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E3 238 Analog VLSI Circuits Lecture 12: Emitter/Source Coupled Pairs Gaurab Banerjee Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore [email protected] Emitter Coupled Pair From KVL around the input loop: For each transistor, assume: Identical transistors (1) Emitter Coupled Pair Also, let αF1= αF2 = αF. Then, from KCL at the emitters: (2) From (1) and (2): => output: Emitter Coupled Pair: Linear Range • Linear input range limited to about +/- 3VT, about 75 mV • In practice, circuit behaves linearly when Vin < VT • Note: For Vid = 0, Vod = 0 -> important in DC coupling of stages Emitter Coupled Pair: With Degeneration Linear range increases as drop across the emitter resistor increases -> local negative feedback Source Coupled Pair Apply KVL around the input loop: Assume M1, M2 are in saturation From the MOS I/V equations: Source Coupled Pair Apply KCL at the common source node -> Id1 + Id2 = ITAIL After solving the Quadratic formula: Source Coupled Pair Range of input voltages over which M1, M2 operate in the active region Large overdrive = Large input range Source Coupled Pair • Vov affects many other parameters in addition to input range: speed, offset, output swing, etc. -> more later • For a fixed current, gm is maximized by minimizing overdrive (analysis before) -> tradeoff between power and linearity Differential Output Voltage: Differential and Common Modes Case 1: Case 2: • Consider the simplified differential pair • Neglect gmbs, gds for simplicity • Assume R1=R2=R, gm1=gm2=gm Differential and Common Modes Now, assume mismatches, i.e., R1 ≠R2 Ideally zero “mode conversion” products -> problematic in cascades of amplifiers