This post is continuation of a previous post Gm - boosted Cross Coupled Pair (XCP) and Negative Impedance
In the previous part we saw how to build Gm-boost circuit and negative resistance by cross-coupled pair. From that post consider figure 2, let us assume the gain A = 1 and output resistances of all transistors are infinite. The differential impedances are given by Zeq as shown in the below figure 1.
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In the previous part we saw how to build Gm-boost circuit and negative resistance by cross-coupled pair. From that post consider figure 2, let us assume the gain A = 1 and output resistances of all transistors are infinite. The differential impedances are given by Zeq as shown in the below figure 1.
Lets play with above circuits. What if we connect these two circuits in parallel? Lets do that and see.
$$Z_{eq} = \frac{2}{g_{m2} - g_{m1}} \to \infty$$
When the transistors are matched the differential impedance tends to infinite value. We can use this as active load to a pmos differential pair as shown in the figure 3. By this way we can get really very very high gain.
The high gain is because of the incremental currents in the load are cancelling each other. But the swing limit of the output will be degraded compared to the normal diode connected nmos load.
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