Lecture 9 OUTLINE BJT Amplifiers contd Commonbase topology

Lecture 9 OUTLINE • BJT Amplifiers (cont’d) – Common-base topology – CB core – CB stage with source resistance – Impact of base resistance Reading: Chapter 5. 3. 2 EE 105 Fall 2007 Lecture 9, Slide 1 Prof. Liu, UC Berkeley

Common Base (CB) Amplifier • The base terminal is biased at a fixed voltage; the input signal is applied to the emitter, and the output signal sensed at the collector. EE 105 Fall 2007 Lecture 9, Slide 2 Prof. Liu, UC Berkeley

Small-Signal Analysis of CB Core • The voltage gain of a CB stage is gm. RC, which is identical to that of a CE stage in magnitude and opposite in phase. EE 105 Fall 2007 Lecture 9, Slide 3 Prof. Liu, UC Berkeley

Tradeoff between Gain and Headroom • To ensure that the BJT operates in active mode, the voltage drop across RC cannot exceed VCC-VBE. EE 105 Fall 2007 Lecture 9, Slide 4 Prof. Liu, UC Berkeley

Simple CB Stage Example VCC = 1. 8 V IC = 0. 2 m. A IS = 5 x 10 -17 A b = 100 EE 105 Fall 2007 Lecture 9, Slide 5 Prof. Liu, UC Berkeley

Input Impedance of a CB Stage • The input impedance of a CB stage is much smaller than that of a CE stage. EE 105 Fall 2007 Lecture 9, Slide 6 Prof. Liu, UC Berkeley

CB Stage with Source Resistance • With the inclusion of a source resistance, the input signal is attenuated before it reaches the emitter of the amplifier; therefore, the voltage gain is lowered. – This effect is similar to CE stage emitter degeneration. EE 105 Fall 2007 Lecture 9, Slide 7 Prof. Liu, UC Berkeley

Practical Example of a CB Stage • An antenna usually has low output impedance; therefore, a correspondingly low input impedance is required for the following stage. EE 105 Fall 2007 Lecture 9, Slide 8 Prof. Liu, UC Berkeley

Output Impedance of a CB Stage • The output impedance of a CB stage is equal to RC in parallel with the impedance looking into the collector. EE 105 Fall 2007 Lecture 9, Slide 9 Prof. Liu, UC Berkeley

Output Impedance: CE vs. CB Stages • The output impedances of emitter-degenerated CE and CB stages are the same. This is because the circuits for small-signal analysis are the same when the input port is grounded. EE 105 Fall 2007 Lecture 9, Slide 10 Prof. Liu, UC Berkeley

Av of CB Stage with Base Resistance (VA = ∞) • With base resistance, the voltage gain degrades. EE 105 Fall 2007 Lecture 9, Slide 11 Prof. Liu, UC Berkeley

Voltage Gain: CE vs. CB Stages • The magnitude of the voltage gain of a CB stage with source and base resistances is the same as that of a CE stage with base resistance and emitter degeneration. EE 105 Fall 2007 Lecture 9, Slide 12 Prof. Liu, UC Berkeley

Rin of CB Stage with Base Resistance (VA = ∞) • The input impedance of a CB stage with base resistance is equal to 1/gm plus RB divided by ( +1). This is in contrast to a degenerated CE stage, in which the resistance in series with the emitter is multiplied by ( +1) when seen from the base. EE 105 Fall 2007 Lecture 9, Slide 13 Prof. Liu, UC Berkeley

Input Impedance Seen at Emitter vs. Base Common Base Stage EE 105 Fall 2007 Common Emitter Stage Lecture 9, Slide 14 Prof. Liu, UC Berkeley

Input Impedance Example • To find RX, we have to first find Req, treat it as the base resistance of Q 2 and divide it by ( +1). EE 105 Fall 2007 Lecture 9, Slide 15 Prof. Liu, UC Berkeley