# Problem Amplitude

#### Tobias12

Joined May 25, 2019
25
Hello community ! I tried to adjust the amplitude of a pulse-width generator with an amplitude between 4 V and 8 V using a potentiommeter and resistances, but when k =0 the amplitude is 3.95 V instead of 4 V even if the values of components are well calculated. For k=1 i have no problem, the amplitude is 8 V. Which may be the problem ? What is the explanation for this ?

#### AlbertHall

Joined Jun 4, 2014
10,764
Schematic?
What is k?

#### MrChips

Joined Oct 2, 2009
22,462
Hello community ! I tried to adjust the amplitude of a pulse-width generator with an amplitude between 4 V and 8 V using a potentiommeter and resistances, but when k =0 the amplitude is 3.95 V instead of 4 V even if the values of components are well calculated. For k=1 i have no problem, the amplitude is 8 V. Which may be the problem ? What is the explanation for this ?
We have no idea.
What is your context? What is k? What potentiometer?

#### Tobias12

Joined May 25, 2019
25
We have no idea.
What is your context? What is k? What potentiometer?
That's the schematic. And k is SET, sorry.

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#### Tobias12

Joined May 25, 2019
25

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#### Papabravo

Joined Feb 24, 2006
14,829
The pot is connected to -5.4 and +10.81 Volts. How is that related to a pulse with an amplitude between 4V and 8V?
Don't the resistor tolerances factor into this?
You have SET on two different pots, which one are we talking about?

#### Tobias12

Joined May 25, 2019
25
The pot is connected to -5.4 and +10.81 Volts. How is that related to a pulse with an amplitude between 4V and 8V?
The adjustment of amplitude is made with last part of circuit where is the second pot. I think the op-amp is the problem. But I'm not sure. For SET=1 it works fine.

#### Papabravo

Joined Feb 24, 2006
14,829
The adjustment of amplitude is made with last part of circuit where is the second pot.
So why would 3.95 V be unexpected?

#### Tobias12

Joined May 25, 2019
25
So why would 3.95 V be unexpected?
It's relatively fine, but i want to know a reason why the ouput result is 3.95V and not 4V.

#### Papabravo

Joined Feb 24, 2006
14,829
Start by computing the values coming out of U3 to the pot in question and tell me what you think is the range of values coming out of the pot.

#### Tobias12

Joined May 25, 2019
25
Start by computing the values coming out of U3 to the pot in question and tell me what you think is the range of values coming out of the pot.
Or it's better to choose other op-amp ?

#### MrChips

Joined Oct 2, 2009
22,462

#### MrChips

Joined Oct 2, 2009
22,462
This is just a simulation.
3.95V seems pretty close to 4V to me. How accurate does it have to be?

#### Tobias12

Joined May 25, 2019
25
This is just a simulation.
3.95V seems pretty close to 4V to me. How accurate does it have to be?
I just want an explanation why is not 4 V even if the calculated values are correct. And one more question. How can I reduce the THD Factor ?

Last edited:

#### Audioguru again

Joined Oct 21, 2019
2,352
Your circuit will not work because the open-collector output of the LM293 has no pullup resistor. Look at its datasheet.
The LM293 has a low output current then your POT2 and R4.2 values are too low.

#### Tobias12

Joined May 25, 2019
25
Your circuit will not work because the open-collector output of the LM293 has no pullup resistor. Look at its datasheet.
The LM293 has a low output current then your POT2 and R4.2 values are too low.
I understand. Can you recommand me other comparator ?

#### Audioguru again

Joined Oct 21, 2019
2,352
The LM293 or LM393 will work fine when it has an output pullup resistor shown in its datasheet, and it drives a non-inverting opamp that has its input attenuated and has a high input resistance.