Inverter / Grounding

gddfp · Joined: Jan 11, 2009 Posts: 21 Location: Belgium

Hi all,

My first post here, please be mild.

I'm just starting out with some basic DIY, learning as much as I possibly can. Still have a long way to go with the most rudimentary of electronics principles, but I'm trying...

Right now I'm breadboarding a simple Inverter, based on a TL072. Real basic stuff: voltage in, inverted voltage out.

I kinda understand the schematic (included), but I think I'm having trouble with grounding. This is supposed to work, but all I get is the same (pos) voltage coming out.
I do not understand if and whereto I have to connect the 0V - to the input & output busses, only one of them, none... ?

This will be part of a separate desktop module, with its own separate PSU. It's not build in one of my modulars or anything.

Can someone please enlighten me how to measure and connect this properly ?

Thank you.

inverter_schema.pdf

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JovianPyx · Posted: Tue May 05, 2009 4:31 pm Post subject:

The ground symbol is the one connected to the smaller diagram lower left. This is the circuit point that should be connected to the input signal "ground". It is also connected to the OpAmp's "+" or non-inverting input. If you place an AC input into this circuit, the output should be inverted (or 180 phase change) with respect to the input and amplified by whatever gain is designed into the the OpAmp circuit. In this case, the gain is -1 because the two resistors (input and feedback) are the same and the minus sign is because it's an inverter.

This is a DC amplifier. You can test it using a DC voltage, like a simple dry cell - which when new produces 1.5 volts. Connect the negative side to ground, the positive side to the inverter's input. The output should read on a DVM as -1.5 volts. If you flip the dry cell around so that + is to ground and the - terminal goes to the inverter input, then the output should be +1.5 volts.

The 0 (zero) volt connection is the ground connection which should go to the opamp + input and to the input signal ground as well as the output signal ground.

In fact, many inverter circuits use a simple wire in place of the 47 K resistor... I believe that's there to make the circuit "more accurate" as an "operational amplifier" - operational as in the arithmetic operation of summation.

gddfp · Joined: Jan 11, 2009 Posts: 21 Location: Belgium

Thanks for the explanation, Scott !

However, I already tried what I think you mean (see new schematic), and then I get +15V at the output... Embarassed

I've included a sketch how I connected the cell -- perhaps that's where I go wrong ?

inverter_schema_2.pdf

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JovianPyx · Posted: Tue May 05, 2009 6:16 pm Post subject:

If things are in fact hooked as you show in the second schematic (which is correct) then the output should be either +9V or -9V depending on the polarity of the battery connection to the plug. It should NOT be 15.

Are you quite sure that both the feedback and input resistors are 100K? If the feedback resistor is larger than the input resistor, you could get positive gain. Or if the input resistor is smaller, the same thing would happen. Let's say that the input resistor is 10K and the feedback is 100K, you'd get a gain of 10 - in this case, the circuit would try to multiply the input voltagbe by the gain of 10, but because the power supply is only 15 volts, it can't produce an output of 90V, so it goes as high as it can or 15 volts.

Make sure the resistor values are really as they are stated in the schematic...

I'm curious, if you reverse the polarity of the battery, does the polarity of the output also reverse? (it should)...

gddfp · Joined: Jan 11, 2009 Posts: 21 Location: Belgium

Hi Scott,

I have solved the problem.

Turned out I made a mistake on the breadboard (this is my first foray in breadboarding as well). The 100k input resistor was connected over the same board column... (so: input <--> 100k <--> pin 2 all on the same column Embarassed

) Rather stupid of me - sorry !

Anyway, it works now, and I want to thank you for your help and explanations. They certainly cleared up most things, plus I learned some more about OpAmps along the way.

Guy

JovianPyx · Posted: Wed May 06, 2009 6:10 am Post subject:

Don't apologize - making mistakes is what breadboards are for! You also now have some practical experience in some troubleshooting and will know what to expect and some things to try in the future. Glad to hear it's working and that you're gaining knowledge. Welcome to "our" little world. Cool