ELE 1110D Lecture review Common-emitter amplifier Some functions of transistors  Current-source  Emitter Follower  Common-emitter amplifier

Vout is biased at middle of V+ AC Gain: G = - R c / R E ELE 1110D Lecture review Common-emitter amplifier 0 (V+)/2 0.1 (V+)/2 + G*0.1 (V+)/2 - G*

ELE 1110D Lecture review Common-emitter amplifier Intrinsic emitter resistance Gain: G = -Rc / r e = -g m Rc Gain is varied with V BE !!! Add R E to reduce the effect of r e re=50 re=25 re=12.5

ELE 1110D Lecture review Common-emitter amplifier Temperature effect  If it is heated up  I c increase, then V BE increase  V BE increase, then Ic increase further  Ic increase will cause further heat up Add R E for compensation  Ic increase will not cause V BE decrease  V BE decrease drive Ic decrease  High AC gain by adding a parallel Capacitor

ELE 1110D Lecture review Current Mirror Act as programmable current source Q1 and Q2 are twins  V BE are the same  Ic 1 = Ic 2 Temperature effect  Temperature increase, cause Ic 1 increase  Voltage drop across 15K resistor increase  V BE decrease, drive the Ic 1 to decrease

ELE 1110D Lecture review Differential Amplifier A differential amplifier is a two-input device.  Only amplify the difference in signal.  By-pass the common signal. For example:  Input signal : 5V, 3V  Common signal : 4V, 4V  Differential signal : 1V, -1V Ideal differential amplifier:  Large differential gain.  Zero common mode gain.

ELE 1110D Lecture review Differential Amplifier Consider no input signals i.e. DC quiescent point  V A = -0.6V.  I TAIL = (-0.6 – (-15)) / 7.5k = 2mA I TAIL is quite constant  Change in V A is small  Large negative voltage Simple current source  By symmetry, I C on both transistor = 1mA.

ELE 1110D Lecture review Differential Amplifier Consider only the differential signals For example,  dV IN /2 = 1V  Assume the simple current source has large impedance (open circuit)  Current across the branch = 2 / ( ) = 10mA  V A = 1- 10mA*100 = 0V In general,  Due to symmetric of 2 R E,  V A = 0V.  Therefore, practically, the differential signals have no effect at A.