SMALL SIGNAL ANALYSIS OF CE AMPLIFIER 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE INTRODUCTION One of the primary uses of a transistor is to amplify ac signals. This could be an audio signal or perhaps some high frequency radio signal. It has to be able to do this without distorting the original input. 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

Definition of Small Signal ac input signal voltages and currents are in the order ±10% of Q-point voltages and currents. eg If dc current is 10mA, the ac current (peak to peak) is less than 0.1 mA. 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE HOW BJT AMPLIFIES? When a weak ac signal is given to the base, a small ac base current start flowing. Due to BJT, a much larger ac current flow through RC 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE R1, R2, RE – form the biasing circuit & stabilization. C1(Coupling capacitor) – couple the signal to the base. C2(Bypass capacitor) - use in parallel with RE to provide low reactance path to amplified ac signal C3(Coupling capacitor)- couple the signal to the next stage 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE Circuit Diagram Common-Emitter Biasing(CE) input = VBE & IB output = VCE & IC + _ IC VCE IB VCC = Common-emitter current gain = = Common-base current gain = The relationships between the two parameters are: 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

Collector-Current Curves Common-Emitter Collector-Current Curves IC Active Region IB Saturation Region Cutoff Region IB = 0 Region of Operation Description Active Small base current controls a large collector current Saturation VCE(sat) ~ 0.2V, VCE increases with IC Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0. VCE 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

DC Analysis of Transistor Circuits Common-Emitter Circuit Common-emitter circuit with an npn transistor Assume B-E junction: forward biased  V drop is the cut-in / turn-on V [VBE (on)] IC represented as a dependent I source (function of IB) Neglect reverse-biased junction leakage current & Early effect 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE Common- Emitter Transistor Characteristics And The Collector-emitter Load Line 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE Amplifier Operation The boundary between cutoff and saturation is called the linear region. A transistor which operates in the linear region is called a linear amplifier. Only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of input phase. Fig 6-2 amplifier circuit 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

dc equivalent circuit of transistor amplifier Only dc condition are to be considered No signal is applied (All ac source reduce to zero) dc cannot flow through a capacitor (open circuit) To calculate the dc currents & voltages 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

DC Equivalent for the BJT Amplifier DC Equivalent Circuit All capacitors in the original amplifier circuit are replaced by open circuits, disconnecting vI, RI, and R3 from the circuit and leaving RE intact. The the transistor Q will be replaced by its DC model. 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

ac equivalent circuit of transistor amplifier Only ac condition are to be considered dc voltage not important, considered zero or ground. (VCC=0) the capacitors used to couple or bypass the ac signal. (short circuit) 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

AC Equivalent for the BJT Amplifier The coupling and bypass capacitors are replaced by short circuits. The DC voltage supplies are replaced with short circuits, which in this case connect to ground. 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

Small-signal hybrid- equivalent circuit Using common-emitter current gain (β) parameter 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

Small-signal equivalent circuit Small-signal hybrid- equivalent circuit Small-signal equivalent circuit r Vs RB RC Vo Ic Ib gmVbe Vbe Vce + - Output signal voltage 7.7.17 EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE Input signal voltage