Folded cascode stage#
Fig. 9 Theoretical folded cascode configuration#
In Fig. 9 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 Fig. 9 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 Fig. 10.
The amplication of this simple folded cascode amplifier stage is defined by:
the \(g_m\) of the input transitor \(T_1\)
the conductance of the load resistor \(g_L\)
the output conductance of the folded cascode stage \(g_{casc}\)
the capacitive load \(C_L\)
Fig. 10 Practical folded cascode amplifier stage configuration#
The important next step is obviously the determination fo \(g_{casc}\). We use Fig. 11 for elaborating this output impedance.
Fig. 11 Circuit block under consideration for measuring the folded cascode output impedance#