836 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



Bo has changed by ±6/i22r mhos from its value at the center of the band. 

 Using the same value of circuit capacitance as above, the indicated band- 

 width is about 7.4 mc. The measured response on an amplifier with this 

 value of load impedance indicates a gain of 16.1 db at 30 mc, with the 

 frequencies at the half power point separated by 7.6 mc. 



Often it is desirable to build tuned amplifiers to work between like 

 impedances in which case at least the output network must perform both 

 the function of selectivity and impedance transformation. An example 

 of a simple network to perform these functions is shown on Fig. 17. The 

 impedance transforming properties of such a circuit are w^ell know^n. 

 With a given value of load resistance, the load admittance presented to 

 the transistor can be made to have a given value at a certain frequency. 

 However, since the circuit performs both the function of impedance 

 transformation and selectivity the bandwidth is determined by the out- 

 put impedance selected. This circuit does not present a fixed value of 

 conductance as a function of frequency but for frequencies near the 

 maximum gain it is a fair approximation to assume it constant. The out- 

 put circuit of Fig. 17 w^as designed to present a load admittance such that 

 G2 + JB2 = 3/i22r — i2/i22r at 30 mc. This is tJhe same condition as com- 

 puted in Table II so one would expect the same value of maximum gain. 

 However, in order to present the proper value of load impedance, a total 

 load capacitance of about 4 /^/xf must be used. This indicates a bandwidth 

 of 3.3 mc between the half power points. The measured response of this 

 amplifier is shown on Fig. 18 as the solid line. The points indicate the 

 computed maximum gain and the frequencies at which the gain is down 

 3db. 



-4.5V 



+ 15V 



Fig. 17 — Simple tuned amplifier. The output circuit performs both the func- 

 tions of impedance transformation and selectivity. 



