Introduction to LTSpice circuit simulation

We have done some in the early stage STM32 Combined with the embedded program development of various sensors , At present, it has been basically completed . The goal at this stage , We need to build a sensor module by ourselves , There are some circuit structures involved in the sensor module , We need to do this before the actual mass production , Do some circuit simulation experiments , So as to verify whether the circuit structure we built is reasonable and efficient . Based on this demand , We need to learn circuit simulation software —— LTspice

1. LTspice Introduce

There are many kinds of circuit simulation software in the market , We choose LTspice Because of him free . We are directly in the Official website download LTspice , install , start-up , You can use .

The official website is quoted here LTspice As a reference ：
LTspice It's a high performance SPICE Simulator software , Including schematic capture graphical interface . Schematic diagrams can be detected to produce simulation results , adopt LTspice Built in waveform viewer for easy exploration . And others SPICE Solution compared to ,LTspice The enhancements and models of the improved analog circuit simulation .

2. Circuit simulation

Circuit simulation actually involves a lot of basic knowledge about circuit principles , Without or remembering the basic operating principle of the relevant circuit elements , Hastily conduct simulation experiments , There will be ambiguous understanding problems . So , We need to learn to use LTspice In the process of , Keep learning or reviewing the basic knowledge of circuit principle . Here we choose to open a section , It is used to record the basic knowledge of circuit principle learned or reviewed .（ The circuit principle is based on the basic circuit components ）

2.1 Circuit principle

Voltage source

Voltage source , Ideal voltage source , It doesn't exist in real life . So why do we often mention this name , We call the power supply used in our experiment a voltage source . That's because , The internal resistance of the power supply used in our experiment is very small , The external load impedance is very large , Then the internal resistance of the power supply is very small relative to the load impedance . In this huge internal and external impedance gap , When we change the external impedance , Load current changes , The voltage on both sides of the power supply will not change . therefore , The voltage source has one characteristic , The voltage at both ends of the voltage source is determined , It does not change with the change of the current flowing through him .

Current source ：

Current source , Ideal current source , It also doesn't exist in real life . The current source used in our experiment , Its internal resistance is very large relative to the impedance of the load . When the difference between internal and external impedance is so large , Load impedance changes , The current in the circuit basically does not change , Because the current in the circuit is controlled by the current source （ voltage / resistance ） decision . therefore , The current source has one characteristic , The output current of the voltage source is constant . Sum up , We can find out , The origin of current source and voltage source , It is a definition produced by the great difference between the internal resistance of the power supply and the load impedance . Let's have a look , The internal resistance of the current source is infinite , The internal resistance of the voltage source is infinitely small .

How to make the ideal current source and voltage source in the laboratory is also an engineering problem worth thinking about .

RC Discharge circuit ：

We use it LTspice Make one RC The discharge circuit is as follows ：

In the initial state , capacitance C1 by 1v,R1 by 1K, Capacitance from 0ms Start discharging at any time , The following discharge curve is obtained ：

capacitance C1 Start discharging , Voltage on both sides v+ Gradual decline . meanwhile , Due to the constant discharge of the capacitor , The capacitor voltage drops ,R1 remain unchanged , The current in the circuit also drops gradually .（-1mA To 0mA）

Next we'll think about , What affects capacitance C1 The discharge rate . According to a diagram in the textbook of circuit principle , as follows ：

We found that , capacitance C1 Discharge rate and time constant τ（tao） relevant ,τ For the definition of ：τ = R*C. According to this theory , The capacitance is about 3τ The discharge ends at this time point .

We go back to LTspice It's done RC Discharge circuit , Calculate by formula ,3τ = 3 * R * C = 3 * 1K * 1u = 3ms. in other words C1 stay 3ms The basic discharge ends at ,C1 The voltage at both ends is close to 0V, This verifies that the theory in the book is consistent with the phenomenon obtained by our simulation .

How to speed up capacitance C1 The discharge rate , We need to lower τ, That is to reduce R*C Value . In the case of keeping a variable constant , Lower another .

Think about it in a different way , from R and C The circuit can realize the filtering effect . Why? ？ Add such a circuit to a standard sine wave , It can attenuate sine wave , The sine wave will gradually weaken to 0, That is, it has the effect of filtering . Here's the picture ：

Voltage source review again ：

We usually use a voltage source to generate some DC voltage in the laboratory , Or produce some varying voltage , Such as ： square wave , Triangle wave , Sine wave . Their simulation method is very simple , Directly select the voltage source symbol , And configure waveform parameters , We draw the following figure ：

DC voltage 0.5V （ Red ）, Throughout 10ms In a straight line in time .
square wave （ Sky blue ）,2ms For a cycle , high voltage 0.5V continued 1ms , Low voltage 0V continued 1ms.
Triangle wave （ Blue ）,2ms For a cycle , The rising edge is 1ms, The falling edge is also 1ms.
Sine wave （ Pink ）,1ms For a cycle , The frequency is 1KHz.

look , This very standard voltage waveform is very easy to draw . So what if we want to draw the waveform of a specific function ？ such as , We want one Step signal To test our system input . At this time, we need to define the input in the form of function . First , According to the circuit theory, we know that the step function is u(t). We are LTspice Select... From the catalog of Arbitrary behavioral voltage source Components , And give him a step function ：V=u(time). For the convenience of observation , Let's write the function as ：V=0.5*u(time-2m), Set the voltage amplitude to 0.5V, The step signal shifts to the right 2ms, The following step signal waveform is drawn ：

thus , We customize V=f(t) function , The voltage waveform of any function can be obtained , As system input , To test our system performance .

xxx：