# 13.pdf - Integrated Circuit Biasing and Active Loads Part 5...

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Integrated Circuit Biasing and Active Loads Part 5 Chapter 10 Reference: Neamen 4th ed.

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4.2) Multi-MOSFET Current Source Circuits Among multi-MOSFET current-source circuits are: 1. Cascode current mirror 2. Wilson current mirror 3. Wide-Swing current mirror JEJ Oct 2013 2
o Increased output resistance, R O o I REF is established using another transistor o When all transistors are matched: I O = I REF 4.2.1) Cascode Current Mirror Figure 10.18: MOSFET cascode current mirror JEJ Oct 2013 3

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o To determine output resistance at the drain of M 4 , use the small-signal equivalent circuit. o I REF constant gate voltages to M 1 and M 3 , and hence M 2 and M 4 , are constant Equivalent to an ac short circuit . o • Therefore, the ac equivalent circuit for calculating the output resistance is shown in Figure 10.19(a). The small signal equivalent circuit is given in Fig 10.19(b). The small-signal resistance looking into the drain of M 2 is r O2 . 4.2.1) Cascode Current Mirror JEJ Oct 2013 4
4.2.1) Cascode Current Mirror Figure 10.19: Equivalent circuits of the MOSFET cascode current mirror for determining R O JEJ Oct 2013 5

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From Fig 10.19(b), summing currents at output node yields: 𝐼 𝑥 = 𝑔 𝑚 𝑉 𝑔?4 + ? 𝑥 −(−? 𝑔𝑠4 ) ? 𝑂4 Also 𝑉 𝑔?4 = −𝐼 𝑥 ? ?2 Thus 𝐼 𝑥 + ? 𝑂2 ? 𝑂4 𝐼 𝑥 + 𝑔 𝑚 ? ?2 𝐼 𝑥 = ? 𝑥 ? 𝑂4 Finally 𝑅 ? = ? 𝑥 𝐼 𝑥 = ? ?4 + ? ?2 (1 + 𝑔 𝑚 ? ?4 ) 4.2.1) Cascode Current Mirror JEJ Oct 2013 6
Normally 𝑔 𝑚 ? ?4 ≫ 1 Thus 𝑅 ?

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