PROPERTIES AND APPLICATIONS OF fl-p-n TRANSISTORS 



553 



collector current will increase by 1/(1 — a) microamperes for each micror 

 ampere which flows through the bias resistor. Since the current in the bias 

 resistor will be approximately Eb/R, it is a simple matter to compute the 

 required value of bias resistor once the desired collector current is known. 

 Figure 20 shows a two-stage audio amplifier which gives approximately 

 90 db gain. The circuit is shown in Fig. 21. 



Fig. 21 — Circuit of the amplifier shown in Fig. 20. 



Fig. 22 — The grounded collector connection of a transistor and the equivalent circuit. 



The Grounded- Collector Stage 



Although the power gain obtainable from this connection is relatively 

 low — of the order of 15 or 20 db — it has very interesting possibilities in 

 producing very high input impedances or very low output impedances. 

 If it is worked into a fairly high load impedance, the input impedance may 

 be several megohms, or if it is worked from a source of moderately low 

 impedance (a few thousand ohms), the output impedance may be of the 

 order of 25 ohms or lower. 



For this type of stage, which is shown schematically in Fig. 22, 



i?ii = n-}- rc= ISA {lOy ohms 



Ri2 = Tc- Tm = 0.288 (10)6 ohms 

 R21 = rc= 13.4 (10)6 ohms 

 R22 = re-\- fc- fm = 0.288 (10)6 ohms 



(48) 



