The answer to your question is "Yes". Opamps do have a limit on the amount
they can amplify a signal.  But it is usually way more than 20x.

What is going on here?

First, as others mentioned, if the output gets near the power supply rails,
it will distort (flatten out usually). So check the peak values and see if
this is the case.  If so, reduce the magnitude of your input signal (the ac
value).

Reality check: you want a gain of 100.  Your opamp can only produce some
limited maximum undistorted signal at the output. Let's suppose you have
+5V and Gnd rails. The '358 can go a little better than 3Vpp in this case.
Thus your input will need to be NO MORE than 100 times less which is
30mVpp. With a gain of 100, any more input will square the sinewave into a
distorted square wave.  Adjust your signal depending on what actual power
supply numbers you have.

Next, the AC gain numbers of voltage mode op amps decreases with frequency.
 The '358 GBW is 1MHz which means that at 10KHz, you will have only a gain
of 100 OPEN LOOP.

It wasn't clear to me what sort of input biasing scheme you are using.  To
get maximum signal swing, you will need your input bias point located in
the middle of what the output can do.  Since the output swings closer to
gnd  but 1.5v from the positive rail, you don't want an input bias point of
2.5; rather, something a little lower. This assumes you are using +5 and
gnd for power; if you  have a negative rail, things will work better. With
+/- supplies, you can use gnd usually as your bias point.  Remember that
the amplifier will amplify DC so if you have a gain of 100, any DC offsets
will be multiplied by 100.


Finally, Ian, you are doing the RIGHT thing: using a simulator.  Trying
this with hardware with a very limited knowledge base and perhaps no
oscilloscope or signal generators, etc. will be an exercise in frustration.
 As others have mentioned about the value of the MPLAB sim for software, an
simulator for circuits is a great idea. Plus you don't let out any smoke or
burn fingers.  You can build circuits all day and see what they will do
without having to buy and destroy any parts.

hth,
Tom M.

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu