Difference in using OPAMP and TIAOPAMP

[Govardhan]

Hello,
I am working with the pulse signals in the range of 0.5 to 3Hz.
I am using the phototransistor output(of around 10mv with freq components in 0.1 to 3Hz) to feed to the highpass filter stage and then to the lowpass filter.But I am confused in selecting between the Opamp (MCP6001 http://www.futurlec.com/SFMicrochip/MCP6004.shtml )and TIA opamp (OPA381 http://www.ti.com/lit/ds/symlink/opa381.pdf) for the lowpass filter stage which should have a fc=3Hz and gain needed is 101.
I got a couple of doubts of
1)Is it possible or can we use the TIA opamp in non-inverting mode to use with above implementation of LP filter as datasheet doesn't mention anything abou the non-inverting mode.
2)which one suits best for my apllication between opamp and TIA opamp.
Can someone explain the abotu things.
thanks.

 

[Laplace]

1)Is it possible or can we use the TIA opamp in non-inverting mode to use with above implementation of LP filter as datasheet doesn't mention anything about the non-inverting mode.

The TIA datasheet makes the following statement in the section BASIC OPERATION: "The OPA381 is used in inverting configurations, with the noninverting input used as a fixed biasing point."

So there is good reason the datasheet doesn't mention anything about the non-inverting mode. The OPA381 has been optimized for use as a transimpedance amplifier and is not suitable to perform your intended function in the non-inverting mode.

 

[Govardhan]

Thanks Laplace.

 

[Govardhan]

Best way to increase the gain of of an OP-AMP

Hello,
I am working with the pulse signals which has a frequency range of 0.5 to 3Hz.
I am feeding the photodiode output to a simple HP filter followed by a LPfilter as shown in the attached.
I am currently using a gain of 101 where the gain of opmap can be calculated as 1+rf/ri=1+680k/6.8K=101(seen in the image attached) but now I want to change the gain to 200 or above but I am confused with the ways to do it.Gain can be increaed as
1)Increasing the feedback resistance while decreasing the feedback capacitor or
2)just decreasing the grounding resistor(6.8k)

Can anyone explain the difference between the two methods in the resulting output.
And which can be best to use if the pulse signal(output of phototransistor) has amplitude of 100mV with the frequencies inteh range of 0.5 to 3Hz.

 

[Laplace]

In the attachment I took the node equation for the LPF circuit and derived the transfer function, from that calculated the magnitude vs frequency plot. H(ω) represents the initial component values giving 40 dB of low-pass gain. H1(ω) adds 12 dB gain by lowering the value of R2. H2(ω) adds 12 dB gain by increasing R3 and lowering C2 to maintain the same RC time constant. Notice that H1(ω) and H2(ω) have an identical magnitude plot vs frequency although neither approaches a gain of one as fast as the initial for increasing frequency.

 

[Govardhan]

Hi laplace,
can you suggest a method form the two above to increase the gain that doesn't show any effect on on the output pulse signal like the offset voltage addition or others.