# Lets take a look on a linear amplifier

#### rjenkinsgb

##### Well-Known Member
you can't CONTROL both at the same time, which was what you asked for.
If you wish a smart microcontroller can do!
No it can not!

For a given load resistance, the relationship between voltage and current is given by Ohms Law.
That is a simple fact and nothing can change it.

It's one of the most fundamental principles of electronics.

I = V / R
V = I * R
R = V / I

They are inseparable; that's why it is called a "law" of physics, the relationship is mathematically defined.

And you changed the subject rather than answer the question:

Yet again:
You need to say exactly what you are trying to achieve and what load you are intending to drive, what frequency response range, and at what power or voltage / current levels!

As I see it, there i no point continuing this thread unless you fully answer that question.

#### MD MUBDIUL HASAN

##### Member
No it can not!

For a given load resistance, the relationship between voltage and current is given by Ohms Law.
That is a simple fact and nothing can change it.

It's one of the most fundamental principles of electronics.

I = V / R
V = I * R
R = V / I

They are inseparable; that's why it is called a "law" of physics, the relationship is mathematically defined.

And you changed the subject rather than answer the question:

Yet again:
You need to say exactly what you are trying to achieve and what load you are intending to drive, what frequency response range, and at what power or voltage / current levels!

As I see it, there i no point continuing this thread unless you fully answer that question.
Let me go for some paper works.
Once I finish, I will post here.

#### MD MUBDIUL HASAN

##### Member
Please attach the schematic with parts values of the circuit you are talking about.
Okay, my circuit looks like (to you) for school going student's home work.
Take a look on attachment.
Discuss it for more knowledge!

Looks like in both case "current feedback amplifier" is used!

Last edited:

#### audioguru

##### Well-Known Member
Why do you show two amplifiers? An amplifier amplifies BOTH the voltage AND the current.

Your circuits have these serious problems:
1) The little 2SC1815 and little 2SA1015 transistors have current and power ratings much too small to be the output transistors of an audio power amplifier.
2) The output transistors should have their bases (not collectors) connected to the diodes.
3) You wrongly show ordinary diodes trying to limit the power supply voltages to the opamp instead of 27V Zener diodes.
4) Your 1st amplifier has a voltage gain too small at only 1+ (270k/220k)= 2.23 times.
5) Your 2nd amplifier has a voltage gain much too small at only 1/14 x [1+ (4.7k/1k)]= 0.41 times without a load and less when there is a load.
6) The series 0.5 ohm resistors are not used in in series with a speaker. They should be in series with the emitters of the output transistors.
7) The four diodes might be used if the output transistors are power darlingtons but you do not show darlingtons.
8) Your 472 capacitor parallel with the 4.7k negative feedback resistor cuts all high frequencies at 7240Hz and above.

#### unclejed613

##### Well-Known Member
At this time TIP147, TIP142 shows steady state.
It succeed the aging test. But, at higher level of feedback amplifiers input, it shows high heat!
you need thermal compensation to control the idle current in the output stage. silicon transistors have a negative temperature coefficient, so the Vbe is reduced 2.2mV for every deg C of temperature increase. if the bias voltage remains the same, this turns on the transistor on harder, increasing the idle current, and increasing the temperature even more. the cure for this is a bias clamp, which acts like an adjustable zener diode. the transistor in this bias circuit is usually mounted on the heat sink between the output transistors so that it can compensate for the change in temperature by reducing the bias voltage of the output transistors.

#### rjenkinsgb

##### Well-Known Member
1: You do not give the complete schematic showing how the output transistors interconnect with the driver, power and the load.
It's impossible to know how the output will react to the driver stages without that.

2: If both drive circuits you show are connected to the same output circuit, one will have an overriding effect on the other and not provide separate and simultaneous control of voltage and current. As you have been repeatedly told, that would be impossible.

It's no different to a bench PSU that has voltage and current controls - each limits the other, they do not control simultaneously.

#### unclejed613

##### Well-Known Member
3) You wrongly show ordinary diodes trying to limit the power supply voltages to the opamp instead of 27V Zener diodes.
quite a few far east manufacturers use a standard diode symbol with a "B" next to it. the usual zener symbol isn't standardized everywhere. with +/-24V supply rails, they don't do anything anyway, unless +/-30V is planned later.

to OP: there have been a few amplifiers made over the years that combined current and voltage feedback, and provided a control that shifted the balance between the two. this basically made the output impedance of the amplifier controllable. the Yamaha amplifier that had this feature was variable from +8 ohms, to -4 ohms. many owners of these amplifiers either had a specific use in mind (such as driving loaded horns), or wanted something to experiment with.

