# Lets take a look on a linear amplifier

1. ## Lets take a look on a linear amplifier

Hi there,

Hope my post find you well.
I like to get attention the knowledgeble person in this post !
So, kindly avoid this post if you wont have experience.

I wanna post a circuit that may be explained by Dr. Kolar, a famous Swiss or Austrian proffesor.

I need to know this application well, because I want to develope a 3-phase-calibrator.
My application needs to amplify signal to a certain level. Class D switching is necessary!

Here we go ....
Explain kindly

1. Why we need to think SNR for this case?
2. Why we need to think frequency gain voltage booster? Loop gain!
3. Explain impedences, transcondactance etc for whole circuit.
4. How the voltage and current is being controlled here?

Thanks a lot!

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2. ## Re: Lets take a look on a linear amplifier

Originally Posted by Hasan2017
Hope my post find you well.
Well.. it found.
Originally Posted by Hasan2017
I need to know this application well, because I want to develope a 3-phase-calibrator.
A what? A 3-phase calibrator? Like this one: https://us.flukecal.com/products/ele...wer-calibrator ??
Originally Posted by Hasan2017
Class D switching is necessary!
Then go with a class D circuit. You posted a class B/AB amplifier.
Originally Posted by Hasan2017
1. Why we need to think SNR for this case?
Until the application is not clear nobody will answer this to you.
Originally Posted by Hasan2017
2. Why we need to think frequency gain voltage booster? Loop gain!
Do you mean "why we need loop gain" ? I guess you should learn fundamentals from a book, not go forward with a complex circuit like above.
Originally Posted by Hasan2017
3. Explain impedences, transcondactance etc for whole circuit.
No way. There are too many. Input/output impedances, open/closed loop node impedances, mosfet/bjt transconductances/impedances... Open a book rather, please.
Originally Posted by Hasan2017
4. How the voltage and current is being controlled here?
Hmm, which???

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3. ## Re: Lets take a look on a linear amplifier

Mr. Frankrose,

Yes, people said "its not Class D, it might be Class AB. Its may be 200W.
Nothing Class D about it, but you can of course feed it with a square wave signal if you like.

Power stage quiescent current is set with the "~10 V" voltage in the middle (which should be thermally coupled/compensated with the output MOSFETs)p.

My current design might be class AB type.
I am not worried to much about PWM generation ruther I am thinking how can I move class AB to Class D amplification.
A lot of the design effort relates to maintaining linearity and avoiding crossover distortion.

I am not going to run a loud speaker, yes its same as your link!
2 amplifiers output will feed to CT and PT! Lets consider load will be connected in their output!

For CT, current transformer primary winding has 80turns, 16.5 V/1.5 A, secondary has 20Turns, 4.125v/6A output!
For PT, Potential transformer, Primary has 80turns, 15v/2A rating input, secondary has centertap!
At secondary 960turns, 360v/0.332 A and 960turns 180v/0.166A output.

STM32 control board is used for making suitable signal to OPA551.

1. The amp input is plus and minus 7volt AC, 45-65Hz( 1 Hz has been converted to 1024)
2. DAC 8812 has used in input control board.
3. 2 channel signal (not sure PWM) has made, one for current, one for voltage.
4. PT output may be 15VAC.

I am using TIP147, TIP142, C1815, A1015. They are properly biasd like cascaded 3 stage, may be AB class network. More symmetric way, like one pairs AB class output is connected to others pair. Last pairs base is connected to first pairs base! At last a voltage divider of 5 ohm has been made between 2nd and 3rd outputs!

Now lets come to the point, I am talking about the feedback amplifier

1. My voltage and current amp circuit is slightly different.

2. In voltage amp, at OPA551, input signal has feed with reference to "ground". A series RC element like 1k, 1uF after that a 220k (one end grounded) are connected to " non- inverting" input. Also a 220k one end ground is conneted to"inverting" input. A parallel RC element 270k, 0.33uF(may be) is connected to feedback path. A10 ohm, 0.1 uF RC element in series also appears in the output of the amp! Output signal also has ground reference.

