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Summary of the standard op-amp amplifiers

The Op-Amp Circuits under test.

We have went through a lot of information about the standard Op-Amp amplifiers so before going on and complicating things even more it is time to summarize what we have learned. To keep things as practical as possible I will do this from the point of view of the source circuit (the circuit feeding our amplifier) and the load circuit (the circuit being driven by our amplifier.)

Throughout this I will use the terms Low Z, and High Z, to represent Low Impedance and High Impedance.  For now impedance is resistance. We will have to go into AC theory to explain the total concept of impedance and we will get there eventually.  In DC impedance is resistance.  Any time varying signal contains an AC component, but often that signal varies so slowly it is assumed to be DC.  That will be the case of the temperature measurements we are working toward for now.  (Yes, I know I am presenting this kind of in the middle… I am doing so to keep it more practical than theoretical.)

The Load:  The op-amp cannot handle Low Z loads.   For the chip we used the bottom limit is 2KΩ.  That also includes the resistance in the feedback circuit as described in the last post, “The Resistance seen by the Op-Amp in our circuits.”  If we need to drive a lower resistance load, we will have to have an external amplifier to do the job.  Again we will get there eventually, but the nice news is our op-amp circuits will provide all we need to drive those circuits.

The op-amp output can be considered to be linear from -10V to +10V with a +/-15 V power supply.   Again, this is more than enough for most uses.  The one other thing that applies to all circuits is if high accuracy is needed, then it will probably be necessary for the feedback resistor, Rf, or the gain resistor to be a variable resistor.  Remember, standard resistors are normally sold with a +/- 5% tolerance.  Although it is possible to buy 1% tolerance resistors normally you are better off install an adjustment into the circuit and adjust to get the exact gain using tests and measurements.

The Source:  We have two things to worry about with respect to the source:  The source impedance, and the source voltage and we will consider both of those at the same time.

Low to Medium Z & Strong Signal Source:  If the source impedance is 10KΩ or lower and the signal from it is greater than 0.1 V we probably have nothing to worry about.  If we are concerned about high accuracy we may need to zero out the voltage offset, but that is relatively easy to do by using a voltage divider with a variable resistor, potentiometer, and a summing amplifier.

Low to Medium Z & Weak Signal Source: In this case we may have to worry about voltage offset affecting our signal.  As described above that is relatively easy to do.  The only other additional worry is that to keep the resistors in our feedback circuit in a reasonable range I would probably not attempt to have the gain of any one stage above 100 and if necessary add multiple stages.  So for example, lets say the source was 1 mV and the output needed to be 1V.  I would split the 1000:1 desired gain into two stages of about 31.6:1 each.  This also would easily allow one stage to be a summing amplifier for the voltage offset.

A Voltage Follower Op-Amp Circuit

 High Z Source:  Anytime we have a source with high impedance we should consider the input stage will need to be a non-inverting amplifier.  If we need to couple this high impedance source to a low impedance load, but we need no gain a voltage follower circuit can be used.   This is an non-inverting amplifier with a feedback resistor of 0Ω and a gain of 1.

High Z source with a strong signal.  Although there may be a problem with current offset, it probably is of no significant value if the input voltage is greater than 100 mV.  Also in our terminology High Z is a relative term.  If the source is 100KΩ the offset will not be much.   If the source is 100MΩ we probably have a big problem and may have to consider some things in the next section.

High Z source with a weak signal.  This is the worst of all situations.  There are many recommendations of what to do in this situation.  The first thing to consider is buying an Op-amp that is a better fit than the one we used.  There are Op-amps with FET inputs and even MOSFET inputs.  Yeah I know I have not explained what these terms mean yet, again we will get there, but for now just know that test draw extremely small currents.  There are also compensated Op-Amps designed exactly for this situation.  As in everything there is a trade off.  I have read these Op-amps are slower and cost more.   The other thing that can be done is to try to make the resistance seen by each input of the amplifier the same.

At this point we are not into a situation where we need to worry about those problems and as we baby-step through our op-amp designs we will take it one step at a time.

The next electrical post will be about temperature measurement.  That will lead us into a whole new set of problems but our standard amplifier can be modified to meet those and we will have our first useful circuit.

Announcements:

I finally am freed up enough from other commitments to be able to do some site work and the site may be down some during this weekend while I do some maintenance and modification.   Please bear with me while I attempt to make things better.

If you have found this post and other information on this site useful, please consider subscribing using one of the methods on the home page and soon to be on a subscribe page.  (One of the maintenance things I talked about,)

Gary


 

 

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