PROPERTIES AND APPLICATIONS OF fl-p-fl TRANSISTORS 535 



the supply voltage is large compared to the few hundredths of a volt drop 

 across the emitter circuit). 



One can also draw some interesting conclusions from the static charac- 

 teristics about the large signal operation of the transistor. If the load is 

 resistive, the instantaneous operating point will swing up and down along a 

 straight line such as the load line shown in the upper plot of Fig. 4. This par- 

 ticular load line corresponds to an a-c. load resistance of 10,000 ohms. 

 Suppose that the steady collector biases are 20 volts and 2 milliamperes 

 so that the drain from the power supply is 40 milliwatts. Now consider the 

 permissible swings of collector voltage and current. Since the collector char- 

 acteristics are quite straight and evenly spaced over a wide range of cur- 

 rent and voltage values, the output signal can swing nearly down to zero 

 collector volts and nearly up to zero collector current without distortion. 

 The limit on the lower end is imposed by the fact that the collector charac- 

 teristics begin to be curved when Vc is less than about 0.1 volts; and the limit 

 on the upper end is imposed by the fact that the collector current does not 

 drop completely to zero when le drops to zero. The lower limit of collector 

 current is, in this case, about 50 microamperes and, since this amount of 

 current in 10,000 ohms corresponds to 0.5 volts, this means that the instan- 

 taneous collector voltage is limited to swings between 39.5 volts and 0.1 

 volts. Starting from a quiescent value of 20 volts, the permissible positive 

 swing is then 19.5 volts and the permissible negative swing is 19.9 volts. 

 Reducing the quiescent voltage to 19.8 volts (and keeping the same load 

 line) makes it possible to obtain a peak swing of 19.7 volts which corre- 

 sponds to 19.45 milliwatts of signal delivered to the load. This gives a col- 

 lector circuit efficiency of 48.5% out of a possible 50%. Some transistors 

 take even less collector current when the emitter current is zero and hence 

 permit even higher efficiencies. 



These computations of efficiency have all been based on the assumption 

 of sinusoidal current applied to the emitter. It will be shown in a later sec- 

 tion that the emitter resistance varies with emitter current, however, and 

 this means that to realize high efficiency with low distortion it is necessary 

 to drive the emitter from a high impedance source. 



Operation with Small Power Consumption 



For small signal applications the transistor represented by the charac- 

 teristics of Fig. 4 can deliver useful gain at very much lower voltages and 

 currents than those used in the example above. In order to show this, the 

 characteristics of Fig. 5 have been plotted for a range of collector voltage 

 extending up to only 2 volts and for a range of collector currents extending 



