简 介: 对于网络上看到的一个最为简单的音频振荡电路进行测试,发现它的确具有工作可能性。并对于它的工作原理进行初步分析。

关键词振荡电路单结晶体管负阻抗

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单管振荡器
文章目录
电路来源
构建电路
电路原理图
搭建测试电路
测试结果
基本振荡波形
更换电路器件参数
问题分析

§01 单管振荡器


一、电路来源

  在 吊打三极管 展示了一个由单个NPN三极管、两个电阻,一个电容,一个LED和9V干电池组成的LED闪烁振荡电路。它是利用了三极管反向击穿时所呈现的“负阻抗”特性产生的间歇振荡器现象。

▲ 图1.1.1 电路工作示意动图

  今天(2021-09-26)看到 Instructables 网站上给出了另外一个简单的振荡电路( Simplest Oscillator (Transmitter) )。这个电路与上面的电路相同之处,都是不按常理出牌。下面这个电路从常理上来看,它不会产生振荡的。

▲ 图1.1.2 简单音频振荡电路

  那么这个电路是否真的能够振荡? 它的工作原理又是什么呢?

二、构建电路

1、电路原理图

  原网站给出的电路图没有给出电路中的元器件。下面电路图中给出了实验电路以及各元器件的参数。

▲ 图1.2.0 电路图原理图以及原器件参数

2、搭建测试电路

  在面包板上搭建了上述简单的实验电路。那么它是否真的可以进行工作呢?

▲ 图1.2.1 :面包板上搭建的实验电路

三、测试结果

1、基本振荡波形

  打开+5V电源,动圈是喇叭中便发出蜂鸣声音。电路震荡的频率并不是非常稳定,当手触碰喇叭,或者面包板上的元器件的时候,振荡频率都会发生变化。

▲ 图1.2.2 电路中三极管的发射极(蓝色)与基极(青色)电压波形

  可以看到实际施加在扬声器上的波形是间歇高频振荡波形。这个高频信号频率与包络线也会随着喇叭的不同位置以及测试示波器探头是否接入有关系。

▲ 图1.2.3 单个脉冲内的高频振荡波形

  下面是示波器探头不通过引线直接抵触在三极管E级测试的的波形。

▲ 图1.2.4 DJ个脉冲内的高频信号

  使用示波器测量脉冲中高频振荡频率,居然达到了惊人的315MHz ! 这的确出乎人的意料。

▲ 图1.2.5 高频波形

2、更换电路器件参数

(1) 三极管

  将三极管9018更换成 8050,它的截止频率为100MHz,低于上面测试脉冲高频信号的频率。更换之后,电路便没有脉冲输出了。

(2) 电容C1

  更换电容C1,可以改变脉冲低频成分的频率。增加C1,低频频率降低;减少C1,增加低频频率。

(3) 扬声器

  测试了两款扬声器。左边的扬声器的标称阻抗为4欧姆,右边的扬声器的标称阻抗也是4Ω。但是在接入电路之后,左边的扬声器没有振荡,右边的扬声器会产生震荡。

▲ 131.不同的扬声器

  使用SmartTweezer测量两个扬声器的交流阻抗(10kHz下的电阻与电感)。可以看到两个扬声器的主要差别在于电感量不同。

左边扬声器:
电阻:8.6Ω
电感:230uH
右边扬声器:
电阻:7.5Ω
电感:64uH

§02 问题分析


  上面的初步实验验证了上述振荡电路的能够工作,但问题来了:它为什么能够工作?原理是什么?

  初步分析,这个电路应该是由扬声器及其引线中的电感成分,与三极管的Cbe,Cbc杂散电容形成了单管电容振荡电路。形成高频振荡之后,三极管的基极输入电阻也呈现出“负阻”特性,再由电路中的 R1、C1与三极管基极的负阻抗特性组成了间歇振荡电路。

  这种利用负阻抗特性形成的间歇振荡器,最初在单结晶体管振荡电路中应用最为广泛。下面是基本的单结晶体管振荡电路图。

▲ 图2.1 单结晶体管振荡电路

  上述电路的工作的基本条件就是需要能够形成高频振荡,之后才能够形成音频振荡。而高频振荡中需要 应用到三极管以及扬声器的杂散参数,因此不同的三极管与扬声器对于高频振荡形成有影响。有的参数可以工作,有的不工作。


■ 相关文献链接:

  • 吊打三极管
  • Instructables
  • Simplest Oscillator (Transmitter)

● 相关图表链接:

  • 图1.1.1 电路工作示意动图
  • 图1.1.2 简单音频振荡电路
  • 图1.2.0 电路图原理图以及原器件参数
  • 图1.2.1 :面包板上搭建的实验电路
  • 图1.2.2 电路中三极管的发射极(蓝色)与基极(青色)电压波形
  • 图1.2.3 单个脉冲内的高频振荡波形
  • 图1.2.4 DJ个脉冲内的高频信号
  • 图1.2.5 高频波形
  • 131.不同的扬声器
  • 图2.1 单结晶体管振荡电路

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