Hello. I am designing an amplifier which amplifies the signal of a heartbeat( around 1Hz) that is collected from a piezoelectric sensor. I aim to have a ~1V amplitude sine wave output at 1 Hz, and the waveform of other frequency will be filtered out. But right now, I only have an output of 0.16V amplitude sine wave.

Is there any way to increase the gain of the circuit and have a higher voltage output?


Figure 1 Circuit Diagram


Figure 2 Circuit Diagram (with notes)


Figure 3 Output Waveform

 

hi almaz,
Welcome to AC,
Please post your LTSpice asc file.
E

 

Do not use rectangular boxes to represent op-amps in a schematic diagram.
Use a 3-pin or 5-pin model instead.

 

Here is the .asc file. I think you also need to download the PSPICE model file for OPA1641 and OPA4187, and I dont know if I can attach the library files here. But I can give you the link to download the files.

OPA4187: https://www.ti.com/product/OPA4187?...verything&usecase=GPN#design-tools-simulation

OPA1641:https://www.ti.com/product/OPA1641?keyMatch=OPA1641&tisearch=search-everything&usecase=GPN-ALT

 

Do not use rectangular boxes to represent op-amps in a schematic diagram.
Use a 3-pin or 5-pin model instead.


View attachment 297593

Thank you. I will change that.

 

hi almaz.
Built the models, this is what I see with your asc file, please check for accuracy.
E

 

Here is the schematic without the rectangular boxes.

 

hi almaz.
Built the models, this is what I see with your asc file, please check for accuracy.
E
View attachment 297596

This is correct.

 

hi,
You have designed an oscillator, with positive feed back.???

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BTW: I will continue with Block symbols.

 

hi almaz,
Your 1st stage is not working correctly, used a 1mV 60Hz sine as test.
The gain is far too high.
E

 

hi,
You have designed an oscillator, with positive feed back.???

E

BTW: I will continue with Block symbols.
View attachment 297599

It is a charge amplifier with differential inputs and a band pass filter. The model of the piezoelectric sensor is an appoximation of the real one.

 

Do you think 192meg (R1) is a practical value?

 

hi almaz,
Your 1st stage is not working correctly, used a 1mV 60Hz sine as test.
The gain is far too high.
E
View attachment 297604

I think need you need to put a capacitor of 0.55nF in series with the voltage source to make it work like a piezoelectric sensor.
Maybe the links below can help you:
Understanding and Modeling Piezoelectric Sensors - Technical Articles (allaboutcircuits.com)
https://en.wikipedia.org/wiki/Piezoelectric_sensor

 

Do you think 192meg (R1) is a practical value?

I think so. I have seen people use 1000MOhm feedback resistor in their charge amplifiers.

 

The charge amplifier in the simulation has a DC gain of zillions that amplifies the opamp DC input offset voltage.

 

hi almaz,
Cleaned up your original asc circuit, it should make it easier to follow any changes you have to make in order to get it working as required.
Correct the excessive gain in the 1st stage.

Don't worry about using boxes for the symbols.
E

 

The charge amplifier in the simulation has a DC gain of zillions that amplifies the opamp DC input offset voltage.

Agreed. What’s this obsession with charge amplifiers for piezo transducers. What’s wrong with an ordinary non-inverting op-amp stage, using a FET op-amp to get a high-impedance input?
Do the filters need to be so complicated? Has the TS quantified the interference signals that need to be removed?

 

hi almaz,
Cleaned up your original asc circuit, it should make it easier to follow any changes you have to make in order to get it working as required.
Correct the excessive gain in the 1st stage.

Don't worry about using boxes for the symbols.
E
View attachment 297647

But If I use another model (the one with the voltage source) of the sensor, the maximum gain will be around 10dB

 

hi almaz,
Your design is over complex, this is an example of a simpler design.
Note the use of an actual Heart beat wav file.
E

 

hi almaz,
Your design is over complex, this is an example of a simpler design.
Note the use of an actual Heart beat wav file.
E
View attachment 297664

I will look into it. Thank you