3. In current amp, at OPA551, input signal is feed with reference to " ground". A series RC element like 13k, 1uF after that a 1k ( one end grounded) are connected to " non inverting" input. Also a 1k single ended gound is also conneted to "inverting" input. A parallel RC element (4.7k and 47uF) is connected to feedback path. A 10 ohm, 0.1 uF RC element in series are also appears between the output of the amp! In this case 2 shunt element 5W 0.5 ohm are connected and grounded in output side. No reference ground output here!

4. ## Re: Lets take a look on a linear amplifier

Your amplifiers are class-AB that waste a lot of power by making heat. The circuit in a class-D amplifier switches on and off at a high frequency with PWM then produce a low amount of heat.
Class-D amplifier ICs have been available for 23 years and Texas Instruments make some ICs with an output power of hundreds of watts.
Here is what is in a class-D amplifier:

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5. ## Re: Lets take a look on a linear amplifier

There may be good reasons to implement a linear (class AB) amplifier for your purposes, I don't apply to decide yes or no.

You have implicitly specified amplifier output voltage and current by mentioning measurement transformers to be connected to it. I'm not quite sure what you are exactly asking in this thread, and I don't understand the relation between post #1 and post #3 schematics. You may consider a feedback topology that includes the transformer, increasing complexity by involving separate DC and AC paths but potentially increasing output accuracy and signal linearity a lot.

6. ## Re: Lets take a look on a linear amplifier

Originally Posted by Hasan2017
For CT, current transformer primary winding has 80turns, 16.5 V/1.5 A, secondary has 20Turns, 4.125v/6A output!
For PT, Potential transformer, Primary has 80turns, 15v/2A rating input, secondary has centertap!
At secondary 960turns, 360v/0.332 A and 960turns 180v/0.166A output.
Not clear. Centertap has 40turns if you have an autotransformer with 80turns. What is the 960turns?
Originally Posted by Hasan2017
I am using TIP147, TIP142, C1815, A1015. They are properly biasd like cascaded 3 stage, may be AB class network. More symmetric way, like one pairs AB class output is connected to others pair. Last pairs base is connected to first pairs base! At last a voltage divider of 5 ohm has been made between 2nd and 3rd outputs!

On both schematic you connect low impedance control signals to transistor collectors. Normally outputs are not connected together, I don't get it why you doing that.
Actually a lot of information is missing, I think you assume we know what did you do from the beginings when you have started your project, but we have no idea for example why did you change circuit as FvM mentioned, so please share more intels (system level block diagramms, how you imagined this project, what are the targets, specifications, why did you decide next to the last schematics, etc.).

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7. ## Re: Lets take a look on a linear amplifier

Originally Posted by Audioguru
Your amplifiers are class-AB that waste a lot of power by making heat. The circuit in a class-D amplifier switches on and off at a high frequency with PWM then produce a low amount of heat.
Class-D amplifier ICs have been available for 23 years and Texas Instruments make some ICs with an output power of hundreds of watts.
Here is what is in a class-D amplifier:
Once again I am blessed to find you here!
May be that technology need more effort to make sequencial PWM for firing the gates!
Here is some way to overcome "heating" and crossover distrortion!

8. ## Re: Lets take a look on a linear amplifier

Originally Posted by Hasan2017
Once again I am blessed to find you here!
May be that technology need more effort to make sequencial PWM for firing the gates!
Here is some way to overcome "heating" and crossover distrortion!
An amplifier does not "sequentially fire gates". Instead it simply amplifies the input signal's voltage and current.

A linear amplifier has a lot of heating but a class-D high frequency switching amplifier produces low heating.

A class-B amplifier and a class-AB amplifier biased wrong produces crossover distortion but a class-D amplifier does not.

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9. ## Re: Lets take a look on a linear amplifier

Originally Posted by Audioguru
An amplifier does not "sequentially fire gates". Instead it simply amplifies the input signal's voltage and current.

A linear amplifier has a lot of heating but a class-D high frequency switching amplifier produces low heating.

A class-B amplifier and a class-AB amplifier biased wrong produces crossover distortion but a class-D amplifier does not.
Yes. well said. Depending on switching topology may we need more.

10. ## Re: Lets take a look on a linear amplifier

Originally Posted by frankrose
Not clear. Centertap has 40turns if you have an autotransformer with 80turns. What is the 960turns?

