Peak detector is not working when there is change in voltage.

[veerubiji]

I am trying to design a peak detector in Multisim simulation tool. I have designed buffered peak detector to get peak voltage of sine signal. The designed circuit is working when signal voltage is constant, for example when signal voltage changes from 1V to 0.5V then it is giving 2V as peak detector output. The designed circuit as shown below.

In the above circuit I am using switch to change the voltage of sine wave from 1V to 0.5V. I am using one more switch to clip negative cycle of sine wave, then i am using buffered peak detector to get maximum voltage.

When i apply 1V sine wave it is working perfectly and giving peak voltage as DC. The problem is when I change signal voltage from 1V to 0.5V then also it is giving 1V as peak. ANy one suggest me what mistake i am doing. I want to design peak detector that will give max voltage even there is voltage variations.View attachment 68759

 

[ChrisP58]

You need some load resistance across C1. Without something to bleed off the charge, it will hold the highest peak that it has ever seen.

 

[crutschow]

Yes, the peak detector is working as it's designed, it's staying at the highest (peak) voltage it has seen. If you want to detect a lower peak voltage then you need a reset switch across the capacitor to reset the voltage to zero or a resistor to bleed off the voltage, as Chris stated.

 

[veerubiji]

You need some load resistance across C1. Without something to bleed off the charge, it will hold the highest peak that it has ever seen.

Thanks for your reply,
I have small doubt if i add resistor to discharge capacitor C1, is there any formula to calculate charging and discharging time. My voltage will vary at every 100 ms thus peak detector has to give max peak, the voltage will vary in-between 0-2V at every 100ms. So how to find the exact values of capacitor and resistor combination? is it 1/2*pi*R*C?

 

[ronv]

You can use T=RC and set the resistor for 5 time constants (20 ms) to discharge the capacitor. The charge time is limited by the op amp and will be fast - so no worry.

 

[crutschow]

How long do you need a stable peak voltage before going to the next measurement?

 

[veerubiji]

How long do you need a stable peak voltage before going to the next measurement?

I am changing the phase of the signal and reading max peak, so it has to hold at least 100 ms before going to next measurement.

 

[ronv]

If you need to start a new reading before each sample you can discharge the cap with a small FET like a 2N7000. So the sequence would be Discharge, shift phase, release discharge, wait a couple of ms. then sample.

I don't think the LT101 will be fast enough for your 125Khz. signal.