Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 18 Lecture 18: Bipolar Single Stage Amplifiers Prof. Niknejad

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Lecture Outline BJT Amps BJT Biasing Common Emitter Amp Common Base Amp Common Collector Amp – AKA Emitter Follower β Multiplier Concept Emitter Degeneration

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Bipolar Amplifiers Common-emitter amplifier: Biasing: adjust V BIAS = V BE so that I C = I SUP.

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Small-Signal Two-Port Model Parameters: (I C = 1 mA, β =100, V A = 3V)

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Base Amplifier To find I BIAS, note that I BIAS = I E = - (1/  F )I C Common-base current gain A i = -  F

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley CB Input Resistance Summing currents at the input node: small

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley CB Output Resistance note polarity put r oc back in after finding v t / i t Same topology as CG amplifier, but with r  || R S rather than R S

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Output Impedance Details First draw small signal equivalent circuit with transistor and simplify as much as possible Then (if needed) add the small signal equivalent circuit If frequency is low, get rid of caps!

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Output Impedance Calculation

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Base Two-Port Model Why did we consider it a current amp? Current Amp : Unity Current Gain (-1) Small Input Impedance Large (huge!) Output Impedance

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Collector Amplifier DC Bias: output is one “V BE drop” down from input “Emitter Follower”

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Collector Input Resistance

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Collector Output Resistance Divider between v t and v  :

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Collector Output Res. (cont) Looking into base of emitter follower: load impedance larger by factor β+1 Looking into emitter of follower: “source” impedance smaller by factor β+1

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Collector Voltage Gain KCL at the output node: note v  = v t - v out

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Common-Collector Two-Port Model typo: R L Voltage Amp : Unity Voltage Gain (+1) Large Input Impedance Small Output Impedance

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Summary of Two-Port Parameters

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Typical “Discrete” Biasing A good biasing scheme must be relatively insensitive to transistor parameters (vary with process and temperature) In this scheme, the base current is given by: The emitter current:

EECS 105 Fall 2003, Lecture 18Prof. A. Niknejad Department of EECS University of California, Berkeley Gain for “Discrete” Design Let’s derive it by intuition Input impedance can be made large enough by design Device acts like follower, emitter=base This signal generates a collector current Can be made large to couple All of source to input (even with R S )