Presentation on theme: "SMALL SIGNAL BJT AMPLIFIER"— Presentation transcript:
1 SMALL SIGNAL BJT AMPLIFIER SMALL SIGNAL OPERATION
2 INTRODUCTIONOne 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.Most transistors amplifiers are designed to operate in the linear regionThe common-emitter amplifier has high voltage and current gain
3 Definition of Small Signal ac input signal voltages and currents are in the order ±10% of Q-point voltages and currents.egIf dc current is 10mA, the ac current (peak to peak) is less than 0.1 mA.
4 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 RCTherefore a large voltage appear across the collector circuit.
5 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 signalC3(Coupling capacitor)- couple the signal to the next stage
6 Circuit Diagram IC VCE IB VCC Common-Emitter Biasing(CE) input = VBE & IBoutput = VCE & IC+_ICVCEIBVCC= Common-emitter current gain == Common-base current gain =The relationships between the two parameters are:
7 ExampleGiven the common-emitter circuit below with IB = 25A, VCC = 15V and = 100. Find the ideal collector current. = 100 = IC/IBIC = 100 * IB = 100 * (25x10-6A) = 2.5 mA
8 Collector-Current Curves Common-EmitterCollector-Current CurvesICActive RegionIBSaturation RegionCutoff RegionIB = 0Regionof OperationDescriptionActiveSmall base current controls a large collector currentSaturationVCE(sat) ~ 0.2V, VCE increases with ICCutoffAchieved by reducing IB to 0, Ideally, IC will also equal 0.VCE
9 DC Analysis of Transistor Circuits Common-Emitter Circuit Common-emitter circuit with an npn transistorAssume 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
10 dc equivalent circuit, with piecewise linear parameters
11 Base-emitter Junction Characteristics And The Input Load Line
12 Common- Emitter Transistor Characteristics And The Collector-emitter Load Line
14 Kirchoff’s voltage law equation (around B-E loop): Both load line & quiescent IB change as either or both VBB & RB changeKirchoff’s voltage law equation (around C-E loop):IC & VCE relationship represents DC load line
15 Amplifier OperationThe 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
16 ExampleAssume Q-point value of iB=10A, ac signal peak value is 5A and β=100.So at any instant, iC =100iBiBiC=100iBVCE=VCC-iCRCOutput voltage15A1.5mA4 VPositive peak value10A1.0mA6 VSignal current zero5A0.5mA8 VNegative peak value
17 dc equivalent circuit of transistor amplifier Only dc condition are to be consideredNo signal is applied (All ac source reduce to zero)dc cannot flow through a capacitor (open circuit)To calculate the dc currents & voltages
18 DC Equivalent for the BJT Amplifier DC Equivalent CircuitAll 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.
19 ac equivalent circuit of transistor amplifier Only ac condition are to be considereddc voltage not important, considered zero or ground. (VCC=0)the capacitors used to couple or bypass the ac signal. (short circuit)
20 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.
21 (continued)By combining parallel resistors into equivalent RB and R, the equivalent ACcircuit above is constructed. Here, the transistor will be replaced by itsequivalent small-signal AC model (to be developed).
22 Graphical Analysis & AC Equivalent Circuit EquationsInput loopOutput loopRCRBvsvOvcevbeicib+-0.026 VAC equivalent circuit of C-E with npn transistor