Analysis of internal circuit of operational amplifier

1、 First, let's take a look at the internal diagram of an op amp , The following figure for 741 Simplified diagram of operational amplifier , Divided into input stages , Darlington amplifier , Output stage .

  Input stage ：

  Important relationships ：ic1=ic3=ic4, obtain ：io=ic4-ic2 namely io=ic1-ic2, And then there is ic1+ic2=IA,

According to the saturation voltage and thermal current of the triode VT The relationship between ：ic = Is * exp ( Veb / VT )

  therefore ic1 / ic2 = exp [ (Vn-Vp) / VT ] We can draw a conclusion that

                                 io=IA*Tanh[ (Vn-Vp) / 2VT ]

This is also from the above io The output shows that when Vn=Vp When , In theory io Should be 0 Of ,

2、 Here's a 741 Internal schematic diagram of operational amplifier , Input 500hz The sine wave of , The magnification is 10 times （1+9k/1k）, The output amplitude is 10v Of 500hz Sine wave .

3、 Note that the internal circuit of the operational amplifier has a frequency compensation capacitor , The picture above shows C1,30p, This capacitor is used to compensate the zero point of the operational amplifier , If this capacitor is not added, a zero point will be generated .

As shown in the figure below ：

Add this 30p After capacitance