Just 10kHz input in simulation.  In reality it would only be 5.  Is this
where I am going wrong.  As Richard has suggested, am I reaching a slew-rate
limit ?

Ian

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Herbert Graf
Sent: Friday, 21 February 2003 12:51 pm
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Differential Amplifiers


I'd have to agree, about ANY op amp out there can amplify more than 100x,
assuming you're not close to unity gain frequency. What frequencies are we
talking about? TTYL

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Richard Prosser
> Sent: February 20, 2003 20:46
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Differential Amplifiers
>
>
> 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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