APPLICATION OF CONVENTIONAL TECHNIQUES 127 
improvement is not included in the design method and is not known 
until after a tentative design is completed. 
Although the clamp, or zero-order hold, is the most common hold 
circuit used in sampled-data control systems, the method of approximat- 
20 logi9 IGHN| 


l 
S 
Phase margin, 180 -ARG HGN(ju) 
—20 
Fia. 6.8. Bode plot for lag compensation. 
ing the sample and hold operation by a continuous network is obviously 
not limited to that case. Asa matter of fact, the higher-order-hold net- 
works lead to generally simpler approximate continuous networks than 
that necessary for the clamp. The transfer function of the first-order 
hold, for example, is 
1 = Ge 
which may be reduced to 
° 2 
H,(jo) = TL + joe? (a (6.9) 
for real frequencies. If this hold circuit is followed by a plant with the 
transfer function G(jw), then the pulse transfer function of the combina- 
