2. NPN- Cascode: Voltage gain
3. PNP- Cascode: Voltage gain
4. NPN- Emitter Follower:
5. PNP- Emitter Follower
This presentation avoids big text and uses pictures instead. Growing step by step the circuit diagramm will declare itself. To understand the complex function of an Operational Amplifier, you need some knowledge about the transistor and amplifier only. Please remember the basic partial circuits:
1. NPN- Amplifier
2. NPN- Cascode: Voltage gain
3. PNP- Cascode: Voltage gain
4. NPN- Emitter Follower:
5. PNP- Emitter Follower
NPN- Differential amplifier
Perhaps this is the most important block 
of an Operational Amplifier. Advantage over the Emitter- grounded
one-transistor amplifier are:
improved linearity
independence from temperature effects
reduction of power supply influences
symmetrical signal input has improved noise immunity
more Basics
7. NPN- Current Mirror: 
I out = I in - 2x Ib
8. Current Generator:
I out depending from the Ratio of the Resistors
9. Improved NPN Current Mirror:
The little base current of the added
transistor brings the only difference between I in and I out.
The following chapters are without english declarations having german text only, sorry.
But I think, the circuit diagrams are helpful without text too. By adding partial circuits
step by step you will understand the function easier than if you see the chaos of an total
circuit diagram as usual in the datasheets. On the other side the often shown simlified
diagrams are dangerous for users not using the standard function.
Attention: Please for the following do not scroll but click
on < or
> !
If you click back/next all images will be on the same place,
advantageous to compare the circuit diagrams.