## 可能出现基础问题

### 自我介绍(self-introduction)

Good morning, nice to meet you here. my name is xxx. I’m a senior student in xxx university. My major is Electronic Information Engineering.I like making or controlling machines.That’s why I choose my major. my objective is obtain a master of degree hardware design and development in German.

I want to apply for the universities such as Karlsruher Institut für Technologie,which has a good academic reputation and excellent professors.Also university of stuttgart and Technical University of Munich are included. They are all very good universities.

German universities have advanced equipments and good study enviroment, which can help me learn my professional knowledge better. At the same time, I want to experience international cross-culture. As you konw, word is becoming samller and closer due to the quickly developing technology.

My parents will pay the tuition for me. And they have already prepared enough money for my aboard study.

## 专业课程资料准备

### 电路

In this course, I mainly learned the basic components of the circuit and some laws and theorems of the circuit analysic.

#### 电路的基本元器件（basic components of the circuit）

##### 电阻（resistor）

When the voltage and current take the correlation reference direction,the voltage and current at both of resistor obey Ohm’s laws at any given moment. Ohm’s law states that the current across the resistor is directly proportional to the voltage at the both ends of resistor and inversely proportional to the resistor.

R是电阻元件的参数，是一个正常的实数，单位是欧姆。电阻的倒数称为电导，i = G*u，电导的单位是西门子，R和G都是电阻元件的参数。

R is the parameter of resistor and it’s a positive constant real number. The unit of resistance is ohm. The reciprocal of resistance is conductance and the conductance is simens. R and G are both the parameter of resistor.

The characteristics of resistor is called volt-ampere characteristics . The characteristics curve is a straight line passing through the origin the slope of the line is related to the resistance of component.

When the terminal voltage of a linear resistance element is no matter any value,the current flows through it is always zreo. So the circuit opens.

when the current of a liner resistance element is no matter any value,the terminal voltage of it is always zero. So the circuit is short out.

Correlation reference direction: The reference direction of the current flows the positive end of the voltage into the circuit and flows out from the negative end. Then the reference direction of the two are the same, so the reference direction is related. The reference direction is specified artificially.Because the actual direction of the current or voltage is unknown, after setting the reference direction, the actual direction direction of the current can be reflected by the plus and minus of the value.

##### 电容（capacitor）

A capacitor consists of two metal plates spaced apart in different mediums(such as mica or air). When the voltage is added to the two polar plate, the same number of positive and negative charges are accumulated on the two polar and an electric field is established in the medium, which has the electric field energy. After the power is removed, the charge can remain concentrated on the plate and the electric field continues to exist. Thus, a capacitor is an energy storage component that can hold charges.

The capacity of a capacitor store charges is called capacitance. The capacitance is equal to the ration of the amount of charges carried by the capacitor to the voltage on the two plates of the capatior. The unit of capacitance is Farad.

i=du/dt=C*du/dt，表明电流和电压的变化率成正比。当电压不随时间变化时，电流为零。所以电容在直流情况下，其两端的电压值恒定，相当于开路。也就是说电容有隔断直流的作用。

If the current and voltage of the capacitor component take the correlation reference direction, the relation between the current and the voltage can be obtained, which indicates that the current is directly proportional to the rate of the voltage. When the voltage does not change with time, the current is zero. Therefore, when the capacitor is under the circumstance of DC , the voltage at both ends is constant, which is equivalent to open circuit. That is to say, the capacitor has the function of blocking the direct current.

The capacitors in series are similar to resistors in parallel. The paraller connection of capation is similar to the series of risistors.

When the capacitor is connected to the DC power supply, there is current flowing through the circuit. Two pieces of metal plates produce repectively equal but opposite charges. When the capacitor is charging, an equal number but opposite charges will be produced on the two plates. The voltage at the two ends of the capacitor increases gradually. Once the voltage increases to equal the supply voltage, the capacitor is charged and there is no current flowing through the circuit.

The image of capacitor charging is consistent with the zero-state response of the RC first-order circuit. Zero-state response is the response that caused by the input signal when the inital state is zero.

When the connection between the capacitor an the power source is cut off, the capacitor is discharged through a resistor. The voltage between the metal plates will gradually drop to zreo. (Under the action of electric field, the positive and negative charges on the two metal plates are constantly nertralized, so the voltage drops continuously.)

##### 电感（inductor）

An inductor is a device that converts electrical energy into magnetic energy and stores it. For example, a winding coil with an iron core. After the coil is charged with electricity, the magnetic flux will be generated inside the coil. The ration of the coil’s magnetic flux to the current is the inductance. The unit of inductance is Henry.

The indutors in series are similar to the serices of resistors and the shunt of inductance is similar to that of resistors in parallel.

When the inductor is connected to the DC source, the sudden increacse in the current will cause the inductor to generate a induced electromotive force that is opposite to the power supply. The induced electromotive fotce hinders the changes of the current and the slowed current will cause the decrease of voltage . When the current is stable, there is no induced electromotive force generated. The voltage drops to zero volt and the energy storage of inductor is finished. VL is zero volt, so in the DC circuit. the inductor flows through the inductor can’t mutate.

When the inductor is disconnected from the power source, the voltage drops to a negative value instantaneously. But at the same time the current remains the same flow direction and size. Inductors release energy through resistors and VL increases gradually to zero while iL gradually decreased to zero.

#### 分析电路的定律定理（laws and theorems of the circuit analysic）

##### KCL，基尔霍夫电流定理(Kirchoff’s Current of Law)

For any node in the circuit, the sum of currents flowing into the node at any time equals to the sum of currents flows out the node.

##### KVL，基尔霍夫电压定理(Kirchoff’s Voltage of Law)

Along any closed loop, the algebraic sum of all branch voltage at any time is identically vanishing. Before in summing of voltages, we need to specify a circuitous direction of the circuit,the voltage is positive. Othewise, the voltage is negative.

##### 叠加定理(Superposition Theorem)

Superposition theorem states that in the linear resistance circuit, a voltage or current is the supersition of valtage or current produced by individual independent power separately applied to the circuit. The superposition theorem applies only to linear circuit, but not to nonliear circuits. In the subcircuit of superimposed circuit, the non-working voltage source is set to zero and replaced by the short circuit and replaced by the short circuit. The non-working current source is set to zero and replaced by the open circuit.

##### 戴维南定理(Thevenin’s Theorem/Norton’s Theorem)

A port with independent sources and linear resistors can be equivalent replaced by a series combinations of valtage source ande resistors or parallel combination of current source and resistor for extermal circuits. The voltage of voltage source equal to the open-circuit voltage of prot. The resistor equals to the input resistors, when all the indepent sources ant the port are set to zero. The current of current source equal to the short-circuit current of the port.

##### 电压源和电流源（Voltage Source & Current Source）

Voltage source is an ideal circuit compnent, the voltage of the voltage source is independent of the current through the componet and always keep a given time function. When it is a constant value, this voltage source is called a DC voltage source and is represented by Us.

The current source is also an ideal circuit element. The current of the current source is indepent of the component’s terminal voltage and can always be maintained as a given time function. current source in the volt-ampere characteristic curve is a line that does not pass through the origin and is paralled to the voltage axis. When is(t) changes with the time, the line will move along.

When i(t)=0，the current source is equivalent to open-circuit. It is pointless to open the current source, which is not equal to zero.

##### 电路分析实验内容

In experimental course, we verified the superposition theorem and Norton theorem and observed the response waveform of RC first-order circuit.

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