5Input and Output Impedance Unlike the op-amp, transistor amplifiers have significant output impedances and finite input impedancesRIN can be comparable with the source resistance of the input signalROUT can be comparable with the load resistance
6Input ImpedanceiINiBInput impedance, rIN, is the ratio of the small signal input voltage and the small signal input currentiRB
8Output Impedance One way to measure rOUT is: Short the input to 0 V Output now looks like just rOUT
9Output Impedance (cont) Applying Kirchoff’s current law:By Ohm’s law:
10Coupling CapacitorsCapacitor COUT is needed to remove the d.c. component of the collector voltageCapacitor CIN is needed to allow the base voltage to be offset from 0VIn both cases this is known as couplingBoth capacitors are chosen to look like short circuits at operating frequenciesTheir reactance will, however, become significant at low frequencies
12Cut-Off FrequencyCut-off frequency, or –3dB point, is when the gain of the amplifier falls by a factor of Ö2If the cut-off frequency, fC, is specified and rIN has been calculated:NB. This assumes that COUT still looks like a short circuit
13COUTFor the lower cut-off frequency calculation to be valid, COUT should still look like a short circuit at fCTypically, choose:
14Emitter Capacitor For the highest voltage gain, vBE But, vIN vE where, Also,
15Emitter Capacitor (cont) For CE to not interfere at fC:vBRWhere,vINvETo make sure, choose,NB. Use rE (=VT/IC) not RE for this calculation!
16SummaryIn the context of the common-emitter amplifier we have covered:Small signal analysisMutual conductanceInput/output impedanceCoupling capacitor requirements and cut-off frequenciesNext time:Applying the same principles to the differential amplifierIt’s actually a much easier circuit to analyse – honest!Make sure you’re happy with the fundamentals by then!