Wien Bridge Oscillator Op Amp Lamp Stabilized; Crossover Distortion with TL072 Not NE5532

Thread Starter

brockrwood

Joined Oct 23, 2016
47
I built a wien bridge audio frequency oscillator using an op amp, a lamp for automatic gain control, and the required matched pairs of plastic film capacitors and resistors for the "bridge". I used a workbench, regulated power supply for the + and - 9V DC to the circuit.

Here is the schematic:

wien_bridge_op_amp_experiment_rev_02.jpg

Here is a picture of the circuit on the breadboard:

opamp_lamp_wien_bridge_oscillator_on_breadboard.jpg

(Yes, I am pretty proud of the soldering job on the lamp. ;) )

The potentiometer on the right was not hooked up when I took the picture. When listening to the oscillator, I connected the pot to the output of the oscillator. I then fed the output of the pot to a small pair of self-powered computer speakers to listen.

(Sorry about the massive, 1,000 uF, 50V power supply filter caps. I didn't have any 1,000 uF caps handy that had a lower rated voltage.)

I tried both a TL072 op amp with JFET inputs as well as an NE5532 op amp with regular BJT inputs. (I only used one op amp on each chip.)

Each op amp produced a nice sine wave of 440 Hz.

The TL072, however, had a small amount of crossover distortion in its output. The NE5532 did not.

Here is the oscilloscope showing the output of the TL072:

tl072_sine_wave_crossover_distortion.jpg

The crossover distortion is small, but noticeable, right about on the zero crossing line. It is audible, I think. My hearing is not what it used to be, but I think I detect a small amount of audible distortion.

Here is the sine wave of the NE5532 shown on the oscilloscope:

ne5532_sine_wave_no_crossover_distortion.jpg

Nice and smooth. No crossover distortion. Also, when I listen I think I can detect a slight improvement in sound over the TL072. Again, it is subtle.

So, the question: Does the NE5532 have some sort of class AB output stage? It does not say so in the datasheet. Here's a link to the datasheet:

http://www.ti.com/lit/ds/symlink/ne5532.pdf

Here is the link to the TL072 datasheet:

http://www.ti.com/lit/ds/symlink/tl072.pdf

The TL072, according to the datasheet, has low THD and is optimized for "hi-fidelity and audio pre-amplifier applications". Not if there is crossover distortion on the output!

I read on the internet that connecting a resistor from the output to the -V power rail might get rid of the crossover distortion. Didn't work for me. I also tried adding another resistor from the output to the +V power rail. That didn't help either.

Is there any way to get rid of this crossover distortion when I use the TL072 or am I just stuck using the NE5532? I don't mind using the NE5532 but it bothers me that I don't understand why the TL072 has the crossover distortion but the NE5532 does not. o_O
 
Last edited:

Jony130

Joined Feb 17, 2009
5,089
Are you sure that you op-amp is TL072 (fake Ic)? Because this type of crossover distortion is common in Lm358/LM324 op amps.


And to fixe this add a resistor load ressitor between the opamp output and Vee.
 

Attachments

DickCappels

Joined Aug 21, 2008
6,005
It is probably a matter of how much crossover distortion you get rather whether you get any at all. Your experiment demonstrates that the superior NE5532 gives an acceptable amount of crossover in this circuit where the TL072 raises concerns. It is not much crossover distortion at all. If you have any TL062 opamps it would be interesting to see how they compare.
 

atferrari

Joined Jan 6, 2004
3,578
Your post
I built a wien bridge audio frequency oscillator using an op amp, a lamp for automatic gain control, and the required matched pairs of plastic film capacitors and resistors for the "bridge". I used a workbench, regulated power supply for the + and - 9V DC to the circuit.

Here is the schematic:

View attachment 174266

Here is a picture of the circuit on the breadboard:

View attachment 174267

(Yes, I am pretty proud of the soldering job on the lamp. ;) )

The potentiometer on the right was not hooked up when I took the picture. When listening to the oscillator, I connected the pot to the output of the oscillator. I then fed the output of the pot to a small pair of self-powered computer speakers to listen.

(Sorry about the massive, 1,000 uF, 50V power supply filter caps. I didn't have any 1,000 uF caps handy that had a lower rated voltage.)

