654 lag oe 
on the initial rate of decay in response to a step voltage. This can be 
made much greater than the value for an uncompensated amplifier, but is 
applicable only if the time of measurement t' does not exceed the time for 
which the compensation is effectivee 
Although the analyses of response characteristics carried out here have 
been rather simplified and specialized, it is believed that the requirements 
for other types of applied signal and response are best considered along 
similar lines, to which the present discussion may act as a guide. 
(b) Input impedance. -- For certain applications, particularly use 
with piezoelectric gauges, an important factor in determining the low-fre- 
quency response characteristics is the input impedance. The low-frequency 
time constant of a piezoelectric-gauge circuit of capacitance C together 
with an input impedance R is given by the product RKC, which must be of the 
order 50 to 100 times the interval over which a faithful record of the tran- 
sient gauge signal is required. 
The necessary input impedance so determined is usually several megohms 
for necessary or desirable values of the gauge-circuit capacitance C, Grid— 
circuit impedances of this magnitude frequently require special designs of 
the input stage. Usually these designs involve selected tube types and 
operating conditions in cathode-follower circuits, examples of which will be 
found in circuits described in the following. 
(c) Linearity and stability. -- If accurate measurements are to be 
taken from records of transient output signals from an amplifier and indi- 
cating circuit, it is necessary that the sensitivity of the combination 
remain constant over at least short intervals, and that the output signal be 
a linear function of the applied voltage. It is usually desirable to use 
regulated supply voltages, particularly in the low-level stages, together 
with considerable degeneration of the well-known types. Linearity and sta- 
bility are both improved with use of push-pull or balanced amplifier stages, 
which are virtually essential in high-level stages, In the interests of 
linearity these stages should also be considerably overdesigned as compared 
to commercial practice. 
(d) Amplifier circuits; diagrams and descriptions. -- (i) Modified 
DuMont type 208 oscillograph. In much of the work which has been done at 
this laboratory, it has been found possible to adapt the DuMont type 208 
oscillograph and make it a usable instrument for recording transients. The 
