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8.2. Small-Signal Operation of the MOS Differential Pair Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) 8.2.3. The Differential Amplifier with CurrentSource Loads To obtain higher gain, the passive resistances (RD) can be replaced with current sources. Ad = gm1(ro1||ro3) Figure 8.11: (a) Differential amplifier with current-source loads formed by Q3 and Q4. (b) Differential half-circuit of the amplifier in (a). Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) 8.2.4. Cascode Differential Amplifier Gain can be increased via cascode configuration – discussed in Section 7.3. Ad = gm1(Ron||Rop) Ron = (gm3ro3)ro1 Rop = (gm5ro5)ro7 Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) Figure 8.12: (a) Cascode differential amplifier; and (b) its differential half circuit. Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) 8.3.1. Basic Operation To see how the BJT differential pair works, consider the first case of the two bases joined together and connected to a common-mode voltage VCM. Illustrated in Figure 8.16. Since Q1 and Q2 are matched, and assuming an ideal bias current I with infinite output resistance, this current will flow equally through both Oxford University Publishing transistors. Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) Figure 8.16: Different modes of operation of the BJT differential pair: (a) the differential pair with a common-mode input voltage VCM; (b) the differential pair with a “large” differential input signal; (c) the differential pair with a large differential input signal of polarity opposite to that in (b); (d) the differential pair with a small differential input signal vi. Note that we have assumed the bias current source I to be ideal. 8.3.1. Basic Operation Figure 8.16: Different modes of operation of the BJT differential pair: (a) the differential pair with a common-mode To see howinput the BJT differential voltage pair consider VCM ; (b)works, the differential pairthe first with “large” casea of the differential two bases joined input signal; the together and(c)connected to a differential pair with a large common-mode voltage VCM. differential input signal of Illustrated inthat Figure polarity opposite to in 8.16. (d) the pair (b); Since Q1differential and Q2 are matched, with a small differential input andvassuming signal .i Note that an we ideal have bias currentthe I with assumed bias infinite current output resistance, this current will source I to be ideal. flow equally through both Oxford University Publishing transistors. Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)