I tried both a TL072 op amp with JFET inputs as well as an NE5532 op amp with regular BJT inputs. (I only used one op amp on each chip.)

Each op amp produced a nice sine wave of 440 Hz.

The TL072, however, had a small amount of crossover distortion in its output. The NE5532 did not.

Here is the oscilloscope showing the output of the TL072:

View attachment 174268

The crossover distortion is small, but noticeable, right about on the zero crossing line. It is audible, I think. My hearing is not what it used to be, but I think I detect a small amount of audible distortion.

Here is the sine wave of the NE5532 shown on the oscilloscope:

View attachment 174269

Nice and smooth. No crossover distortion. Also, when I listen I think I can detect a slight improvement in sound over the TL072. Again, it is subtle.

So, the question: Does the NE5532 have some sort of class AB output stage? It does not say so in the datasheet. Here's a link to the datasheet:

http://www.ti.com/lit/ds/symlink/ne5532.pdf

Here is the link to the TL072 datasheet:

http://www.ti.com/lit/ds/symlink/tl072.pdf

The TL072, according to the datasheet, has low THD and is optimized for "hi-fidelity and audio pre-amplifier applications". Not if there is crossover distortion on the output!

I read on the internet that connecting a resistor from the output to the -V power rail might get rid of the crossover distortion. Didn't work for me. I also tried adding another resistor from the output to the +V power rail. That didn't help either.

Is there any way to get rid of this crossover distortion when I use the TL072 or am I just stuck using the NE5532? I don't mind using the NE5532 but it bothers me that I don't understand why the TL072 has the crossover distortion but the NE5532 does not. o_O
Regarding the TL072, you took me in surprise, Used it intensively for so many projects (audio generators) and never noticed that distorsion. Intriguing really.
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
Are you sure that you op-amp is TL072 (fake Ic)? Because this type of crossover distortion is common in Lm358/LM324 op amps.


And to fixe this add a resistor load ressitor between the opamp output and Vee.
Thanks!

I tried the load resistor from the output to Vee (-V). It worked but I had to reduce the value of the resistor to a really low level. I used a 470 ohm resistor in series with a 10K pot wired as a variable resistor. I kept rotating the shaft of the pot until crossover distortion disappeared off of hte bottom of the wave form. The value of the pot was only 455 ohms when the crossover distortion finally disappeared off the bottom peak of the wave! That is a total load resistance of 925 ohms.

Here is a picture of the oscilloscope with the crossover distortion moved about half way towards the bottom of the wave:

tl072_sine_wave_crossover_distortion_moved_down_with_load_resistor.jpg

Here is another picture with the load resistance reduced even further. The crossover distortion is now close to the bottom of the wave:

tl072_sine_wave_crossover_distortion_almost_gone_with_load_resistor.jpg

With the load resistance (from output to Vee (-V)) set to 925 ohms, the crossover distortion disappears off the bottom of the wave form. All gone!

tl072_sine_wave_crossover_distortion_gone_with_load_resistor.jpg
So, based on what the narrator says in the video link you posted, I am adding a load to the output to bias the positive going output transistor into just staying on all the time, right? That means that the output transistor for the op amp for the negative going half of the wave does not come on at all? That would be class A output. Which would be very inefficient, correct?

For my purposes, I am not sure if I care if I force the output of the op amp into class A. The current going through the load resistance of 925 ohms would be, lessee, 2 volts AC rms divided by 925 ohms, or 2.16 mA through the load resistor. So the power dissipated would be 2 * .00216216 or 4.32 mW. That's not too bad but it would be a significant drain if the circuit were powered by batteries.

Is my analysis correct?

Here is the internal diagram from the Tl072 datasheet marked up by me showing what I think are the output transistors (a complementary pair in emitter follower configuration):

marked_up_tl072_datasheet_internal_diagram.jpg

If my analysis is correct, is there any downside to forcing the upper transistor of the op amp output to just stay on in class A mode while the lower transistor never turns on?
 

