Re: what is sample and hold circuit and its applications or uses?

A sample and hold circuit acts to remember the value of an analog signal 
at some particular instant of time (i.e., the sampling time, determined by 
some sort of trigger).  The output of the S&H circuit "holds" steady after 
the input is sampled.

The sample and hold circuit in principle is as simple as a capacitor with 
one end connected to ground to an input signal.  When the S&H 
is "triggered", a switch transfers the non-grounded end of the capacitor 
to the output side of the circuit.   In practice, this is dressed up with 
amplifiers and compensation circuits to combat voltage decay due to 
capacitor leakage and output load on the stored charge.

Dynamic RAM memory actually works this way... the capacitor remembers 
whether 0 voltage or the supply voltage was last seen.  Periodic refresh 
is used to restore the charge before it decays to too low a level.   

The sample and hold circuit is useful when one wants to compare the value 
of a signal with the value of that signal (or another signal) at some time 
in the past.  Some types of analog to digital converters use sample and 
hold circuits to "remember" the value of the input signal while giving 
time for the conversion circuit to decide what digital value to output.

Another example of using a S&H is to remember the setpoint of some 
feedback control system.  For example, pre-digital electronic cruise 
control on automobiles would use a "sample and hold" circuit to "remember" 
the current speed (a voltage proportional to speed generated from the 
speedometer) so that the current automobile speed could be compared to the 
set point to determine whether the throttle should be opened wider or 
closed down a bit.

Hope this helps.

Dr. Steve