Lesson 4 Transistor Voltage Amplifier

Amplifiers are necessary in many types of electronic equipment such as radios, oscilloscopes and record players. Often it is a small alternating voltage that has to be amplified. A junction transistor in the common-emitter mode can act as a voltage amplifier if a suitable resistor, called the load, is connected in the collector circuit.

The small alternating voltage, the input ui, is applied to the base-emitter circuit and causes small changes of base current which produce large changes in the collector current flowing through the load.The load converts these current changes into voltage changes which form the alternating output voltage uo (uo being much greater than ui).

A transistor voltage amplifier circuit is shown in Fig.4-1. To see just the voltage amplification occurs, consider first the situation when there is no input, i.e. ui= 0, called the quiescent (quiet) state.

For transistor action to take place the base-emitter junction must always be forward biased. A simple way of ensuring this is to connect a resistor RB, called the base-bias resistor. A steady (DC) base current IB flows from battery “+”, through RB into the base and back to battery “-”, via the emitter. The value of RB can be calculated once the value of IB for the best amplifier performance has been decided.

If VCC is the battery voltage and VBE is the base-emitter junction voltage (always about +0.6V for an n-p-n silicon transistor), then for the base-emitter circuit, since DC voltages add up, we can write

VCC=IB×RB+VBE　（1）

IB causes a much larger collector current IC which produces a voltage drop IC×RL across the load RL. If VCE is the collector-emitter voltage, then for the collector- emitter circuit

VCC=IC×RL+VCE　（2）

When ui is applied and goes positive, it increases the base-emitter voltage slightly (e.g. from +0.60V to +0.61V). When ui swings negative, the base-emitter voltage decreases slightly (e.g. from +0.60V to+0.59V). As a result a small alternating current is superimposed on the quiescent base current IB which in effect becomes a varying DC.

When the base current increases, large proportionate increases occur in the collector current. From equation (2) it follows that there is a corresponding large decrease in the collector-emitter voltage (since VCC is fixed). A decrease of base current causes a large increase of collector-emitter voltage. In practice positive and negative swings of a few milli-volts in ui, can result in a fall or rise of several volts in the voltage across RL and so also in the collector-emitter voltage.

The collector-emitter voltage may be regarded as an alternating voltage superimposed on a steady direct voltage, i.e. on the quiescent value of VCE. Only the alternating part is wanted and capacitor C blocks the direct part but allows the alternating part, i. e. the output uo to pass.

New Words

oscilloscope [ə'siləskəup] n.　示波器

alternating ['ɔ:ltəneitiŋ] adj.　交互的

amplifier ['æmplifaiə] n.　放大器

junction ['dVʌŋkʃn] n.　连接，接合，交叉点，汇合处

resistor [ri'zistə] n.　电阻器

quiescent [kwi'esnt] adj.　静止的

biased ['baiəst] adj.　结果偏倚的，有偏的

silicon ['silikən] n.　硅，硅元素

superimpose [ˌsu:pərim'pəuz] v.　添加，双重

proportionate [prə'pɔ:ʃənət] adj.　成比例的

equation [i'kweiʃn] n.　方程式，等式

Phrases and Expressions

to convert…into　把……转变成

as a result　结果

Notes

1. Often it is a small alternating voltage that has to be amplified.

译文：通常小的交流电压需要放大。

这是一个常见句型，it是形式主语，that has to be amplified引导真正意义上的主语从句。

2. …which produce large changes in the collector current flowing through the load.

(1) which引出base current。(2) flowing through the load是现在分词短语，充当后置定语，修饰collector current。

3. The load converts these current changes into voltage changes which form the alternating output voltage uo (uo being much greater than ui).

译文：负载将交变电流转变成交变电压，即输出交流电压uo（uo比ui大）。

(1) to convert … into … 把……变成……，相当于to change … into… (2) which form the alternating output voltage uo是定语从句，修饰voltage changes。(3) alternating是现在分词，修饰output voltage。

4. A simple way of ensuring this is to connect a resistor RB, called the base-bias resistor.

译文：简单的方法是接一个电阻RB，称为基极偏置电阻。

不定式to connect a resistor RB充当表语，其中called the base-bias resistor是过去分词短语，充当定语。

5. As a result a small alternating current is superimposed on the quiescent base current IB which in effect becomes a varying DC.

译文：结果是一个小的交流电流叠加在静态基极电流IB上并产生一个变化的直流电压。

(1) as a result：“结果”。（2）which in effect becomes a varying DC是定语从句，修饰the quiescent base current IB。

6. … and so also in the collector-emitter voltage.

so这里指can also result in a fall or rise of several volts。

7. The collector-emitter voltage may be regarded as an alternating voltage superimposed on a steady direct voltage, i.e. on the quiescent value of VCE.

译文：集-射间电压可以认为是一个交流电压叠加在静态直流电压（即静态电压VCE）之上。

过去分词短语superimposed on a steady direct voltage, i.e. on the quiescent value of VCE充当定语，修饰an alternating voltage。

Exercises

1. Write T (True) or F (False) beside the following statements about the text.

a. Amplifiers are necessary in many types of electronic equipment such as radios, oscilloscopes and record players. Often it is a large alternating current that has to be amplified.

b. A junction transistor in the common-emitter mode can act as a resistance amplifier.

c. The small alternating voltage is applied to the base-emitter circuit and causes large changes of base current which produce small changes in the collector current flowing through the load.

d. The load converts current changes into voltage changes, which form the alternating output voltage.

e. For transistor action to take place the base-emitter junction must always be forward straight line.

f. When the base current increases, large proportionate increases occur in the collector current.

g. A decrease of base current causes a large increase of collector-emitter voltage.

h. The collector-emitter voltage may be regarded as an alternating voltage superimposed on a steady direct voltage.

2. Match the following terms to appropriate definition or expression.

3. Fill in the missing words according to the text.

a. A junction transistor in the common-emitter mode can act as a _____ if a suitable resistor called the _____, is connected in the collector circuit.

b. The load converts these _____ changes into _____ changes, which form the alternating output voltage uo.

c. A simple way of ensuring this is to connect a resistor RB, called the _____. A steady(DC) base current IB flows from battery “+”, through RB into the base and back to battery “-”, via the _____.

d. The collector-emitter voltage may be regarded as an _____ superimposed on a steady direct voltage, i.e. on the quiescent value of VCE. Only the alternating part is wanted and capacitor C blocks the direct part but allows the _____, i.e. the output uo to pass.

4. Translate the following paragraph into Chinese.

When the base current increases, large proportionate increases occur in the collector current.From equation (2) it follows that there is a corresponding large decrease in the collector-emitter voltage (since VCC is fixed). A decrease of base current causes a large increase of collector-emitter voltage. In practice positive and negative swings of a few milli-volts in ui, can result in a fall or rise of several volts in the voltage across RL and so also in the collector-emitter voltage.

Chinese Translation of Texts（参考译文）