AnalogKid

Joined Aug 1, 2013
8,226
Note that the opamp is characterized at 2K ohm load or greater, while your load is 301 ohms plus the lamp. After the circuit has bee running for a bit, what is the lamp resistance if you measure it quickly after removing it from the circuit?

ak
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
Are you sure that you op-amp is TL072 (fake Ic)?
I am NOT sure if this TL072 is a fake IC or not. I got it at a very cheap price, in a bundle of about 20 TL072's, on ebay, direct from a seller in Asia. Could I have been sold a counterfeit TL072? It is certainly possible. Is there any way to tell if the TL072 is fake? It is marked with the TI logo and says "TL072CP". It also says "71MDBWQ". If it is a fake, the fake TI logo and labeling are well done!

I was thinking about this very issue today, of "counterfeit" op amps and other IC's being sold on ebay for very cheap prices. I wonder if some of the "counterfeit" IC's are not actually fake, but were really made in a TI-authorized factory in Asia. BUT, maybe some of the IC's being sold on ebay were REJECTS that did not meet the specifications and were not intended to be sold as genuine TI products. Is that possible?
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
Note that the opamp is characterized at 2K ohm load or greater, while your load is 301 ohms plus the lamp. After the circuit has bee running for a bit, what is the lamp resistance if you measure it quickly after removing it from the circuit?

ak
Let me check... 104.4 ohms.
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
Note that the opamp is characterized at 2K ohm load or greater, while your load is 301 ohms plus the lamp. After the circuit has bee running for a bit, what is the lamp resistance if you measure it quickly after removing it from the circuit?

ak
So, with no load connected to the output, the only "load" on the output is the lamp plus the trimmer pot of the gain control part of the circuit, for a total of 405.4 ohms? Is that the correct analysis? Are you saying that the crossover distortion might be showing up on the oscilloscope because I need to connect a load resistance of at least 2K ohms to the output of the op amp?
 
Last edited:

Jony130

Joined Feb 17, 2009
5,089
I am adding a load to the output to bias the positive going output transistor into just staying on all the time, right? That means that the output transistor for the op amp for the negative going half of the wave does not come on at all? That would be class A output. Which would be very inefficient, correct?
You summed it up very well.

For my purposes, I am not sure if I care if I force the output of the op amp into class A. The current going through the load resistance of 925 ohms would be, lessee, 2 volts AC rms divided by 925 ohms, or 2.16 mA through the load resistor. So the power dissipated would be 2 * .00216216 or 4.32 mW. That's not too bad but it would be a significant drain if the circuit were powered by batteries.

Is my analysis correct?
Yes.

Is there any way to tell if the TL072 is fake?
Try to check the input common mode voltage range. And if memory serves me correctly in TL072 you should see the phase inversion phenomenon if you violating the input common-mode range (input close to Vee). And you do not see this effect on LM358/LM324.





BUT, maybe some of the IC's being sold on ebay were REJECTS that did not meet the specifications and were not intended to be sold as genuine TI products. Is that possible?
This may be the case also. My TL072 don't show this type of distortion.
 

Jony130

Joined Feb 17, 2009
5,089
Today I tested the input common mode voltage range for LM358 and TL072.

The test setup:

Opamp_0.1.png

Input signal comes form function generator, F = 1kHz and the amplitude is 10V peak to peak. All Opamp are supplied from symmetrical supply +/-5V.
So we can easily exceed the allowed input common mode voltage.

First the TL072 form ST:

Opamp_0.2.png

As you can see in the TL072 phase inversion phenomenon exist.

In LM358 from ST we have this situation:

Opamp_0.3.png

As you can see no phase inversion only clipping for Vin > Vcc - 1.4V. And no crossover distortion due to lack of load current.

For the NE5532 from Philips

Opamp_0.4.png


As for the crossover distortion. For the LM358 the crossover distortion is clearly visible

Opamp_0.6.png

But no sign of visible crossover distortions in the TL072.
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
Today I tested the input common mode voltage range for LM358 and TL072.

The test setup:

View attachment 174367

Input signal comes form function generator, F = 1kHz and the amplitude is 10V peak to peak. All Opamp are supplied from symmetrical supply +/-5V.
So we can easily exceed the allowed input common mode voltage.

First the TL072 form ST:

View attachment 174369

As you can see in the TL072 phase inversion phenomenon exist.

