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Our Op-Amp Circuits Input Impedance.

No Chip (or man) is an island.

In a post awhile back, The Load affects the Process. (Electrical Theory and More, I talked about how the input impedance of a voltmeter could affect the voltage readings of that meter.  That same concept is back again, only life is getting a little more complicated.  This time we have at least 3 layers of complication to consider as shown in the first picture.    This post will primarily deal with the resistor values we have chosen for the gain and feedback resistors (Rg & Rf).  I will touch on some of the problems with the actual Op-AMP I have chosen,  but those will show up a little more in some future posts.

R1 & R2 test values.

In this post we will still be using the same amplifiers used in “Building and testing the simple amplifier Op-AMP Circuits“.  The complication I will be doing is changing the values of the voltage divider providing the input to the amplifiers.   R1 is connected to our +Vcc power source and the junction to R2.  R2 then goes to the common connection of our power source or ground if that is what you prefer.  The junction of R1 and R2 is our input to the amplifiers.    Each pair of resistors in this test have the same ratio to each other so each pair should produce exactly the same voltage drop, but because real resistors have a tolerance, we should expect some variation.  In the table above I show the measured values, the calculated expected values and the percent error from real and calculated.   The lowest value for the R1 resistor was 1KΩ because lower values would require higher wattage resistors than what I have available.  (V * V / R = watts).  When R1 was increased to a 10 MΩ resistor, I had a very high error.  This is probably because the input resistance of my voltmeter is probably in the 10MΩ range and I was “loading” the circuit.

Non-Inverting Amplifier measurements.

The second table shows what happened when I connected the non-inverting amplifier to these various voltage divider junctions.
Things went very well until I got to extremely high resistance values in the voltage divider.   At those values, strange things started to happen.  Instead of loading down the voltage the voltage actually increased!  When we talked about the op-AMP datasheet there was some specifications given for Input Offest Current and Input Bias Current.  Those are what is creating our problem in this circuit.   We will deal with those specifically in a future post.  For now, let me do another “shuck and jive” and just say that is where the problem is coming from and we will deal with it later in some better tests to show what it is.

Of our three layers in the first picture, “the rest of the amplifier circuit” seems to not have any effect on the input resistance of the non-inverting amplifier. This is indeed the case.  The only path from the non-inverting input of the amplifier is through the actual integrated circuit (chip) and the input resistance of this has a typical value of 2 MΩ according to the data sheet.  Things are quickly going to take a turn for the worse.

Voltmeter readings from the Inverting Amplifier

The next table shows what happens when we test the inverting amplifier using the same voltage divider circuits.  Almost all of the voltages at the R1-R2 junction decreased as soon as the amplifier was connected to the voltage divider.  The answer to what happened can be seen by looking at the inverting amplifier circuit.  The input feeds into Rg, a 10K resistor, and the other end of Rg connects to the inverting input lead of the chip.  Because the non-inverting input is at 0 volts, the inverting input must also get to 0 V due to the feedback resistor.   This means our amplifier circuit has an input resistance equal to Rg or 10K.

Calculating the input to the Inverting Amplifier with the Input Resistance taken into account.

In our final table, I put the 10K resistor in parallel with R2 and then do the voltage drop calculations.  Now things are starting to make sense.   The fact that it is making sense is all nice and everything… that means we understand things but it is of little help if we really needed a high input resistance because of the circuit we are having to use to feed this amplifier.   The answer in that case would be to use a non-inverting amplifier to feed this inverting amplifier.   That is very often the case where we may have a summing amplifier and some of the inputs need low gain while others need a high gain.  An amplifier used primarily to to correct impedance problems is called a buffer amplifier.

We still have much more to go through on these simple circuits while we stay in DC electricity.  In the next electrical post we will talk about the output impedance and then on to those offsets I did the shuck and jive about earlier.   It is a long way to go, but hopefully you feel like you are getting fed the right amount each time.

As always if you enjoy this information, please consider subscribing to this blog and if you know anyone else who might be interested share the link.

Thank you for your time…. I hope I made it worth it.

Gary


 

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