icon (F7 or ) will allow you to freely move components around your schematic

while the drag icon (F8 or ) drags the components and any wires that may be connected to

it. In order to move multiple components at once, click on the move icon and select the

components or wires that you want to move by holding left-click and dragging the cursor over

the components you want to move. The same procedure can be done using drag.

A list of useful shortcuts are available in the LTspice_Shortcuts document on the lab website.

Part 2 – DC Operating Point Analysis

DC Operating Point Analysis calculates the behavior of a circuit when a DC voltage or current is

applied to it. The result of this analysis is generally referred as the bias point or quiescent point,

Q-point.

We will be building the same resistor network that we analyzed in the Resistor Networks lab for

our DC analysis. Using the same schematic that you created earlier, place 6 resistors on your

schematic like in the screenshot below.

Figure 1 Inserting components on schematic

Now add a voltage source by clicking on the components icon (F2 or ) and selecting

“voltage” from the menu as seen below.

Figure 2 Selecting Voltage Source

Connect the components using the wire tool (F3 or ) as seen below.

Add the ground (G or ) to the schematic as well.

Figure 3 Resistor Network Schematic

Label the nodes using the label icon (F4 or ). Use the label names as seen in the

screenshot below. Once you select the label icon, you will be asked to enter the name for the

node. Type “Node1” and click OK. You can now place the label by hovering over and clicking the

wire that needs to be labelled. Set the values of the resistor and voltage source as well by right

clicking on the components and entering the values listed in the screenshot below.

Figure 4 Resistor Network Schematic with Net names

You are now ready to run the DC Operating Point Analysis. Click on the Simulation icon ( )

and select the “DC op pnt” tab and click OK. A window will then pop up showing the voltage at

the various nodes and the current through the source and the resistors like in the screenshot

shown below. (Note that most of the values in the screenshot are hidden)

Figure 5 Operating Point Simulation Results

Although it makes sense for the current through the source (V1) to be negative, you may

observe that some resistors have a negative current flowing through them as well. This can be

fixed by moving that resistor and rotating it twice and putting it back to its place. If you rerun

the simulation, you will observe that the current flowing through that resistor is now positive.

Keep in mind that passive resistors do not work this way in the real world and that the direction

of the resistor doesn’t affect the polarity of the current.

Record the voltage and current values in the results sheet.

After running the simulation, you can also view the voltage at each node by simply clicking on

the node. You can then right-click on the voltage value and modify it to show a different value

such as current by selecting the value from the “Displayed Data” window. (Just be sure to

remove the ‘$’ sign which usually aliases to the node voltage you selected) You can also display

your own equations, for example, you can display the power consumed by resistor R2 by typing

in the expression, “ (V(output)-V(node1))*I(R2) “.

Part 3 – Transient Analysis

Transient Analysis, also called time-domain transient analysis, computes the circuit’s response

as a function of time. This analysis divides the time into segments and calculates the voltage

and current levels for each given interval.

We will be building the RC Lowpass filter that we used in the First Order Response lab for our

transient analysis. The two type of transient response that we will be simulating is the step

response.

Create a new schematic and call it “Transient Analysis”. Create the schematic as seen in the

screenshot below.

Figure 6 First Order Circuit Schematic

Set the voltage source to be a pulsed function by right-clicking it and selecting “Advanced”.

Select “PULSE” under Functions and set the parameters to the values shown in the screenshot

below.

Figure 7 Voltage Source Advanced Settings

The above parameters should create a square wave with a 50% duty cycle at a frequency of 1

KHz and an amplitude of 5 Vp-p. You are now ready to run the transient analysis. Click on the

Run icon. Under the Transient Analysis tab, set the stop time to 0.002 (or 2m) seconds and click

ok. Once the simulation is done, hover over the schematic and select the output (Out) and

input (In) nodes using the probe cursor. You should now be able to see the graph for the step

response similar to the one shown below. (Note that the font size and colors were modified to

make the graph more visible)

Figure 8 Transient Analysis Graph

To zoom in to a certain area of the graph, hold left-click and highlight the desired area. You can

zoom to fit your graph by simply right-clicking anywhere on the graph and selecting “Zoom to

Fit”.

