Beginners OpAmp question

Thread Starter

doubledutch1962

Joined Oct 27, 2020
28
For the last day or so I'm been trying, without success to get the below circuit to work which I copied from this YouTube tutorial. He uses an TL072 which I don't have so I tried to use a TL074CN instead. When the circuit is not powered up, Scope Channel 1 shows the 3V square wave, and Scope Channel 2 shows about 1.5V Square wave. When the circuit is powered on: Scope Channel 2 becomes a steady voltage of about 18v.

Clearly I'm doing something wrong but I can't for the life of me figure out what it is. The pin 4 / 11 may seem the wrong way around but I'm assured that VCC is at the bottom of this chip and Gnd at the top - which is unusual. Any thoughts would be greatly appreciated.

2020-12-07_19-22-43.png
 

dl324

Joined Mar 30, 2015
12,255
When the circuit is not powered up, Scope Channel 1 shows the 3V square wave, and Scope Channel 2 shows about 1.5V Square wave. When the circuit is powered on: Scope Channel 2 becomes a steady voltage of about 18v.
Any readings with the circuit not powered are probably something you're picking up from an antenna effect.
I haven't found YouTube to be a good source for reliable information. The videos by college professors teaching are decent, but many so-called experts aren't really as expert as they think they are.

I couldn't get past the first 60 seconds. Graphics were horrible and I found the hairy hands very distracting.
 

Thread Starter

doubledutch1962

Joined Oct 27, 2020
28
Simple procedure that often overlooked check the pinouts they should be pin for pin identical
I thought the TL074 was effectively the same but contains 4 individual OpAmps whilst the TL072 only contains two. So my understanding is that Pin 1,2,3 are identical but on the TL072 Pin 4 is VCC- whilst on the TL074 it's VCC+ so I reversed that in my circuit so the 12V+ goes to Pin 4 and Gnd goes to pin 11.
 

upand_at_them

Joined May 15, 2010
631
*Most* YouTube videos are junk. This guy says "virtual ground", but is setting the non-inverting input at 1/2 supply. Find tutorials by someone who actually knows what they're doing.

Quick hint: The op-amp tries to set the output such that the inputs are the same. One input is clearly set at 1/2 supply.
 

BobTPH

Joined Jun 5, 2013
2,745
Yep, that is exactly what it should show. Can you figure out why?

Hint: Put a ground symbol in your schematic and measure all voltage from it.

Bob
 

MrChips

Joined Oct 2, 2009
22,531
What signal generator are you using?
Is the signal generator grounded?

You don't show a ground connection in the circuit. Hence your circuit would be floating unless the power supply also has a ground reference somewhere.

Try coupling the output of the signal generator via a series capacitor to the input resistor.
 

Thread Starter

doubledutch1962

Joined Oct 27, 2020
28
Hint: Put a ground symbol in your schematic and measure all voltage from it
Apologies that's just the way I drew it the circuit for this post. The top-rail is the + of my desktop powersupply, the bottom rail is the - (Gnd) of my desktop power supply and I'm referencing everything from that bottom rail. That's correct isn't it?
 

Thread Starter

doubledutch1962

Joined Oct 27, 2020
28
What signal generator are you using?
Is the signal generator grounded?

You don't show a ground connection in the circuit. Hence your circuit would be floating unless the power supply also has a ground reference somewhere.

Try coupling the output of the signal generator via a series capacitor to the input resistor.
I'm using a FeelTech Dual Channel Function / Arbitrary waveform generator. Something relatively cheap from Amazon. As per my above reply - the bottom rail is actually connected to the - on my desktop supply and I reference everything to that. I should have drawn that better. Apologies.
 

BobTPH

Joined Jun 5, 2013
2,745
That is the question, is it?

Your circuit is an inverting amplifier, which will invert a signal around 0V. But where in your circuit is 0V. Is it the bottom rail?

Bob
 

BobTPH

Joined Jun 5, 2013
2,745
@MrChips. I am not saying there is no common ground. I am saying that the circuit is erroneous because of what he is assuming is ground. He shows the signal generator negative connected to the bottom rail.

Bob
 

MrChips

Joined Oct 2, 2009
22,531
Well there is your problem. You need to pay attention to what is your reference voltage.

Your non-inverting input is connected to a reference voltage at ½Vsupply.
You need to bias the inverting input and the input signal to this voltage.
 

Audioguru again

Joined Oct 21, 2019
2,405
When the circuit is powered on: Scope Channel 2 becomes a steady voltage of about 18v.
The datasheet shows that the output cannot go higher than about 17V in your circuit.

The pin 4 / 11 may seem the wrong way around but I'm assured that VCC is at the bottom of this chip and Gnd at the top.
Top? Bottom? Pin 4 is for the positive supply (Vcc) and pin 11 is for the negative supply (Vss).

Your circuit does not work because it has no input coupling capacitor and your DC-connected signal generator has its ground at the opamp's negative supply. Then the opamp inverts the negative DC causing the output to go as positive as it can.
The negative supply IS NOT GROUND! The negative supply is ground when an input coupling capacitor is used.

View attachment 224375
[/QUOTE]
 

Attachments

WBahn

Joined Mar 31, 2012
26,398
For the last day or so I'm been trying, without success to get the below circuit to work which I copied from this YouTube tutorial. He uses an TL072 which I don't have so I tried to use a TL074CN instead. When the circuit is not powered up, Scope Channel 1 shows the 3V square wave, and Scope Channel 2 shows about 1.5V Square wave. When the circuit is powered on: Scope Channel 2 becomes a steady voltage of about 18v.

Clearly I'm doing something wrong but I can't for the life of me figure out what it is. The pin 4 / 11 may seem the wrong way around but I'm assured that VCC is at the bottom of this chip and Gnd at the top - which is unusual. Any thoughts would be greatly appreciated.

View attachment 224375
Do a DC analysis of what you expect the voltage at your Scope Channel 2 to be (relative to the negative side of the 18 V supply) as a function of the voltage at your Scope Channel 1 node (again relative to the negative side of the 18 V supply).

Assume that your opamp is capable of reaching either of it's supply rails.

What is the voltage at Scope Channel 1 at which the voltage at Score Channel two just reaches 0 V.

What is the voltage at Scope Channel 1 at which the voltage at Score Channel two just reaches 18 V.

Is ANY part of your input square wave within this voltage range?

If not, what do you expect the output voltage to be?
 

dl324

Joined Mar 30, 2015
12,255
Not sure what you mean by that but you have given me a lot of stuff to think about / work with.
Most will have a greater sense of satisfaction if they're guided to a solution instead of being handed the answer. By making you think about it, it should increase your understanding of opamps. In some cases, the person trying to guide you also learns something in the process.

Give a man a fish, and you feed him for a day. Teach a man to fish, and you feed him for a lifetime...
 

BobTPH

Joined Jun 5, 2013
2,745
Give a man a fish, and you feed him for a day. Teach a man to fish, and you feed him for a lifetime...
Or the other point of view: Give me a fish and I will eat for a day. Teach me how to fish, and I have to work the rest of my life.

Bob
 
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