Lect. 24: CG and Cascode Amplifiers (Razavi 7.3 and 9.1) Common-Gate Amplifier Increase in Vin Decrease in VGS Decrease in ID Increase in Vout ID: - gmVin = g V R Vout: - IDRD m in D Gain: gmRD (Without rO consideration) With small-signal circuit model Rin: 1/gm Rout: RD Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers Common-Gate Amplifier with RS With small-signal model: (Ignore rO) Voltage Gain: gmRD / (1 + gmRS) VX = 1/gm / (1/gm + Rs)Vin = 1 / (1+ gmRs)Vin Vout= gmRDVX Input loading due to small Rin Rin: 1/gm Rout = RD Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers Common-Gate Amplifier with RS and rO It can be shown Rin = (1 + RD/rO )/ (gm + 1/rO) If rO > RD, Rin 1/ gm Rout = [(1+gmrO) + rO] Rs || RD gm rO Rs || RD Voltage Gain = (gm + 1/rO) Rout gm Rout What is this good for? Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers Current amplifier Consider an equivalent circuit for a current amplifier Ais: Short-circuit current gain Small Ri and large Ro desired Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers CG has small Rin and large Rout Ais 1 CG: Current buffer Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers Small signal model with current buffer for CG Rin : Rout :gm2 rO2 Rs||RD gm2 rO2 Rs gm2 rO2 rO1 Voltage Gain: - gm1 gm2 rO1 rO2 If Q and Q are identical under same bias CS + CG 1 2 2 R ~ Ar Cascode Amplifier Agvo~( mr o ) out v2 o Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers CS vs Cascode Amplifier Rout: ro (gmro)ro 2 Avo: - gmro -(gmro) Can be faster for the same voltage gain Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers CS CG Cgd between input and output No direct capacitor between input and output Miller effect! No Miller effect! Cascode: Smaller gain in CS stage is sufficient for achieving the target gain For the same overall gain, cascode is faster than CS! Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers CS vs Cascode Amplifier R : r (g r )r out o m o o Disadvantages: 2 Avo: - gmro -(gmro) - Smaller voltage headroom Can be faster for the same voltage gain Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers Folded Cascode Double Cascode With BJT, CE + CB Cascode Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers Homework: Determine W/L and Vb2 so that Rout=200k in the following cascode circuit. 2 Assume Vt= 0.4V, kn’(=nCox)=100A/V , rO=10/Ibias () for both transistors Use Ibias = 0.5mA, (W/L)1= (W/L)2 and the approximate expression for cascode Rout. Electronic Circuits 1 (13/2) Prof. Woo-Young Choi.