Are you sure that you're not getting close to either of the supply rails? -
Otherwise I can't think of much!
100x gain should be OK provided that the input offsets don't get out of
hand - do both input have the same effective source impedance (or near
enough the same).

What frequency are you simulating your circuit - could you be hitting a
slew rate limit - or the simulated effect of one?

Can you send the circuit or put it on a website somewhere?

Richard P




Hi guys,

I have a generalised op-amp question.  I am having trouble trying to
extrapolate what I need to know from manuals and datasheets, so maybe one
of
you experts here in analog electronics (something I definately am not) can
help me with understanding the concept.  This may seem like a dumb question
to some of you, but here goes...

Do op-amps have a limit on the amount they can amplify a signal ?  What I
am
finding is that if you increase the difference in resistance in the divider
in a differential amplification setup, at a certain point, the signal
becomes non-linear at the peaks.  Increasing the reference voltage into the
op-amp does not seem to help.  At a certain point in the divider value, the
output simply shifts further up from zero, rather than increasing the gain,
and the peaks are flattened out.  Note that this is not the same as getting
the low point to zero on a non rail-to-rail op-amp by providing a negative
reference - I am already doing this.  I am having a great deal of trouble
getting much more than about 20x gain from an LM358.  I would like 100x
gain, but this seems impossible to achieve with this op-amp, even though it
is marketted as a "HiGain" op-amp.

I am pretty sure of my observations.  I am not proto testing it, but SPICE
testing it with CircuitMaker, so I can see a sine wave amplified output
compared to a sine wave input - a good way to graphically see with a SPICE
transient analysis what is going on.

PS: I am good with digital electronics, but I wish I was better at analog
...

Rgs
Ian.

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