Differential pairs are often used as “current switches.” As shown in Fig. 19.69, the circuit routes its tail current to either of the outputs according to the large swings controlling the gates of M1 and M2. Explain what happens at node X during switching. If the tail currents of a large number of

Question

Differential pairs are often used as “current switches.” As shown in Fig. 19.69, the circuit routes its tail current to either of the outputs according to the large swings controlling the gates of [latex] M_1 ext { and } M_2 [/latex]. Explain what happens at node X during switching. If the tail currents of a large number of differential pairs feed from node X, should this voltage be provided externally?

Accepted Answer

Recall from Chapter 4 that for the differential pair to experience complete switching, the differential swing [latex]\left|V_2-V_1 ight|[/latex] must exceed [latex]\sqrt{2}\left(V_{G S}-V_{T H} ight)_{e q}[/latex], where [latex]\left(V_{G S}-V_{T H} ight)_{e q}[/latex] is the overdrive of [latex]M_1 and M_2[/latex] in equilibrium, i.e., if [latex]I_{D 1}=I_{D 2}[/latex]. We denote the voltage at node P when the pair is completely switched by [latex]V_{P 1}[/latex], and in equilibrium by [latex]V_{P 2}[/latex]. Thus,

[latex] V_{P 1}=V_2-\sqrt{2}\left(V_{G S}-V_{T H} ight)_{e q} [/latex] (19.25)

In equilibrium,

[latex] V_{P 2}=\frac{V_1+V_2}{2}-\left(V_{G S}-V_{T H} ight)_{e q} [/latex] (19.26)

Assuming that [latex] V_2-V_1=\sqrt{2}\left(V_{G S}-V_{T H} ight)_{e q}, ext { and hence } V_1=V_2-\sqrt{2}\left(V_{G S}-V_{T H} ight)_{e q} [/latex], we have

[latex] V_{P 2}=V_2-\left(1+\frac{\sqrt{2}}{2} ight)\left(V_{G S}-V_{T H} ight)_{e q} [/latex] (19.27)

Thus, [latex]V_{P 2}[/latex] is lower than [latex]V_{P 1}[/latex] by [latex](1-\sqrt{2} / 2)\left(V_{G S}-V_{T H} ight)_{e q}[/latex], indicating that during switching, [latex]V_P[/latex] drops by this amount. This voltage change is coupled to node X through the gate-drain overlap capacitance of [latex]M_3[/latex], disturbing [latex]I_{D 3}[/latex] and hence [latex]I_{ ext {out } 1} or I_{ ext {out } 2}[/latex].

Question 19.8: Differential pairs are often used as “current switches.” As ...

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