help with amplifier circuit

[FvM]

you get what i want to do.

I hope so. But for the sake of clarity you should really show a diagram of expected output versus input voltage, or may be a table.

 

[mshh]

I hope so. But for the sake of clarity you should really show a diagram of expected output versus input voltage, or may be a table.

this is input output relations ,below 2 volt the input is doubled and greater than 2 volt the output is equal to the input

 

[crutschow]

this is input output relations ,below 2 volt the input is doubled and greater than 2 volt the output is equal to the input

So when the output is 1V you don't care whether the input is 0.5V or 1V since they both would give the same output?

 

[mshh]

this is input output relations ,below 2 volt the input is doubled and greater than 2 volt the output is equal to the input

i had another idea to solve this issue , using microcontroller if signal is less than 2 volt it will be amplified then Q2 will be switched on to pass it .if the signal is greater than 2 v Q1 sitched on and pass it .


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So when the output is 1V you don't care whether the input is 0.5V or 1V since they both would give the same output?

no i didn't say that, every value below 2 volt should be doubled and that mean when te output is 1 the input should be only .5v

 

[crutschow]

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no i didn't say that, every value below 2 volt should be doubled and that mean when te output is 1 the input should be only .5v

So with a 1.99V input, the output will be 3.98V, but when the input is 2V, the output will be 2V? So if the output is 3V how will you know whether the input is 1.5V or 3V since either of those inputs can generate a 3V output??

 

[mshh]

So with a 1.99V input, the output will be 3.98V, but when the input is 2V, the output will be 2V? So if the output is 3V how will you know whether the input is 1.5V or 3V since either of those inputs can generate a 3V output??

aha , i got you . i don't know how to differentiate between them,but the circuit that i proposed don't need this information

 

[crutschow]

aha , i got you . i don't know how to differentiate between them,but the circuit that i proposed don't need this information

No the circuit doesn't but do you need to differentiate between them when you are observing the output?
What are you doing with the output?

 

[mshh]

No the circuit doesn't but do you need to differentiate between them when you are observing the output?
What are you doing with the output?

i don't need to differentiate between them if the operation in the table is done. the output is used to be input for inverter that drives motor

 

[FvM]

aha , i got you . i don't know how to differentiate between them,but the circuit that i proposed don't need this information

O.K., you have designed a circuit and the thread can be closed? But I presume it doesn't actually work and you have still no clue how to make a working circuit that implements the specified characteristic.

As crutschow showed, you still didn't fully specify the transfer function, we have the choice between a curve the steps back from 4 V output to 2V at 2 V input voltage and a curve that keeps constant output of 2V between 1V and 2V input, because you only gave a few points but didn't tell how the curve inbetween looks like. I know why I asked for a diagram, and why I would always start the design by visualizing the transfer curve on paper.

 

[crutschow]

i don't need to differentiate between them if the operation in the table is done. the output is used to be input for inverter that drives motor

And this inverter/motor won't mind the step transition in voltage when the output suddenly switches gain?

 

[FvM]

This simple circuit might work as a practical solution. The voltage source E2 is an OP model.

 

[mshh]

This simple circuit might work as a practical solution. The voltage source E2 is an OP model.

what is the value for v1 and v2 and what do you mean by E2 is op model. and how to modify the trnasition voltage from 2 v to any other like 1volt?. please clarify connections.

 

[crutschow]

If you click on the schematic you will see the values.

You can modify the transition voltage by varying the value of V2. The break-point is approximately V2 + 0.6V

 

[mshh]

what caused this break point, is it Vbe? and why does he connect the emitter to the positive supply of the op amp ? what is 100K? i can't see any resistors here .what is the operation of the pnp here it should work with negative supply on base?

 

[crutschow]

Below is an LTspice simulation of FvM's circuit for values of V3 from 0 to 2V using an LM324 op amp model. The PNP transistor is off until it's Vbe is high enough to turn on the transistor. At the point the transistor basically shorts across R2 thus making the op amp a follower with a gain of 1.

(Note: To use the .asc circuit simulation file, remove the .txt extension from the name).



View attachment Breakpoint gain change amp.asc.txt

Edit: When the emitter becomes more positive than the base, the its Vbe is negative.

 

[mshh]

could i replace the pnp by npn and reverse collector and emitter connections? . i did your circuit in proteus using opamp lm358 but the output greater than 2 is less than input and this difference increased to reach 1 volt with increasing input to 5 volt

 

[crutschow]

could i replace the pnp by npn and reverse collector and emitter connections? . i did your cisrcuit in proteus but the output greater than 2 is less than input and ithis difference increased with increasing input.

No, you can't replace the PNP with an NPN. Why would you want to?

Are you sure the circuit you simulated is the same as mine with identical components, values, and connections? There shouldn't be that much difference in the simulations.

 

[mshh]

the transistor is just a switch here so i think that npn and pnp should do the job. i want to use npn as it is available in my hand now.also it will be fine to know the difference . i did your circuit in proteus using opamp lm358 and bc557

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i tried lm324 and your transistor 2n3906 and it give logical correct results . i got this result when i replaced lm358 with lm324 , what is the reason for that?

 

[FvM]

Base voltage polarity matters, thus you can't use a NPN transistor in this circuit. It's up to you to design a circuit that can use NPN transistors, according to your previous results how many attempts do you expect?

I must admit that my interest is slowly fading...

Happy developing!

 

[mshh]

Base voltage polarity matters, thus you can't use a NPN transistor in this circuit. It's up to you to design a circuit that can use NPN transistors, according to your previous results how many attempts do you expect?

I must admit that my interest is slowly fading...

Happy developing!

i know that the base polarity should be changed with npn but i didn't completely get you, do you mean that my attempt to do the circuit is useless .