![]() As noted previously in the common emitter (CE) configuration the input is between the base and the emitter and the output is between the collector and the emitter.The behaviour of the transistor can be represented by current-voltage (I-V) curves (called the characteristic curves of the device).For a given transistor type data sheets may specify a range of values. is also known as hFE and may appear on data sheets and in some textbooks as such.We must try and allow for these variations in circuit design.Ĭommon Emitter Characteristics For example α = 0.98, β = 49 α = 0.99, β = 99 α = 0.995, β =199.A small change in α causes a much bigger change in ß which means that ß can vary significantly, even from transistor to transistor of the same type.For a practical (non-ideal) transistor this is only true at a particular bias (operating) point.We note, in passing that, if β can be regarded as a constant for a given transistor then.Since reverse saturation current is negligible the second term on the right hand side of this equation can usually be neglected (even though (1- α) is small).Define a common emitter current-transfer ratio Such that:.Hence: which after some rearrangement gives.For the present we consider DC behaviour and assume that we are working in the normal linear amplifier regime with the BE junction forward biased and the CB junction reverse biasedĬommon Emitter Characteristics Treating the transistor as a current node:. ![]() collector emitter voltage and VBE for a d.c. Thus in the CE configuration we would write VCE for a d.c.It is convention to refer all transistor voltages to the ‘common’ terminal.iB).This will be less common.Ġ Transistor Amplifier Basics ib + IB = iB And an upper case subscript for thetotal time varying signal (i.e.We will use a lower case subscript for thea.c.signal bit (e.g.levels this subscript will be in upper case. is it the base current or the collector current?). These primary quantities will also need a subscript identifier (e.g.We will use a lower case letter for a time varying (a.c.) quantity (e.g.We will use a capital (upper case) letter for a d.c.As with dynamic resistance in diodes we will be dealing with a.c.This is therefore dealt with explicitly ‘up front’.It is critical to understand the notation used for voltages and currents in the following discussion of transistor amplifiers.
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