Electronics > Beginners

Op Amp Full Wave rectifier with Hi Pass question

<< < (5/5)

Zero999:

--- Quote from: alexg on June 26, 2018, 03:37:46 am ---
--- Quote from: Audioguru on June 26, 2018, 03:23:09 am ---I do not see how the circuit you found can work properly.
You do not need a virtual ground if a single supply opamp (maybe a lousy old LM358?) is used in a circuit like this one that works:

--- End quote ---

It just does :), its a single supply circuit so diode is not required.

--- End quote ---
The diode is required, otherwise the op-amp's lower output transistor will turn on, taking the output voltage to zero.


--- Quote from: alexg on July 02, 2018, 04:22:12 pm ---
--- Quote from: Audioguru on June 26, 2018, 04:35:43 am ---Like all opamps, the opamp you are using has an absolute maximum negative input of only -0.3V when its negative supply (Vv-) is 0V.
In the circuit I posted, the inputs NEVER go negative (unless they have a very high level) so use this opamp in the circuit I posted.

--- End quote ---

I am just curious what effect will this have on the op amp of you let negative input go below -0.3V when supply is 0? It seems to work but does it mean that I am slowly killing it with every impulse below -0.3V ?  I know datasheet says "Exposure to maximum rating conditions for extended periods may affect device reliability." and I understand that but I am curious if I am just lucky to have circuit that works fine even beyond max ratings or I am slowly killing op amp?

--- End quote ---

--- Quote from: Audioguru on July 02, 2018, 10:13:16 pm ---Your input voltage is exceeding the maximum allowed input voltage so of course you are damaging the opamp.

--- End quote ---
That depends on the op-amp. The LM358/324 has protection diodes and if the input current is limited to a safe level, with series resistors, as is the case with the schematic posted previously, then nothing bad will happen. The input diodes will simply conduct, causing the input to sit at 0.6.

-0.3 was specified on the data sheet because, that's the maximum negative voltage the inputs can be connected to, over the entire temperature range, without causing a significant current to flow through the protection diodes.

alexg:

--- Quote from: alexg on July 26, 2018, 03:12:01 pm ---
--- Quote from: oPossum on July 23, 2018, 03:14:09 am ---Schottky diodes have high reverse leakage. Try a silicon diode like 1N4148 or 1N914 instead.

--- End quote ---
Worked like a charm, thank you.

--- End quote ---
Well, if I heat the board from 25degrees to about 40degrees,  upper half-wave is shorter again I suppose because diode's reverse current leakage increases, oops.

oPossum:
I think changing the 200k resistors to a lower value will reduce that problem. There will always be some small difference between the two halves of the sine wave because a perfect diode does not exist and that circuit is sensitive to reverse leakage.

alexg:

--- Quote from: oPossum on July 27, 2018, 04:06:02 pm ---I think changing the 200k resistors to a lower value will reduce that problem. There will always be some small difference between the two halves of the sine wave because a perfect diode does not exist and that circuit is sensitive to reverse leakage.

--- End quote ---
At the time of measurement those resistors were 20k already so I suppose I will have to deal with waveform difference at higher temperatures.

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod