晶体管放大作用必须满足的条件: 内部条件: 晶体管结构上的特点 外部条件: 晶体管的发射结加正向电压,集电结加反向电压。P区接正,N区接负; N区接正,P区接负。 (一)晶体管内部载流子的运动 1.发射区向基区注入电子的过程 载流子运动主要表现为发射区向基区注入电子,形成电流iE≈iEn. 2.电子在基区的扩散过程 注入的电子在扩散的过程中,极少数与基区的空穴复合形成iB,绝大部分扩散到集电结边界。 3.电子被集电级收集的过程 电子受到集电结上的电场吸引而迅速漂移过集电结,形成iCn. 集电结反向偏置必然要使集电区与基区的少子漂移,形成ICBO。
(二)晶体管的电流分配关系 |
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晶体管各极的电流构成: |
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发射极电流 iE 大部分流入集电极形成 iCn。 |
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令 |
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令 |
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则 |
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(1)当 iB=0 时, iC=ICEO 。 称ICEO为穿透电流
(2)当 ICEO≈0, 则iC≈iB 即
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代表iB对iC的控制作用, |
越大,控制作用越强。 |
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(三)晶体管的放大作用 |
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本质: 电流控制作用,即iB对iC或iE对iC的控制作用。 |
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在集电极回路中串接一个负载电阻,就可以在负载电阻两端得到相应的幅度较大的变化电压。 |
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集电极回路中接入的负载电阻上,仍可得到较大的输出信号电压。 |
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(四)关于PNP型晶体管 |
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1.电源极性不同:NPN型电流从集电极流向发射极 2.电流方向不同:PNP型电流从发射极流向集电极
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