#### MD MUBDIUL HASAN

##### Member
Why do you show two amplifiers? An amplifier amplifies BOTH the voltage AND the current.
May be your sharp eyes skip what I wanted to develop. I am not going to run a loud speaker.
2 amplifiers output will feed to CT and PT! Lets consider load will be connected in their output!
Cant you find a clue that , using single OPAMP an "unbalanced loading effect" could arise or impedance mismatching could cause failure.
Suggest an opamp for duel action then!

Your circuits have these serious problems:
1) The little 2SC1815 and little 2SA1015 transistors have current and power ratings much too small to be the output transistors of an audio power amplifier.
Even they are small power rating, these are helping to balance for the next stage. Cant you see voltage divider is used in each side!
2) The output transistors should have their bases (not collectors) connected to the diodes.
2SC1815 and little 2SA1015 has common emiter connection.

3) You wrongly show ordinary diodes trying to limit the power supply voltages to the opamp instead of 27V Zener diodes
.
Yes, mistake, suppose its zenerdiode's symbol.
4) Your 1st amplifier has a voltage gain too small at only 1+ (270k/220k)= 2.23 times.
Do you think, this amplifier is in following structure, then dont you think its enough?

5) Your 2nd amplifier has a voltage gain much too small at only 1/14 x [1+ (4.7k/1k)]= 0.41 times without a load and less when there is a load.
Interesting to see why this time gain equation is different.

6) The series 0.5 ohm resistors are not used in in series with a speaker. They should be in series with the emitters of the output transistors.
Imagine that a "CT" is used here in output.
7) The four diodes might be used if the output transistors are power darlingtons but you do not show darlingtons.
Yes, I told you before. Lets understand the this amplifier first!
8) Your 472 capacitor parallel with the 4.7k negative feedback resistor cuts all high frequencies at 7240Hz and above.
Right at this moment this capacitor is absent! its MC type. Why we use this? For high frequency attinuation?

#### rjenkinsgb

##### Well-Known Member
May be your sharp eyes skip what I wanted to develop.
We are not mind readers and you keep evading the question of exactly what you ARE trying to achieve.

I've had enough of the timewasting, subject closed as far as I am concerned.

#### MD MUBDIUL HASAN

##### Member
I've had enough of the timewasting, subject closed as far as I am concerned.
Kindly wait. Things are not easy to explain. As you have raised your voice for ClassD instead of ClassB, from your part keep your mind here! And also read whole post conversation carefully.

Right at this moment for your kind information ...

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 will be post all calculation in the feedback amplifier circuit!

#### audioguru

##### Well-Known Member
What is "CT"?
What is "PT"?
What is an "MC" capacitor?

#### MD MUBDIUL HASAN

##### Member
Thank you sir Audioguru.

What is "CT"?
Its current transformer. Primary winding has 80turns, 16.5 V/1.5 A, secondary has 20Turns, 4.125v/6A output!
What is "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.
What is an "MC" capacitor?
Multilayer Ceramic Chip Capacitor.

#### audioguru

##### Well-Known Member
I have never needed or used a current transformer and I have never needed or used a mains electricity inverter circuit.
58 years ago I built the only amplifier I made that used transformers. It used vacuum tubes. The transformers caused phase shifts.

#### MD MUBDIUL HASAN

##### Member
I have never needed or used a current transformer and I have never needed or used a mains electricity inverter circuit.
58 years ago I built the only amplifier I made that used transformers. It used vacuum tubes. The transformers caused phase shifts.
Again I salut you! Old is gold always!
Not necessay to know everything, need to be focused on a particular area.

Looking at your comment on posted feedback amplifier, could you tell me more about " current amplifier"?

#### audioguru

##### Well-Known Member
Your current amplifier input has a 13k resistor in series with a 1k resistor to ground then it attenuates the signal to 1/14th its original level. Why?
Why are you using transformers?

#### Nigel Goodwin

##### Super Moderator
Your current amplifier input has a 13k resistor in series with a 1k resistor to ground then it attenuates the signal to 1/14th its original level. Why?
Why are you using transformers?
Why bother AG? - you're assuming it's an audio amplifier, which it most probably isn't - but he refuses to say, or give any details worth knowing.

#### audioguru

##### Well-Known Member
He wrote "sinewave, 45Hz to 65Hz" which are audio signals.

#### Nigel Goodwin

##### Super Moderator
He wrote "sinewave, 45Hz to 65Hz" which are audio signals.
Have you ever seen an audio amplifier with a bandwidth of only 45-65Hz? - that fact alone means it's not an audio amplifier.

I can't help thinking it's going to the usual stupid over-unity or perpetual motion scheme, which just needs a little 'tweaking' to get it's efficiency from 50% to over 100%

#### audioguru

##### Well-Known Member
Hasan talks about using high power audio amplifiers on at least two other forums to make this thing:

"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 ??"