Still confused about OpAmp gains

BobTPH

Joined Jun 5, 2013
9,256
Do you understand that the capacitor and resistor between your signal generator and the opamp input is a high-pass filter?

With 1K and 0.1 uF, the filter corner frequency is about 1500 Hz. This means that the signal is reduced to about 70% at that frequency and reduced by more for all lower frequencies.

This is why we need to know the frequency of your signal in order to interpret the results.

Depending on the actual frequency, your lower than expected output voltage might well be correct.

Get back to the point where you are measuring a lower than expected gain, then change the capacitor to 10 uF and see what happens.

Bob
 

MrChips

Joined Oct 2, 2009
31,067
There are a number of items that you need to be aware of and be able to check.
Firstly, be aware of the meaning of GND.

Your oscillosope is referenced to GND.
We don't know about your signal generator but you can check this.
I can see that your power supply appears to be "floating".

How to check your GND

Firstly, disconnect the PSU (power supply) completely from your circuit.
Touch the oscilloscope probe to the - terminal of the PSU. You should see AC LINE frequency.
Touch the oscilloscope probe to the + terminal of the PSU. You should see AC LINE frequency.

Connect the oscilloscope probe to the + terminal of the PSU and the ground clip of the probe to the - terminal.
You should see the PSU output voltage on the scope.
Your PSU is floating.

This means that the center connection on voltage divider splitter can be connected to GND of the oscilloscope.
Use this as your 0V (GND) reference in all your circuit designs.

Connect the BLACK wire from your signal source to this GND.

You can also check for continuity on all your GND connections with an ohmmeter.
Check for continuity between the GND post on your PSU and the ground clip of the oscilloscope probe.
Do the same with the BLACK wire from your signal source with respect to a GND that has been confirmed.

Edit: I will show a new circuit schematic later.
 

Audioguru again

Joined Oct 21, 2019
6,782
I disagree with having two grounds.
The "virtual ground" is a simple DC voltage reference, not a ground.
When the power supply ground is the same as the circuit's ground, signal generator ground and 'scope ground then the scope will show the DC and AC output voltages.
 

MrChips

Joined Oct 2, 2009
31,067
I disagree with having two grounds.
The "virtual ground" is a simple DC voltage reference, not a ground.
When the power supply ground is the same as the circuit's ground, signal generator ground and 'scope ground then the scope will show the DC and AC output voltages.
I never said anything about two grounds.
I will show a circuit schematic later.
 

MrChips

Joined Oct 2, 2009
31,067
The first thing to recognize is the fact that you have a floating single voltage PSU.
We want to turn this into a dual voltage (split rail) supply.
Notice that the halfway voltage reference point is now tied to earth GND.

Do not do this if your power supply is not floating.

I will show how the rest of the circuit is to be connected.
Split PSU.jpg
 

BobTPH

Joined Jun 5, 2013
9,256
How about sticking with one circuit until he gets it to work. Part of the problem here is multiple posters pushing in different directions and confusing him.

Bob
 

MrChips

Joined Oct 2, 2009
31,067
The problem is TS has to first understand GND.

Once you understand how the split power supply works you are less likely to get your grounds mixed up.
All grounds are connected and signals and voltages are referenced to earth ground. There is no second GND in this circuit.

For a stiffer split power supply I would change R1 and R2 to 1kΩ.

Opamp test circuit.jpg
 

Audioguru again

Joined Oct 21, 2019
6,782
The first thing to recognize is the fact that you have a floating single voltage PSU.
We want to turn this into a dual voltage (split rail) supply.
Notice that the halfway voltage reference point is now tied to earth GND.

Do not do this if your power supply is not floating.

