CHAPTER 5. TRANSISTOR CIRCUITS88We now turn to the description of some simple ampliﬁers that use a single bipolar tran-sistor. Our goal will be to estimate the voltage gains, current gains, input impedances,output impedances and corner frequencies of the ampliﬁers. The characteristics of a perfectampliﬁer are as follows:• inﬁnite input impedance (the ampliﬁer draws no current),• zero output impedance,• inﬁnite voltage and current gain, and• stable, linear, etc.5.2The Common Emitter AmpliﬁerThe common emitter conﬁguration is the most versatile of the three.It has low inputimpedance, moderate output impedance, voltage gain and current gain.The input andoutput are often capacitively coupled. Before performing an AC analysis we will discuss DCbiasing.5.2.1DC BiasingDC biasing is setting up a circuit to operate a transistor at a desired operating point on itscharacteristic curve. Three bias networks for the common emitter ampliﬁer are shown inﬁgure 5.7.In ﬁgure 5.7a the only path for DC bias current into the base is through RB. VCC is apower supply voltage which is generally greater than 10 V such that VPN can be ignored. TheDC voltage at the collector should be large enough to provide at least a 2 V drop betweencollector and emitter and clearly must be less than VCC. In the absence of other circuitrequirements, a convenient algebraic choice for VC is VCC/2. DC circuit analysis results inthe following relative sizes of the two resistors:RB =2hFERC.(5.19)Although the circuit works reasonably well, the fact that hFE is quite variable amongsamples leads to a bad design. A well-designed circuit should have an operating point thatis less dependent on this parameter.Figure 5.7b shows a network with the base-biasing resistor connected to the collectorinstead of VCC. RF acts as a negative feedback resistor since it feeds the collector currentback into the base. Analysis givesRF = hFERC.(5.20)Therefore a change in hFE has only half the eﬀect of the previous design.A more common bias stabilization technique employs a series resistor between the emitterand ground. This circuit has about the same sensitivity to changes in hFE as the previouscircuit.