🚀 go-pugleaf

In article <1992Nov5.012546.17925@doug.cae.wisc.edu> kolstad@cae.wisc.edu (Joel Kolstad) writes:
>Hi Mike,
>
>In article <1992Nov05.043724.130701@zeus.calpoly.edu> mruddick@joule.elee.calpoly.edu (Michael Ruddick) writes:
>>Be careful...you should only have to use one op-amp for what you are doing.
>>It sounds like you want the op-amp to amplify your signal as much as
>>it can, then pass it to another one that does the same thing.  Op-amps
>>don't work like that...

>However, I did want to point out that there's a very good reason (in general)
>for using two cascaded op amps (providing *x*y gain=*xy gain) rather than
>using just one (providing *xy) -- the gain bandwidth product.  You can
>show mathematically, in not too long of a time, that the closed loop
>gain bandwidth product of two cascaded op amps is greater than that of
>a single one for the same gain conditions.

	That's true, BUT one is not cascading OP AMPS in this case,
rather one is cascading limited-gain amplifiers.  You have to close
TWO feedback loops on two cascaded op amps, or the frequency
compensation becomes very difficult.  Two cascaded limited-gain
amplifiers is a very common configuration: the first is called
the 'preamplifier', and is selected for input impedance and 
low noise.  The second amplifier (power amp) would have requirements for
driving the load, for dynamic range, etc.

	Because the two amplifiers have different requirements,
it is more common to find a single-transistor preamp and op amp
output stage, or op amp input stage and power transistor output buffer.

	John Whitmore