Lecture 11 ANNOUNCEMENTS Prob 5 of Pre Pre Lab Lab 5 has been clarified Download new version HW 6 has been posted Review session 3 5PM Friday 10 5 in 306 Soda HP Auditorium Midterm 1 Thursday 10 11 Material of Lectures 1 10 HW 2 6 Chapters 2 4 5 2 pgs of notes double double sided sided 8 5 8 5 11 11 calculator allowed OUTLINE Review off BJT Amplifiers lf Cascode Stage Reading Chapter 9 1 EE105 Fall 2007 Lecture 11 Slide 1 Prof Liu UC Berkeley Review BJT Amplifier Design A BJT amplifier circuit should be designed to 1 ensure that the BJT operates p in the active mode 2 allow the desired level of DC current to flow and 3 couple to a small signal input source and to an output load Proper DC biasing is required DC analysis using large signal BJT model Key amplifier parameters AC analysis using small signal BJT model Voltage gain Av vout vin Input resistance Rin resistance seen between the input node and ground with output terminal floating Output O t t resistance it Rout resistance it seen between b t th output the t t node and ground with input terminal grounded EE105 Fall 2007 Lecture 11 Slide 2 Prof Liu UC Berkeley Large Signal vs Small Signal Models The large signal model is used to determine the DC operating p g point p VBE VCE IB IC of the BJT The small signal model is used to determine how the output responds to an input signal EE105 Fall 2007 Lecture 11 Slide 3 Prof Liu UC Berkeley Small Signal Models for Independent Sources The voltage across an independent voltage source does not varyy with time Its small signal g voltage g is always y zero It is regarded as a short circuit for small signal analysis Large Signal Model Small Signal Model The current through an independent current source does not vary with time Its small signal current is always zero It is i regarded d d as an open circuit i it for f small signal ll i l analysis l i EE105 Fall 2007 Lecture 11 Slide 4 Prof Liu UC Berkeley Comparison of Amplifier Topologies Common Emitter Large L Av 0 Degraded by RE Degraded by RB 1 Moderate Rin Increased by RB Increased byy RE 1 Common Base Large L Av 0 Degraded by RE and RS Degraded by RB 1 Small Rin Increased by RB 1 Decreased byy RE Rout RC Rout RC ro degrades Av Rout o t ro degrades Av Rout o t but impedance seen looking into the collector can be boosted byy emitter degeneration EE105 Fall 2007 but impedance seen looking into the collector can be boosted byy emitter degeneration Lecture 11 Slide 5 Emitter Follower 0 Av 1 Degraded by RB 1 Large Rin i due to RE 1 Small Rout Effect of source impedance is reduced by 1 D db by RE Decreased ro decreases Av Rin and Rout Prof Liu UC Berkeley Common Emitter Stage VA VA RC vout 1 RB vin RE gm 1 Rin R B r 1 R E Rout RC EE105 Fall 2007 Rout RC rO 1 gm RE r Lecture 11 Slide 6 Prof Liu UC Berkeley Common Base Stage VA vout RC RE 1 RB RS RE vin RS RE gm 1 1 RB Rin RE gm 1 Rout RC EE105 Fall 2007 VA Rout RC rO 1 gm RE r Lecture 11 Slide 7 Prof Liu UC Berkeley Emitter Follower VA VA vout RE vin R 1 RS E gm 1 Rin r 1 RE 1 Rs Rout RE gm 1 EE105 Fall 2007 vout RE rO vin R r 1 RS E O gm 1 Rin r 1 RE rO Rout Lecture 11 Slide 8 Rs 1 RE rO 1 gm Prof Liu UC Berkeley Ideal Current Source Circuit Symbol I V Characteristic Equivalent Circuit An ideal current source has infinite output impedance impedance How can we increase the output impedance of a BJT that is used d as a current source EE105 Fall 2007 Lecture 11 Slide 9 Prof Liu UC Berkeley Boosting the Output Impedance Recall that emitter degeneration boosts the impedance seen lookingg into the collector This improves the gain of the CE or CB amplifier However headroom is reduced Rout 1 g m RE r rO RE r EE105 Fall 2007 Lecture 11 Slide 10 Prof Liu UC Berkeley Cascode Stage In order to relax the trade off between output impedance and voltage headroom headroom we can use a transistor instead of a degeneration resistor Rout 1 g m rO 2 r 1 rO1 rO 2 r 1 Rout g m1rO1 rO 2 r 1 I C 2 I E1 I C 2 if 1 1 VCE for Q2 can be as low as 0 4V 0 4V soft saturation EE105 Fall 2007 Lecture 11 Slide 11 Prof Liu UC Berkeley Maximum Cascode Output Impedance The maximum output impedance of a cascode is limited by r 1 If rO 2 r 1 Rout max g m1rO1r 1 1rO1 EE105 Fall 2007 Lecture 11 Slide 12 Prof Liu UC Berkeley PNP Cascode Stage Rout 1 g m1 rO 2 r 1 rO1 rO 2 r 1 Rout g m1rO1 rO 2 r 1 EE105 Fall 2007 Lecture 11 Slide 13 Prof Liu UC Berkeley False Cascodes When the emitter of Q1 is connected to the emitter of Q2 it it ss not a cascode since Q2 is a diode diode connected connected device instead of a current source R out R out EE105 Fall 2007 1 1 1 g m 1 rO 2 r 1 rO 1 rO 2 r 1 g m2 g m2 g m1 1 1 rO 1 2 rO 1 g m2 g m2 Lecture 11 Slide 14 Prof Liu UC Berkeley Short Circuit Transconductance The short circuit transconductance of a circuit is a measure of its strength in converting an input voltage signal into an output current signal iout Gm vin EE105 Fall 2007 Lecture 11 Slide 15 vout 0 Prof Liu UC Berkeley Voltage Gain of a Linear Circuit By representing a linear circuit with its Norton equivalent the relationship between Vout and Vin can equivalent be expressed by the product of Gm and Rout Norton Equivalent Circuit Computation of short circuit output current EE105 Fall 2007 v out iout Rout G m vin Rout v out vin G m Rout Lecture 11 Slide 16 Prof Liu UC Berkeley Example Determination of Voltage Gain Determination of Gm iout Gm vin g m1 vout 0 Determination of Rout Rout vx ro1 ix Av g m1rO1 EE105 Fall 2007 Lecture 11 Slide 17 Prof Liu UC Berkeley Comparison of CE and Cascode Stages Since the output impedance of the cascode is higher than that of a CE stage stage its voltage gain is also higher higher vout g m1vin rO1 VA Av g m1rO1 VT EE105 Fall 2007 Av g m1rO 2 g m 2 rO1 r 2 Lecture 11 Slide 18 Prof Liu UC Berkeley Voltage Gain of Cascode Amplifier Since rO is much larger than 1 gm most …
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