TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 317 



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FREQUENCY IN CYCLES PER SECOND 



Fig. 14 — - Loop current transmission of the integrator. 



The negative feedback in the integrator has been shaped by means of 

 local feedback and interstage networks as described in Section 2.2. The 

 loop current transmission has been calculated from (13), (14), (15), and 

 (A6) and is plotted in Fig. 14. The transmission is determined under the 

 assumption that the alphas of the transistors are 0.985 and the alpha- 

 cutoff frequencies are three megacycles. Since the feedback above 800 

 cycles per second falls off at a rate of 9 db per octave, the analysis in 

 Section 4.1 using (23), is conservative. The integrator has a 44° phase 

 margin and a 20 db gain margin. In order to insure sufficient feedback 

 between 30 and 800 cycles per second and adequate margins against 

 instability, the transistors used in the integrator should have alphas in 

 the range 0.98 to 0.99 and alpha-cutoff frequencies equal to or greater 

 than 2.5 megacycles. 



The silicon diodes Di and D2 are rec^uired in order to prevent the 

 integrator from overloading. For output voltages between —4.0 and 21 

 volts the diodes are reverse biased and represent very high resistances, of 

 the order of 10,000 megohms. If the output voltage does not lie in this 

 range, then one of the diodes is forward biased and has a low resistance, 

 of the order of 100 ohms. The integrator is then effectively a dc amplifier 

 with a voltage gain of approximately 0.1. The silicon diodes affect the 