In LM358 from ST we have this situation:

View attachment 174370

As you can see no phase inversion only clipping for Vin > Vcc - 1.4V. And no crossover distortion due to lack of load current.

For the NE5532 from Philips

View attachment 174372


As for the crossover distortion. For the LM358 the crossover distortion is clearly visible

View attachment 174373

But no sign of visible crossover distortions in the TL072.
Wow! This is great! Thank you! I think it is time to buy some op amps that AREN’T 10 for a dollar!
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
But no sign of visible crossover distortions in the TL072.
OK, I just ordered 10 TL072's, 10 LM358's, and 10 NE5532's (Texas Instruments) from Arrow. Free overnight shipping. Who knew? Great deal. I will test these against the chips I got on ebay and see how they compare.

Arrow's free overnight shipping (FedEx) in the USA is hard to pass up.
 

KL7AJ

Joined Nov 4, 2008
2,224
I built a wien bridge audio frequency oscillator using an op amp, a lamp for automatic gain control, and the required matched pairs of plastic film capacitors and resistors for the "bridge". I used a workbench, regulated power supply for the + and - 9V DC to the circuit.

Here is the schematic:

View attachment 174266

Here is a picture of the circuit on the breadboard:

View attachment 174267

(Yes, I am pretty proud of the soldering job on the lamp. ;) )

The potentiometer on the right was not hooked up when I took the picture. When listening to the oscillator, I connected the pot to the output of the oscillator. I then fed the output of the pot to a small pair of self-powered computer speakers to listen.

(Sorry about the massive, 1,000 uF, 50V power supply filter caps. I didn't have any 1,000 uF caps handy that had a lower rated voltage.)

I tried both a TL072 op amp with JFET inputs as well as an NE5532 op amp with regular BJT inputs. (I only used one op amp on each chip.)

Each op amp produced a nice sine wave of 440 Hz.

The TL072, however, had a small amount of crossover distortion in its output. The NE5532 did not.

Here is the oscilloscope showing the output of the TL072:

View attachment 174268

The crossover distortion is small, but noticeable, right about on the zero crossing line. It is audible, I think. My hearing is not what it used to be, but I think I detect a small amount of audible distortion.

Here is the sine wave of the NE5532 shown on the oscilloscope:

View attachment 174269

Nice and smooth. No crossover distortion. Also, when I listen I think I can detect a slight improvement in sound over the TL072. Again, it is subtle.

So, the question: Does the NE5532 have some sort of class AB output stage? It does not say so in the datasheet. Here's a link to the datasheet:

http://www.ti.com/lit/ds/symlink/ne5532.pdf

Here is the link to the TL072 datasheet:

http://www.ti.com/lit/ds/symlink/tl072.pdf

The TL072, according to the datasheet, has low THD and is optimized for "hi-fidelity and audio pre-amplifier applications". Not if there is crossover distortion on the output!

I read on the internet that connecting a resistor from the output to the -V power rail might get rid of the crossover distortion. Didn't work for me. I also tried adding another resistor from the output to the +V power rail. That didn't help either.

Is there any way to get rid of this crossover distortion when I use the TL072 or am I just stuck using the NE5532? I don't mind using the NE5532 but it bothers me that I don't understand why the TL072 has the crossover distortion but the NE5532 does not. o_O
You may reduce it by running slightly higher rail voltage.
 

Thread Starter

brockrwood

Joined Oct 23, 2016
47
I think that generally, the crap shoot of buying cheap, possibly counterfeit parts from China is likely not worth the small savings in cost for a hobbyist. :rolleyes:
Agreed.

It is hard enough when you are trying to learn the principles of a new circuit, as a hobbyist, without having to worry about whether you are using fake parts that don't work right. How do you know if you are not understanding the circuit or if you *are* understanding the circuit but a fake part is not working as it should? Not worth the headache.

I just did not know that the problem of fake parts was so pervasive a problem.

Here’s a problem: I want to buy some TDA7052A’s (audio amp IC). The chips are no longer made and the big electronics distributors in the USA no longer carry them. The vendors on ebay, most of them in China, are my only source for the TDA7052A. For that part I have no choice but to throw the crap shoot dice and hope for 7 or 11!
 
Last edited:
Top