We will now measure the time constant from the simulation similar to what was done in the

First Order Response lab. Since we only need the output trace for this, right-click “V(In)” and

select “Delete this Trace”. Cursors can be placed on the graph by clicking on the corresponding

title. Bring up two cursors on the output by clicking V(out) twice. Measure the Decay time by

placing one cursor at the start of the decay and the other cursor halfway (2.5 V) and record the

Difference in the Horizontal value in the Cursor pop-up window. To set the cursor close to 2.5 V,

you will need to zoom in to the graph at 2.500 V in order to increase the resolution. Once

zoomed-in it will be easier to drag the cursor to the desired value. The cursor seen in the

screenshot below was set close to 2.5 V this way.

Figure 9 Transient Analysis Graph Zoomed In

Calculate the time constant from the measured decay time and enter your answer in the results

sheet. If you forgot how to calculate the time constant, refer back to the First Order Response

Screencast.

Part 4 – DC Sweep Analysis

DC Sweep Analysis is used to calculate a circuits’ bias point over a range of values. This

procedure allows you to simulate a circuit many times, sweeping the DC values within a

predetermined range. You can control the source values by choosing the start and stop values

and the increment for the DC range. The bias point of the circuit is calculated for each value of

the sweep.

For this simulation, we will use the non-inverting the Operational Amplifier (Op-amp)

configuration we used in the Operational Amplifiers 1 lab.

In order to use the LME49710 SPICE model, the model and symbol file must be added to the

appropriate folders. Download the LME49710.lib and LME49710.asy files from the lab website.

Paste the LME49710.lib file under “This PC -> Documents -> LTspiceXVII -> lib -> sub” and paste

the LME49710.asy file under “This PC -> Documents -> LTspiceXVII -> lib -> sym -> Opamps”.

Note: Once the files are placed in those folders you may need to restart LTspice in order to be

able to see the LME49710 Op-amp under Components.

Create a new schematic and call it “DC Sweep Analysis”. Open components (F2 or ) and go

to [Opamps] and select the LME49710 Op-amp. Place the Op-amp in the schematic and build

the schematic seen in the screenshot below.

Figure 10 Non-Inverting Operational Amplifier Schematic

In order to run the DC Sweep, click on Run and select DC Sweep tab and set the parameters as

shown in the screenshot below.

Figure 11 Edit Simulation Command Window

Note that “V3” which is the input source is selected as the 1st source to sweep. This may not be

true in your schematic based on what source you chose to use for your power rails versus your

input.

Run the simulation and probe the Output node. You will observe that your output begins to

saturate at a certain voltage. Ideally, the Op-amp would saturate at +/- 12 V. Record the voltage

at which your Op-amp saturates, in your results sheet.

Part 5 – AC Analysis

AC Analysis is used to calculate the small-signal response of a circuit. In AC Analysis, the DC

operating point is first calculated to obtain linear, small-signal models for all nonlinear

components. Then, the equivalent circuit is analyzed from a start to a stop frequency. The

result of an AC Analysis is displayed in two parts: gain versus frequency and phase versus

frequency.

For this simulation, we will use the same non-inverting the Op-amp configuration we used in

the previous section. Go back to your old “DC Sweep Analysis” schematic and save as “AC

Analysis”. Set your input source to have an AC amplitude of 1 by right-clicking on your source

and “Advanced” as seen in the screenshot below.

Figure 12 Voltage Source Advanced Settings

In order to modify or change the type of simulation from DC Sweep to AC analysis, you can go

to Simulate -> Edit Simulation cmd or by simply right-clicking the DC Sweep spice directive line

on the schematic. Select the AC analysis tab and set the parameters as shown below.

Figure 13 Edit Simulation Command Window

Run the simulation and select the Output node. Use the cursors to find the cutoff frequency.

Record the DC gain (V/V) and frequency at the cutoff. Calculate the Gain-Bandwidth Product

(GBWP).

If you forgot how to measure the cutoff frequency and how to calculate the GBWP, refer back

to the Operational Amplifiers 1 Screencast.

*** END of LAB ***