E3 238 Analog VLSI Circuits Lecture 10: CB/CG Amplifiers, Emitter/Source Followers Gaurab Banerjee Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore [email protected] Common Base Amplifier Note: vbe = -vin Apply KCL at vout: gm >> go Non-inverting gain stage, same magnitude as CE stage Common Gate Amplifier Note: vgs = -vin KCL at vout: Non inverting gain, same magnitude as CS stage Note: Both CE/CS and CB/CG amplifiers have their gain equations in the form: • • (Effective trans-conductance)/(Effective output conductance) • Conductance adds in parallel networks -> easier to work with Small Signal Terminal Impedances Common Base Amplifier: Common impedance analysis technique -> apply test voltage, determine test current Small Signal Terminal Impedances From (2): From (1): very small Approx. = gm A few substitutions... Derive this! Small Signal Terminal Impedances Common Gate Amplifier: rin => obtained by applying a test voltage at the input KCL at input: Small Signal Terminal Impedances Common Collector Amplifier (Emitter Follower) Common Collector Amplifier (Emitter Follower) Gain < 1 Terminal Impedance Values Good voltage buffer -> Ri large, Ro small Input Resistance: Emitter resistance “multiplied up” and added to base -> High input resistance! Terminal Impedance Values Output Resistance: Note: We ignore RL here, assuming RL >> Ro Low impedance, dominates Resistance in base is divided by (β0+1) and reflected to the emitter => low output impedance Common Drain Amplifier (Source Follower) Body and Source not at the same potential Gain < 1 Order of magnitude => gm >> gmbs >> gds => av ≈ 1 Common Drain Amplifier (Source Follower) Small, due to high o/p impedance -> high gain Large, due to low o/p impedance -> low gain So, by inspection, the MOS CD amplifier has a low output impedance given by: Derive!