On both schematic you connect low impedance control signals to transistor collectors. Normally outputs are not connected together, I don't get it why you doing that.
Actually a lot of information is missing, I think you assume we know what did you do from the beginings when you have started your project, but we have no idea for example why did you change circuit as FvM mentioned, so please share more intels (system level block diagramms, how you imagined this project, what are the targets, specifications, why did you decide next to the last schematics, etc.).
Just think the primary inputs should be the power rating out of class D amplifier. Consider the circuit protection for " high frequency impedance and oscillation for using transformer".

11. ## Re: Lets take a look on a linear amplifier

Low frequency and high frequency phase shifts caused by a transformer will occur if the transformer is inside the negative feedback loop (when the output of the transformer provides the negative feedback). Then the amplifier must be compensated for these phase shifts.

An audio amplifier is designed to have a load impedance that is 2 ohms to 16 ohms. What are the impedances of your loaded transformers?

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12. ## Re: Lets take a look on a linear amplifier

Originally Posted by Audioguru
Low frequency and high frequency phase shifts caused by a transformer will occur if the transformer is inside the negative feedback loop (when the output of the transformer provides the negative feedback). Then the amplifier must be compensated for these phase shifts.

An audio amplifier is designed to have a load impedance that is 2 ohms to 16 ohms. What are the impedances of your loaded transformers?
People also curious about" core saturation" effect.
"A product with a Class-D audio power amplifier (APA) driving an output transformer with inadequate
low-frequency performance may shut down when its output is stepped from zero to maximum at the start
of a sine cycle. Shutdown is triggered by short circuit protection (SCP), after the first half cycle of the sine
output. The root cause is saturation of the transformer core."

If you look back in feedback amplifier diagram ( may not clear to you), you would see 10 ohm has connected. Let me study it again. CT output has taken from " Hall sensor"

13. ## Re: Lets take a look on a linear amplifier

You are using an audio amplifier circuit in another kind of circuit that has output transformers. But audio amplifier circuits have not used output transformers for at least 55 years.
If the core of an audio amplifier output transformer saturates then the transformer's current rating is too low.

14. ## Re: Lets take a look on a linear amplifier

But audio amplifier circuits have not used output transformers for at least 55 years
Really low ! But need a range for idea.
If you read all information you could see,
360v/0.332 A and 180v/0.166A outputs are from PT.
4.125v/6A output is available from CT.

Then why dont we look for other type amplifier?

15. ## Re: Lets take a look on a linear amplifier

Hi,

If the core of an audio amplifier output transformer saturates then the transformer's current rating is too low.
In my eyes saturation of a transformer has nothing to do with current rating.
Saturation depends on voltage rating and frequency rating.

Transformers are very sensitive on DC at the input. Only a very small amount of DC may cause a transformer to saturate.

Klaus

16. ## Re: Lets take a look on a linear amplifier

When the signal current is at a peak then it is probably much higher current than a little DC offset current. At 55Hz then the peaks occur at 110Hz.

17. ## Re: Lets take a look on a linear amplifier

Originally Posted by KlausST
Hi,

In my eyes saturation of a transformer has nothing to do with current rating.
Saturation depends on voltage rating and frequency rating.

Transformers are very sensitive on DC at the input. Only a very small amount of DC may cause a transformer to saturate.

Klaus
Nice feedback indeed.
Could you suggest me an article for this issue?

18. ## Re: Lets take a look on a linear amplifier

Hi,

I don't know which information you are looking for.
Thus I suggest an internet search:
https://lmgtfy.com/?q=cause+of+trans...ore+saturation

Klaus

19. ## Re: Lets take a look on a linear amplifier

Originally Posted by KlausST
Hi,

I don't know which information you are looking for.
Thus I suggest an internet search:
https://lmgtfy.com/?q=cause+of+trans...ore+saturation

Klaus
Dont you think for this kind of low frequency 60Hz transformer case ClassD type is useless?
As you have said " voltage facts" ruther than low current!

20. ## Re: Lets take a look on a linear amplifier

Hi,

I did low frequency (30..70Hz) sinwave via classD, then filtering, then transformer to get about 200V RMS as measurement signal.
It worked great, about no heating, high efficiency, good signal quality.

Klaus

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