introduction to ltspice - mitltspice • developed in 1998 by mike engelhardt at linear technology...

6.101 Spring 2020 Lecture 4

Introduction to LTspiceAcknowledgment: LTspice material based in part by Devon Rosner (6.101 TA 2014), Engineer, Linear Technology

SPICE

• Simulation Program with Integrated Circuit Emphasis

• Developed in 1973 by Laurence Nagel at UC Berkeley’s Electronics Research Laboratory

• Dependent on user defined device models

6.101 Spring 2020 Lecture 4 2

Netlists


Components

Commands

3

LTspice• Developed in 1998 by Mike Engelhardt at Linear

Technology Corporation• GUI, simulator, and schematic -> netlist for SPICE • FREE and comes with tons of models


You do this Ltspice makes this

4

Getting Started


THAT’S IT!

These buttons are where you will live

5

Component to Menu Item Matchup


Net Labels


By labeling nets you can avoid a giant mess of wires. Always use these for at least your power supplies. When you start making large circuits, your power supplies will provide energy all over your schematic.

7

Adding Other Components


Devices besides basic resistors, capacitors, and inductors are found from this button

8

Op-Amps


There are no “ideal” op-amps in reality. BUT, there are in LTspice.

PAY CLOSEATTENTIONTO THE TEXT

You must literally include.lib opamp.subin your netlistor schematicas a SPICE directive.

9

Op-AmpsThough listed as “ideal” there are still 2 parametersyou can tweak.


Open Loop Gain: As this number approaches infinity, the Op Amp becomes more “ideal”. Look at some Op Ampdata sheets to see some real open loop gains.

Gain Bandwidth: As this number approaches infinity, the Op Amp becomes more “ideal”. To check if this is highenough, multiply your desired Closed Loop Gain by your highest desired output frequency.

Op-AmpsTo more accurately model a real Op Amp not available in LTspice, UniversalOpamp2 has many tweakable parameters.


Open loop gain, gain bandwidth, slew rate, current limit, rail-rail voltage, input voltage offset, phase margin, Rin, etc.

Editing Components


Just right click thecomponent

12

Editing Components


This is the basic voltage source menu.Use this for DC sources such as powersupplies or bias voltages.

But what about this?

13

Editing Components


Voltage sources can produce many test signals.PWL can be used to construct any signal.

14

Selecting Device Model


There are no “ideal” BJT’s, MOSFET’s, etc. You can select a model (provided by LTspice), download models, or create your own.

15

Simulation: TransientTransient simulation gives Voltage and/or Current vs.time.


These are transient parameters for a voltage source

16

Simulation: Transient


This is all youreally need

17

Random Tangent: Parameters


This is a parameter

You MUST define all of your parameters. The “list” command allows you to choose multiple values (simulation simulates each value separately).

18

What Should My Circuit Do?• The very first step to any simulation is to know how

your circuit should behave. Simulation is a verification tool NOT A CIRCUIT SOLVER.

• So how should this circuit behave?


Here’s Where You Write the Solution


Here’s Where You Write the Solution


vx

vo

i1

i2

i3

A DOUBLE POLE!!

21

Expected Behavior• Double pole is at:• We expect frequencies up to this point to be

large, but frequencies above to quickly drop off due to the -40 dB/decade characteristic of the double pole


Transient Simulation


Hover over the desired voltage node to be probed and click when you see this symbol

**This is the current probe

23



1 kHz10 kHz100 kHz1 MHz

24

AC Simulation


This is the ACparameter.Just set theamplitude to 1

AC simulation gives Voltage and/or Current vs.frequency.

25

AC Simulation


Extra Fun: Math in LTspice


Remember:

27



It’s the same as before!

28

Even More Fun


*Note: You can try out some math functions in the simulator window, too! (ex: V(Vo)/V(Vi)).

AC Simulation


Temperature as a Variable

• PTAT current source


Temperature as a Variable


Including External Models


• PFET model• Includes

parameters to describe MOS device physics

Making Things Pretty


Making Things PrettyBob Reay of Linear Technology has provided a nifty tool on his website to give LTspice circuits an even better makeover: http://reaylabs.com/tools/SchematicViewer/SchematicViewer.html


Before:

Making Things PrettyBob Reay of Linear Technology has provided a nifty tool on his website to give LTspice circuits an even better makeover: http://reaylabs.com/tools/SchematicViewer/SchematicViewer.html


After:

LTspice SecretsMany aspects and functions of LTspice are not documented. You can learn lots of interesting undocumented capabilities of LTspicefrom:http://ltwiki.org/?title=Undocumented_LTspice

Of particular interest should be B-sources. These allow you to make devices such as non-linear resistors whose value is determined from a function of voltage, current, if statements, constants, etc. Though you cannot build these, they may be useful to model a part not available in LTspice, or to model a special function in your circuit you have not designed yet.


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LTSPICE SHORTCUTS ON A MAC 11/5/2013 REV 3

a DRAW CIRCLE b BUS TERMINATION g GROUND l DRAW LINE s ADD SPICE DIRECTIVE (right click for HELP ME EDIT) t ADD TEXT COMMENT w DRAW BOX z H HIDE LTSPICE z L SPICE LOG z N NEW SCHEMATIC z O OPEN z Q QUIT LTSPICE z S SAVE z Z UNDO ⇧z Z REDO z M MINIMIZE ⌥z M MINIMIZE ALL z W CLOSE ⌥z W CLOSE ALL z P PRINT ⇧z P page seupt F2 COMPONENT F3 WIRE F4 NET NAME F5 DELETE F6 DUPLICATE F7 MOVE (CNTRL-R to rotate, CNTRL-E to mirror) F8 DRAG (CNTRL-R to rotate, CNTRL-E to mirror) F9 UNDO ⇧F9 REDO SPACE BAR ZOOM TO FIT 2 FINGER PINCH ZOOM IN 2 FINGER SPREAD ZOOM OUT Here are the modifier key symbols you may see in OS X menus: z COMMAND ⌥ ALT OR OPTION ⇧ SHIFT

introduction to ltspice - mitltspice • developed in 1998 by mike engelhardt at linear technology...

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