a large value. The point is that because gmrO is large almost regardless of the technology, the tabulated gain and the output resistance for both the MOSFET and the bipolar cascode are very large. That fact has implications in the discussion that follows. Low-frequency design The g-parameters found in the above formulas can be used to construct a small-signal voltage amplifier with the same gain, input and output resistance as the original cascode (an equivalent circuit). This circuit applies only at frequencies low enough that the transistor parasitic capacitances do not matter. The figure shows the original cascode (Fig. 1) and the equivalent voltage amplifier or g-equivalent two-port (Fig. 4). The equivalent circuit allows easier calculations of the behavior of the circuit for different drivers and loads. In the figure a Thévenin equivalent voltage source with Thévenin resistance RS drives the amplifier, and at the output a simple load resistor RL is attached. Using the equivalent circuit, the input voltage to the amplifier is (see Voltage divider): , which shows the importance of using a driver with resistance RS << Rin to avoid attenuation of the signal entering the amplifier. From the above amplifier characteristics, we see that Rin is infinite for the MOSFET cascode, so no attenuation of input signal occurs in that case. The BJT cascode is more restrictive because Rin = rπ2. In a similar fashion, the output signal from the equivalent circuit is . In low-frequency circuits, a high voltage gain is typically desired, hence the importance of using a load with resistance RL >> Rout to avoid attenuation of the signal reaching the load. The formulas for Rout can be used either to design an amplifier with a sufficiently small output resistance compared to the load or, if that cannot be done, to decide upon a modified circuit, for example, to add a voltage follower that matches the load better. The earlier estimate showed that the cascode output resistance is very large. The implication is that many load resistances will not satisfy the condition RL >> Rout (an important exception is driving a MOSFET as the load, which has infinite low frequency input impedance). However, the failure to satisfy the condition RL >> Rout is not catastrophic because the cascode gain also is very large. If the designer is willing, the large gain can be sacrificed to allow a low load resistance; for RL <<