In {numref}Afig1 we also add a cascode transistor (in red) between the input transistor and the output node. This corresponds as a consequence to a classical cascode stage. However, there is one major difference: the cascode transistor is of the opposite polarity when compared to the input transistors. In {numref}Afig1 the input transistor is a pMOS tranistor and the cascode transistor is an nMOS transitor. Obviously, this also alters the current flow, and in order to maintain the current flow between power ($V_{dd}$) and ground, an addition current source ($I_b$) needs to be added.
For the practical implementation of this folded cascode, we replace the current source ($I_b$) with the transistor $T_3$, as can be seen in {numref}Afig2.
The amplication of this simple folded cascode amplifier stage is defined by:
$$ A=\frac{g_{m1}}{g_L+g_{casc}+j \omega C_L} $$
The important next step is obviously the determination fo $g_{casc}$. We use {numref}Afig3 for elaborating this output impedance.