EE105 Fall 2005 Microelectronic Devices and Circuits Lecture 29 BJT Biasing Summary Announcements Lab 9 reports due now Check your homework lab grades next week Reading Chapter 9 6 10 7 2 Final December 20 12 30 3 30pm Bechtel Aud Sibley 2 1 Lecture Material Last lecture BJT amplifiers common emitter common collector common base This lecture BJT biasing Example amplifier Review 3 PNP Transistor Emitter P VEB VEC Base N IB Collector P IE IC 4 2 5 6 3 Discrete VHF Amplifier 7 Multi Stage Voltage Amplifier 8 4 Cutting Through the Complexity Two Approaches 1 Eliminate background transistors to reduce clutter 2 Identify the signal path between the input and output 9 First Approach Find I V Sources 10 5 What s Left Voltage at base of Q2 is set by totem pole 11 Second Approach Find Signal Path 12 6 Identifying the Stages First stage or two stages CS CB cascode Second stage or two stages CD CC voltage buffer Why does this make sense for a voltage amplifier 13 Find Key Two Port Parameters Output resistance of cascode Rout CS CB roc ro 2 1 g m 2 r 2 RS 2 roc Rup ro 6 1 g m 6RS 6 14 7 Two Port Parameters Cont 15 Output Resistance and Voltage Gain Source resistance of the CC stage is the output resistance of the CD stage small Rout Rout CC R 1 1 1 S CC gm4 o g m 4 g m 3 o g m 4 1 Open circuit voltage gain Av last two stages have nearly unity gain Av g m1 o ro 2 ro 6 1 g m 6ro 7 16 8 Output Swing VOUT MIN Minimum output voltage M10 M3 and Q2 are suspects M10 goes into triode when VOUT 0 5 V M3 goes into triode when VSD3 0 5 V VOUT 0 5 V 0 7 V 0 2 V Q2 goes into saturation when VCE2 0 1 V or VBC2 0 6 V VOUT VB2 VBC2 VSG3 VBE4 2 V 0 6 V 1 5 V 0 7 V VOUT 2 2 V 17 Output Swing VOUT MAX Maximum output voltage Q4 M5 and M6 are suspects Q4 goes into saturation when VCE4 0 1 V VOUT 4 9 V M5 goes triode when VSD5 0 5 V VOUT 3 8 V M6 goes triode when VSD6 0 5 V VOUT VS6 0 5 V VSG3 VBE4 3 5 0 5 1 5 0 7 V 3 8 V 18 9 Insight into the Frequency Response 19 Qualitative Insight Could always do brute force open circuit time constants CS CB is a wideband stage so is the CD CC buffer Look for large RTxCx products high impedance nodes are likely candidates 20 10 Node X High impedance node is node X look at RTxCx Capacitance Cx Cgd6 C 2 Cgd3 CM3 Miller for CD stage M3 21 Finding the Miller Capacitance CM3 Cgs3 X CD RL3 R L3 Gain across Cgs3 AvC gs 3 1 g m 3 R L 3 RL3 Rin4 22 11 Dominant Pole of Voltage Amplifier Th venin resistance for CX RTx Rout 2 R in 3 Rout CB R in CD RTx roc ro 2 1 g m 2 r 2 RS 2 ro 6 1 g m 6ro 7 ro 2 o Dominant pole 1 1 RTx C x 23 12
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