SOME CIKCL rr ASI'KCTS Oi THE IKASSISlUR Hi 



development is still at an early stage, there are considerable variations 

 between units. 



Sinc;le Stage Amplifiers. SrAiiii.irv, 

 Electron' Tube Analogy 



An amplifier can be built in a straightforward manner by using the emitter 

 as input electrode and collector as output electrode, the base being common 

 to the two circuits. This amplifier is therefore called the grounded base 

 amplifier. Figure 7 shows a schematic circuit using the average parameters 

 just mentioned, working between 500 ohms and 20,000 ohms. The ampli- 

 fier has an operating power gain of 17 db, power output Class A 10 milli- 

 watts, noise figure at 1000 cycles 60 db with a variation inversely with fre- 

 quency, and frequency response down 3 db at 5 megacycles. 



Fig. 6 — Equivalent circuit parameter values. 



Some comments are in order on how this amplifier compares with an 

 electron tube amplifier. First of all, the amplifying function and the manner 

 of analyzing it from the circuit point of view are very similar, even though 

 the internal mechanisms are markedly different. Secondly, there are quali- 

 tative differences in circuit behavior, which are set forth on Fig. 8. The 

 base resistance rb acts as a positive feedback element which, under adverse 

 conditions, can cause the circuit to oscillate. A necessary condition for 

 stability is that the circuit determinant shall be positive, and this can be 

 written as follows: 



^ < 1 + ^ + ?^ (1) 



■^c Rb Rc 



Here the quantity r,„ is the net mutual resistance of the transistor, and 

 the capital R's are the total resistances in the corresponding leads, internal 

 and external. One can see several features, as follows: 



1. If Rb = 0, the circuit can be stable. 



2. If Rb > 0, as usual, the circuit can be stable if the emitter and collector 

 lead resistances are large enough or if rm is not too large. In other 

 words, resistance in the base lead tends toward instability if rn, is 

 large; resistance in emitter or collector leads tends toward stability. 



