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The Non-Inverting Op-Amp Amplifier

Non-Inverting Op-amp Amplifier

Today we are going to look at our first electronic circuit. First I probably need to talk a little about this device.  An Op-amp stand for Operational Amplifier.  Before the days of integrated circuits these were made of individual transistors and formed a block where simple connections made lots of things possible.  Once integrated circuits came into being these blocks of transistors became very small.   The simplest Op-amp  only require 5 pins for connections, the three shown in the diagram and two more for a + and – power source.

You will notice in the diagram there are two inputs.  These are labeled the inverting input, shown with the – sign, and the non-inverting input, shown with the + sign.  Just like we did when we started learning electricity, we are going to keep everything simple in the beginning.   Our ideal Op-amp has the following properties.  It has infinite input resistance, just like our ideal voltmeter, so no current flows into our out of those inputs.  It has 0 output resistance, just like the ideal source.  And a new property is that it has infinite gain.   That means that if the non-inverting input is the smallest amount greater than the inverting input the output will go to + infinity and likewise in the reverse direction.  Obviously, none of this is true, but it makes life a lot simpler in the beginning.  We will start complicating things soon enough.

Gain equations for the Non-Inverting Amplifier.

The thing we are always interesting in with an amplifier is gain.  Voltage gain is defined as Vout/Vin.  The resistors on the outside of the amplifier set the gain.  This is done by forming a Voltage divider.   Imagine a small voltage is applied to the + input.  Vout will start to climb.  As it does it creates a voltage across Rf and Rg and the Voltage on the – input will be Vout* Rg/(Rf+Rg).  Vout will stop increasing once the – input = +input.   This is all summarised in the equations in the 2nd picture.

If the resistors are in the range of 1K to 100K Ohm range the circuit works very close to the ideal calculations.  This type of circuit is called an analog circuit, because it does not chop the signals up and turn them into numbers for a computer like digital circuits do.   So why are we talking about “out-of-date” circuits? We all know the world has gone digital.  It turns out that digital circuits require a certain voltage range to be effective.  Analog circuits are necessary to get the signal in range before converting it to digital.  Similarly, analog amplifies the signal after it comes out of the digital device to be used in the real world.  As an example, the speakers plugged into a computer have internal amplifiers.

As promised, I have created the drawings for circuits to obtain the values for the current ranges on the Simpson meter.  For the most part I am not going to talk about these because the drawings have enough information on them to show how I did the calculations.   Simpson probably designed the circuits this way to get resistors in the reasonable range. The most interesting range is the 10A range.   There they went back to only 255mV drop across the meter at full range. The shunt resistor is a very small value and to obtain that value they probably used copper wire (about 4.7 inches of 28 Awg wire.) and they cut the wire to the length necessary to scale the meter.  I bring this up because many of the values in the previous post were this kind of range of resistance and would have required a lot of work to calibrate.  Also, this shows one more reality that we have not dealt with yet.  All conductors have resistance.  Normally it is assumed to be negligible and normally it is, but sometimes it becomes very important.

































If you are curious and have access to a spreadsheet program I have the spreadsheet I used to calculate this, including the wire length for the 10 A shunt available. Simpson-meter calcs.

Next time I do an electronics post will have two more of the standard op-amp circuits.


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