EE105 Fall 2006 Microelectronic Devices and Circuits Prof Jan M Rabaey jan eecs Lecture 20 Cascode Administrative Midterm next Th Covering everything up to but NOT INCLUDING multi stage amplifiers Review session next week on Tu evening 2 1 Method of Open Circuit Time Constants cntd General two pole transfer function 1 j A j A0 1 j A j A0 a1 z1 1 j p1 p2 N j 1 a1 j a2 j 2 1 1 p1 p 2 a2 1 p1 p 2 Sum of time constants 3 Higher Order Time Constants Coefficient a1 a1 R011C1 R022C2 Coefficient a2 a2 R211C1R022C2 R011C1R122C2 This is exact If p1 p2 a1 1 p1 a2 a1 p 2 4 2 Example Calculation Cgd Rs vout vs Cgs vgs gmvgs ro roc RL 5 The Cascode Configuration Common source common gate cascade is one version of a cascode all have shared supplies DC bias Two port model first stage has no current supply of its own 6 3 Cascode Two Port Model CS1 CG2 Output resistance of first stage Rout CS ro1 Rout roc 2 1 g m ro1 ro 2 Gm g m1 R in Why is the cascode such an important configuration 7 Miller Capacitance of Input Stage Find the Miller capacitance for Cgd1 Input resistance to common gate second stage is low gain across Cgd1 is small 8 4 Two Port Model with Capacitors CM 1 AvCgd 1 Cgd 1 Miller capacitance AvCgd 1 g m1 g 1 ro1 m1 1 gm2 gm2 CM 2C gd 1 9 Other contributions 10 5 Improved Current Sources Goal increase roc Approach look at amplifier output resistance results to see topologies that boost resistance 11 Cascode or Stacked Current Source Insight VGS2 constant AND VDS2 constant Small Signal Resistance roc 12 6 Drawback of Cascode I Source Minimum output voltage for all transistors saturated VOUT MIN VDS 4 SAT VS 4 VDS 4 SAT VGS 2 iOUT vOUT 13 14 7 Complete Amplifier Schematic 15 Complete Amplifier Schematic Goals gm1 1 mS Rout 10 M 16 8 Device Sizes M1 select W L 1 200 2 to meet specified gm1 1 mS find VBIAS 1 2 V Cascode current supply devices select VSG 1 5 V W L 4 W L 4B W L 3 W L 3B 64 2 M2 select W L 2 50 2 to meet specified Rout 10 M find VGS2 1 4 V Match M2 with diode connected device M2B Assuming perfect matching and zero input voltage what is VOUT 17 Output Voltage Swing Maximum VOUT Minimum VOUT 18 9 Two Port Model Find output resistance Rout n 1 20 V 1 p 1 50 V 1 at L 2 m ron 100 A 20 V 1 1 200 k rop 500 k gm2 2ID 2 2 100 A 500 S VGS 2 VTn 1 4V 1V gm3 2 ID 3 2 100 A 400 S VSG 3 VTp 1 5V 1V Rout roc ro 2 1 g m 2RS 2 ro 3 1 g m 3RS 3 ro 2 1 g m 2ro1 19 Some Interesting Questions 20 10 21 22 11 23 24 12 25 26 13 27 28 14
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