I will show how the rest of the circuit is to be connected.
View attachment 225239
I knew that you would add a second battery or power supply!:rolleyes:
 

Audioguru again

Joined Oct 21, 2019
6,782
Instead of connecting together all the grounds like I did and biasing the very low current + input of the opamp with a "virtual ground", you added a virtual negative battery.
 

sarahMCML

Joined May 11, 2019
416
As MrChips has shown in #29, and as I remarked sometime earlier, the standard method of connecting 'grounds' in circuits using 'psuedo' positive and negative supplies is by connecting all grounds of test devices to the virtual zero point. In this case, the common point between R1 and R2.
This shows that a signal going more positive than zero goes negative of zero on the output. For an inverting circuit such as this, of course!
It removes any confusion when measuring signals, especially DC ones.
 

MrChips

Joined Oct 2, 2009
31,067
It is all relative what you choose to be COMMON. For newcomers this can be confusing.
It is best to keep it simple by making earth GROUND your reference point.
 

MrChips

Joined Oct 2, 2009
31,067
Instead of connecting together all the grounds like I did and biasing the very low current + input of the opamp with a "virtual ground", you added a virtual negative battery.
I did not add a virtual negative battery.
There is only one 18V battery or voltage source.

In your circuit you put the 18V on top of 0V. You can reference the 18V to any voltage you choose.
I chose to put my reference point halfway along 18V.

You know all about this so why are you so persistent in being argumentative?
 

Audioguru again

Joined Oct 21, 2019
6,782
Hi MrChips,
Your circuit with its "grounds" at the virtual ground needs a very stiff high current through the voltage divider.
Mine has an extremely low opamp input current (Jfet opamp inputs) then high value divider resistors using very a low current produce an extremely stiff reference voltage.
 

MrChips

Joined Oct 2, 2009
31,067
Hi MrChips,
Your circuit with its "grounds" at the virtual ground needs a very stiff high current through the voltage divider.
Mine has an extremely low opamp input current (Jfet opamp inputs) then high value divider resistors using very a low current produce an extremely stiff reference voltage.
What are you going on about?
As far as I can see the current in your circuit is the same as mine.
 

Thread Starter

doubledutch1962

Joined Oct 27, 2020
37
Dear People - please don't fall out with each other whilst helping me :->

I am pleased to report that my OpAmp test circuit now seems to work and I now get more or less predictable amplification. I believe there were a couple of key things I had to learn, most of which became clear through @Audioguru again post #17 above:

1) Messy wiring/noise: I never appreciated how much impact that had on what I was trying to do.
2) Combine (1) with the wrong type of capacitors. Changing them to Electrolytic made a huge difference
3) I'm still not 100% sure about this but will try to work that out myself: Post 17 talks about the inductance of my C2 capacitor combining that to my 5K R3 resistor, I think really messed up the Amplification ratio. So when I significantly increased the value of R4 to 500k things became a lot more obvious and I could see real (although clipped) amplification.

For those in a similar boat like me, post #17 also made me investigate further what you need to consider when using OpAmps to build an Audio circuit and I found this post which I thought was terrific and again thought me a lot.

Finally: I realized what a huge knowledge gap there is between me and the other people on this forum and that must be extremely frustrating for some people. Apologies: I'm trying to learn something completely new and some of the comments on this post actually make me wonder if people like me should post here. Most replies, however, have been unbelievably helpful and I thank you for that. I learned a lot.

Thank you and I will continue to watch this post with interest.
 

Ian0

Joined Aug 7, 2020
10,249
It's not the inductance of your C2 that was the problem.
Have you come across those piezo transducers, than you can use as speakers or microphones? https://www.rapidonline.com/kepo-ft-35t-2-6a1-475-piezo-element-300-ohm-2-6-0-5-khz-35mm-51-7477
Some ceramic capacitors are made out of the same stuff, so they turn vibrations into signals.

Not all ceramic capacitors are made the same way - those called "NP0" or "COG" are actually very good for audio, but they are much more expensive. But avoid the rest!
 

BobTPH

Joined Jun 5, 2013
9,256
The entire problem was the too small coupling capacitor and its affect on the gain. Everything else only created confusion.

Bob
 
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