EE105 Fall 2007Lecture 10, Slide 1Prof. Liu, UC Berkeley Lecture 10 OUTLINE BJT Amplifiers (cont’d) – CB stage with biasing – Emitter follower (Common-collector amplifier) – Analysis of emitter follower core – Impact of source resistance – Impact of Early effect – Emitter follower with biasing Reading: Chapter ANNOUNCEMENTS Alan Wu will hold an extra lab session tomorrow (9/28), 2-4PM The post-lab assignment for Experiment #4 has been shortened! 2 pgs of notes (double-sided, 8.5”×11”) allowed for Midterm #1

EE105 Fall 2007Lecture 10, Slide 2Prof. Liu, UC Berkeley Biasing of CB Stage R E is necessary to provide a path for the bias current I E to flow, but it lowers the input impedance.

EE105 Fall 2007Lecture 10, Slide 3Prof. Liu, UC Berkeley Reduction of Input Impedance Due to R E The reduction of input impedance due to i 1 is undesirable because it shunts part of the input current to ground instead of to Q 1 (and R C ).  Choose R E >> 1/g m, i.e. I C R E >> V T

EE105 Fall 2007Lecture 10, Slide 4Prof. Liu, UC Berkeley Creation of V b A resistive voltage divider lowers the gain. To remedy this problem, a capacitor is inserted between the base and ground to short out the resistive voltage divider at the frequency of interest.

EE105 Fall 2007Lecture 10, Slide 5Prof. Liu, UC Berkeley Example of CB Stage with Bias Design a CB stage for A v = 10 and R in = 50 . R in = 50  ≈ 1/g m if R E >> 1/g m  Choose R E = 500  A v = g m R C = 10  R C = 500  I C = g m ·V T = 0.52mA V BE =V T ln(I C /I S )=0.899V V b = I E R E + V BE = 1.16V Choose R 1 and R 2 to provide V b and I 1 >> I B, e.g. I 1 = 52  A C B is chosen so that (1/(  +1))(1/  C B ) is small compared to 1/g m at the frequency of interest. V CC = 2.5V I S = 5x A  = 100 V A = ∞

EE105 Fall 2007Lecture 10, Slide 6Prof. Liu, UC Berkeley Emitter Follower (Common Collector Amplifier)

EE105 Fall 2007Lecture 10, Slide 7Prof. Liu, UC Berkeley Emitter Follower Core When the input voltage (V in ) is increased by  V in, the collector current (and hence the emitter current) increases, so that the output voltage (V out ) is increased. Note that V in and V out differ by V BE.

EE105 Fall 2007Lecture 10, Slide 8Prof. Liu, UC Berkeley Unity-Gain Emitter Follower In integrated circuits, the follower is typically realized as shown below. – The voltage gain is 1 because a constant collector current (= I 1 ) results in a constant V BE ; hence  V out =  V in.

EE105 Fall 2007Lecture 10, Slide 9Prof. Liu, UC Berkeley Small-Signal Model of Emitter Follower The voltage gain is less than 1 and positive.

EE105 Fall 2007Lecture 10, Slide 10Prof. Liu, UC Berkeley Emitter Follower as a Voltage Divider

EE105 Fall 2007Lecture 10, Slide 11Prof. Liu, UC Berkeley Emitter Follower with Source Resistance

EE105 Fall 2007Lecture 10, Slide 12Prof. Liu, UC Berkeley Input Impedance of Emitter Follower The input impedance of an emitter follower is the same as that of a CE stage with emitter degeneration (whose input impedance does not depend on the resistance between the collector and V CC ).

EE105 Fall 2007Lecture 10, Slide 13Prof. Liu, UC Berkeley Effect of BJT Current Gain There is a current gain of (  +1) from base to emitter. Effectively, the load resistance seen from the base is multiplied by (  +1).

EE105 Fall 2007Lecture 10, Slide 14Prof. Liu, UC Berkeley Emitter Follower as a Buffer The emitter follower is suited for use as a buffer between a CE stage and a small load resistance, to alleviate the problem of gain degradation.

EE105 Fall 2007Lecture 10, Slide 15Prof. Liu, UC Berkeley Output Impedance of Emitter Follower An emitter follower effectively lowers the source impedance by a factor of  +1, for improved driving capability. The follower is a good “voltage buffer” because it has high input impedance and low output impedance.

EE105 Fall 2007Lecture 10, Slide 16Prof. Liu, UC Berkeley Emitter Follower with Early Effect Since r O is in parallel with R E, its effect can be easily incorporated into the equations for the voltage gain and the input and output impedances.

EE105 Fall 2007Lecture 10, Slide 17Prof. Liu, UC Berkeley Emitter Follower with Biasing A biasing technique similar to that used for the CE stage can be used for the emitter follower. Note that V B can be biased to be close to V CC because the collector is biased at V CC.

EE105 Fall 2007Lecture 10, Slide 18Prof. Liu, UC Berkeley Supply-Independent Biasing By putting an independent current source at the emitter, the bias point (I C, V BE ) is fixed, regardless of the supply voltage value.

EE105 Fall 2007Lecture 10, Slide 19Prof. Liu, UC Berkeley Summary of Amplifier Topologies The three amplifier topologies studied thus far have different properties and are used on different occasions. CE and CB stages have voltage gain with magnitude greater than one; the emitter follower’s voltage gain is at most one.

EE105 Fall 2007Lecture 10, Slide 20Prof. Liu, UC Berkeley Amplifier Example #1 The keys to solving this problem are recognizing the AC ground between R 1 and R 2, and using a Thevenin transformation of the input network. CE stageSmall-signal equivalent circuit Simplified small-signal equivalent circuit

EE105 Fall 2007Lecture 10, Slide 21Prof. Liu, UC Berkeley Amplifier Example #2 AC grounding/shorting and Thevenin transformation are needed to transform this complex circuit into a simple CE stage with emitter degeneration.

EE105 Fall 2007Lecture 10, Slide 22Prof. Liu, UC Berkeley Amplifier Example #3 First, identify R eq, which is the impedance seen at the emitter of Q 2 in parallel with the infinite output impedance of an ideal current source. Second, use the equations for a degenerated CE stage with R E replaced by R eq.

EE105 Fall 2007Lecture 10, Slide 23Prof. Liu, UC Berkeley Amplifier Example #4 Note that C B shorts out R 2 and provides a ground for R 1, at the frequency of interest.  R 1 appears in parallel with R C ; the circuit simplifies to a simple CB stage with source resistance.

EE105 Fall 2007Lecture 10, Slide 24Prof. Liu, UC Berkeley Note that the equivalent base resistance of Q 1 is the parallel connection of R E and the impedance seen at the emitter of Q 2. Amplifier Example #5