Calculating maximum allowable gain of the OPAMP

[patan.gova]

I didn't get this.

All coupling capacitors increase the cutoff frequency so they all must be considered.

You mean if the coupling capacitor is not used then the low frequency components gets into the signal.But there was a HP filter prior to the LP filter won't the HP filter remove the lower freq if so, how the lower freq will be added again in the LP filter(If coupling capacitor is not used).

 

[Audioguru]

A coupling capacitor is a highpass filter. It blocks DC and low frequencies.
If one coupling capacitor has a cutoff frequency of 3Hz then two in series (even if there is an opamp in between) create a cutoff frequency of 6Hz.
A capacitor in series feeding a resistor to ground is an extremely simple highpass filter. If its cutoff frequency is 3Hz then 3Hz is reduced to 0.707 times much higher frequencies, 1.5Hz is 0.5 times, 0.75Hz is 0.25 times and 0.375Hz is 0.125 times which is still a fairly strong signal level etc.
A highpass filter blocks only DC and VERY low frequency signals.

A lowpass filter reduces higher frequencies and passes DC and low frequencies.

 

[patan.gova]

But the addition of coupling capacitor is eating the pulse signal.
Here is the output
1)before adding coupling capacitor of 100uF **broken link removed**
The output has good pulse signal with a Offest voltage of 1.04V
2)After adding coupling capacitor of 100uF **broken link removed**
The offset voltage was removed(red signal lying at 17mV) and also the required pule signal is also removed.
Is there a way to get the pulse signal after removing offset voltage.

 

[patan.gova]

Hello,
Can someone explain a way to get rid of the offset voltage while keeping pulse signals.

 

[patan.gova]

Hello Audioguru,
I am in the half-way of removing offset voltage could you suggest what to do to get the Pulse signals.

 

[Audioguru]

The circuit in the blog does not show an output offset voltage. I think it is using a different LED/phototransistor than you are using because its the second opamp has an saturated output as high as it can go (even when it drives an LED).

Are you the one who is using a green LED instead of an IR LED?

 

[patan.gova]

I tried using IR,red and green but everything is having a offset voltage from the first stage but there is no offset voltage after the second stage.
But the OPAMP I am suing is OP484 and the one embedded blog using is MCP6004.Is it the OPAMP that makes the difference in offset voltages?

 

[patan.gova]

Yes,now I changed the OPAMP to the new OPAMP of MCP6001 but still facing the OFFset voltage and now its around 200mV.
Is there anything that I can add to the circuit or modify so that the Pulse signals i.e., the output of the LP filter of the first stage filtering will be located with a 0 offset voltage.
Thanks.

 

[Audioguru]

An opamp part number has a range of input offset voltage. It can be negative or positive and could be minimum or maximum as shown on its datasheet.
The inputs of an opamp have a bias current that creates an input offset voltage if the input resistor values are not the same.
If the opamp has DC gain then it amplifies its input offset voltage.

If you built the circuit on a breadboard then maybe the many rows of contacts and the wires all over the place are antennas that pickup mains hum that is rectified by the opamp then filtered so you see only the resulting DC. Use a compact pcb maybe in a metal box connected to 0V and shielded input wires.

 

[patan.gova]

Thanks and I guess you might be right because when I simulated with the OPAMP-MCP6001 it was not having an DC offset voltage rather it was just having 0V offset as the pulse signal was lying at 0V with some millivolts amplitude but was exhibiting some DC offset voltage when connected on the breadboard.
But can you try to explain me a bit clearly about this
1)In the datasheet of MCP601 here **broken link removed**
it mentions the Input Offset voltage of +4.5mV and input bias current of 1pA.How this Dc offset comes form this.I didn't got this one"if the input resistor values are not the same."
what the input resistor mean for the opamp and how to calculate them to a use the correct resistor values needed.
2)And this offset value is also increasing by increasing the gain I guess. I remember that you said like the gain can be increased by increasing Rf and decrease the Cf instead of decreasing the ground resistor as the decrease in the value of ground resistor increases the Offset voltage.Isn't it?.

 

[Audioguru]

1)In the datasheet of MCP6001 here it mentions the Input Offset voltage of +4.5mV and input bias current of 1pA. How this Dc offset comes form this. I didn't got this one"if the input resistor values are not the same. What the input resistor mean for the opamp and how to calculate them to a use the correct resistor values needed.

This is a Cmos opamp with extremely low input bias current that does not cause an input offset voltage when the input resistors have different values like an ORDINARY opamp that has much higher input bias current.

2)And this offset value is also increasing by increasing the gain I guess. I remember that you said like the gain can be increased by increasing Rf and decrease the Cf instead of decreasing the ground resistor as the decrease in the value of ground resistor increases the Offset voltage.Isn't it?.

If the amplifier has DC gain then it amplifies the DC input offset voltage. If the feedback resistor to ground has a series capacitor then the DC gain is only 1 and the input offset voltage is not amplified.

I think you must get rid of the breadboard that is very bad for such low level signals.