EECS 105 Fall 2003, Lecture 17 Lecture 17: Common Source/Gate/Drain Amplifiers Prof. Niknejad Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Lecture Outline Department of EECS MOS Common Source Amp Current Source Active Load Common Gate Amp Common Drain Amp University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Common-Source Amplifier Isolate DC level Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Load-Line Analysis to find Q I RD VDD Vout RD Q slope 5V ID 10k 1 10k 0V ID 10k Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Small-Signal Analysis Rin Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port Parameters: Generic Transconductance Amp Rs vs vin Rin Gmvin Rout RL Find Rin, Rout, Gm Rin Gm gm Rout ro || RD Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port CS Model Reattach source and load one-ports: Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Maximize Gain of CS Amp Av gm RD || ro Increase the gm (more current) Increase RD (free? Don’t need to dissipate extra power) Limit: Must keep the device in saturation VDS VDD I D RD VDS ,sat For a fixed current, the load resistor can only be chosen so large To have good swing we’d also like to avoid getting to close to saturation Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Current Source Supply Department of EECS Solution: Use a current source! Current independent of voltage for ideal source University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CS Amp with Current Source Supply Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Load Line for DC Biasing Both the I-source and the transistor are idealized for DC bias analysis Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port Parameters From current source supply Rin Gm gm Rout ro || roc Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad P-Channel CS Amplifier DC bias: VSG = VDD – VBIAS sets drain current –IDp = ISUP Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port Model Parameters Small-signal model for PMOS and for rest of circuit Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Common Gate Amplifier DC bias: I SUP I BIAS I DS Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG as a Current Amplifier: Find Ai iout id it Ai 1 Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Input Resistance vgs vt vt vout it gm vgs gmb vt r o Output voltage: v i (r || R ) i (r || R ) out d oc L t oc L At input: vt roc || RL it it g m vt g mb vt r o Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Approximations… We have this messy result g m g mb 1 ro it 1 r || RL Rin vt 1 oc ro But we don’t need that much precision. Let’s start approximating: g m g mb 1 ro roc || RL RL R in Department of EECS RL 0 ro 1 g m g mb University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Output Resistance vs vs vt g m vgs ( g mb vs ) 0 RS ro 1 1 vt vs gm gmb ro ro RS Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Output Resistance Substituting vs = itRS 1 1 vt it RS gm gmb ro ro RS The output resistance is (vt / it)|| roc Rout Department of EECS ro roc || RS g m ro g mb ro 1 RS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Approximating the CG Rout Rout roc || [ro g m ro RS g mb ro RS RS ] The exact result is complicated, so let’s try to make it simpler: g m 500S g mb 50S ro 200k Rout roc || [ro g m ro RS RS ] Assuming the source resistance is less than ro, Rout roc || [ro g m ro RS ] roc || [ro (1 g m RS )] Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Two-Port Model Function: a current buffer • Low Input Impedance • High Output Impedance Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Common-Drain Amplifier I DS VGS Department of EECS W 1 Cox (VGS VT ) 2 L 2 2 I DS VT W Cox L Weak IDS dependence University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Voltage Gain Note vgs = vt – vout vout g m vgs g mb vout roc || ro vout g m vt vout g mb vout roc || ro Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Voltage Gain (Cont.) vout g m vt vout g mb vout roc || ro KCL at source node: 1 gmb gm vout g mvt roc || ro Voltage gain (for vSB not zero): vout vin gm 1 g mb g m roc || ro vout gm 1 vin g mb g m Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Output Resistance Sum currents at output (source) node: Rout vt ro || roc || it Rout Department of EECS it gmvt g mb vt 1 g m g mb University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Output Resistance (Cont.) ro || roc is much larger than the inverses of the transconductances ignore Rout 1 g m g mb Function: a voltage buffer • High Input Impedance • Low Output Impedance Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Department of EECS Prof. A. Niknejad University of California, Berkeley