TRANSISTOR PULSE REGENERATIVE AMPLIFIERS 



1101 



A third method of couphng, which is attractive for systems using only 

 AND- and OR -type logic, utilizes the reverse characteristic of a break- 

 down diode, Fig. 9 (a). The interesting feature of this diode is the sharp 

 transition between the high and low incremental resistance regions of the 

 reverse characteristic. With this diode it is possible to shift dc levels by 

 an amount ec^ual to the rcA^erse breakdown voltage of the diode, as indi- 

 cated in Fig. 9 (b). In the ciuiescent state D2 operates in the breakdown 

 region and Dl serves to clamp the collector voltage at — F3 ; during the 

 pulse duration D2 operates in the high resistance portion of its reverse 

 characteristic. If the driven circuit has a ^'oltage threshold, like the 

 transistor threshold in Fig. 7 (a), less than — F^ + Vb + Vs and Vs 

 <|Fb|, the circuit operates like a normal AND-type circuit except for 

 the dc level change. For this reason it is convenient with AND-OR logic 

 circuits to include only Dl and R2 in the output circuit of the amplifier 

 and use D2 and Rl as the AXD input elements in the logic circuits. 



The principal advantages of diode coupling are simplicity and the 

 lack of an energy storage element. The limitations are that there is no 

 opportunity to match transistor and output conditions, variations in 



-V 



(a) BREAKDOWN DIODE 



V-I CHARACTERISTIC 



-V. 



If^ 



Dl 



BREAKDOWN 

 DIODE N 

 I 



W 



l+v, 



R1 



-V3) 



OUTPUT 



D2 



(-V3 + VB) 



R2 



-V3 -V2 



(b) COUPLING CIRCUIT UTILIZING 

 A BREAKDOWN DIODE 



Fig. 9 — Direct output coupling circuit. 



