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I'm 74, I am retired after working twenty years as a computer technician and I live in France. I developed small software with Windev and Clarion Softvelocity (timing and ratings races, mountain biking). I build model airplanes and gliders old. I have a CNC (CNC-concept 1500, Tevo Tarantula) and I use CAMBAM and LinuxCNC, Spaceclaim also enjoy DIY and motorsport ... I would love to have an invitation. In advance, thank you. Excuse me, but I am not an expert in English.

I didn't study this kind of topology in university so I was surprised that you put a feedback from second OPAMP to first one.

do you have any material that I can study more to understand your circuit ?

I don't have any reading material on this, but this technique is often used in designing amplifiers for mass-spectrometry, where you deal with very high gains, which reduces the amplifier bandwidth. The CFA helps to improve the final slewrate of the amplifier stage. The slewrate of the CFA should be at least ten times that of the VFA.

If you search papers on pulse amplifiers for mass-spectrometry, you may find something.

@PM3295, your circuit is much better than mine !

I didn't study this kind of topology in university so I was surprised that you put a feedback from second OPAMP to first one.

do you have any material that I can study more to understand your circuit ?

one more question : is there any IC that can do this all this amplifications and automatic gain control for me ?
I found MC1350P but it is IF amplifier.

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Note	Dear All, I've favor of analog circuits with epecially enhanced mcu_functions verry accurracte CV/CC. This forum is many analog circuits discussion, Sincerely SID6582R2

measuring the time interval will not work !   I already thought of it. it will give very very low resolution.

Let me clarify with numbers:

min Distance = min Time x velocity of light

velocity of light = C = 3e8


assume TDC with resolution 30ps. This will give min distance of  30ps x 3e8 m/s = 9 cm.

This mean I can only measure multiple time of 9cm.

[TDC7200PWR]

To begin with, pay attention to TDC7200PWR



PIC controllers have such a CTMU module, it can be used for Precision Time Measurement. (AN1375)

Maybe a fast time-interval counter can work. One pulse edge from one PD to start the counter and the delayed edge on the other PD to stop the counter. From the count, the angle can be calculated.

Hi Folks,

I work on a small project using cheap laser modules and i would like to measure offset of photodiodes from center of laser light source. angle error shall be in range of 0.1 degree.

I prepared 2 figures to show the concept :

I have two photodiodes (red circles) 20cm far from each others and a laser source which has the power curve on the right side.


if the middle point of the two diodes are exactly on the center of the beam this mean they are centered in the beam.


if the two diodes are shifted upwards there exists angle between the center of the beam and middle point.



I have two ways in my mind to solve the problem :

• Measure the difference in received power for each diode
• Measure the difference in phase shift of each diode

Both techniques shall reflect the angle of shift. I don't think that measuring the power would result in good accuracy.
I believe the phase shift shall be a more accurate technique. I have to modulate the light source with Sine wave to be able to implement the measure phase difference.

I don't have good background in analog stuff and I am missing the components names that can help to measure the difference in phase shift between the photodiodes.

any recommendations ?

Thx, TDC7200 seems as a good option.

I was planning to use TDC-GP21 , it is very popular and I can buy it as a board from Aliexpress.

Inserting the capacitor may work for the DC offset, but the type and quality of the capacitor may be an issue for precise amplification. If you look at the comparison between your latest circuit and my modified circuit, you see the obvious problems. The AC gain is affected by the capacitor, and the problem with parasitic/stray capacitance causes pulse shapes that are far from the original.

I find it useful to have some form of adjustable DC shift control of the output signal.