# Using optocouplers to transfer analog signals.

#### Green Bean

Joined Mar 31, 2017
126
I know that a resistive optoisolator can be used to transfer AC, but what about those DIP packaged phototransistor optocouplers? If I'm sending the signal optically to the base of the transistor, what do I need to do to get that same signal out of either the collector or emitter? Would a circuit like this:

be the best way to go or is there a better way? I should mention that the signals I want to send have a DC offset, they never go below ground.

#### AnalogKid

Joined Aug 1, 2013
10,055
There are other threads on this forum about this, but I don't know where. There are optocouplers designed specifically to transfer/isolate analog signals. A common type has one LED and two receivers all in one package. One receiver is the isolated output, and the other one drives the transmitting opamp feedback loop. This makes the driving opamp take out any transfer non-linearities.

ak

#### crutschow

Joined Mar 14, 2008
29,791
If the analog frequencies are in the audio frequency range, and] you can tolerate a possible small amount of distortion in the analog signal, then using a standard opto-isolator in a circuit such as this, may work for you.

Joined Jul 18, 2013
25,211
H11F1/2/3 Analogue fet output.
Max.

#### Bordodynov

Joined May 20, 2015
2,993
See

#### Green Bean

Joined Mar 31, 2017
126
If the analog frequencies are in the audio frequency range, and] you can tolerate a possible small amount of distortion in the analog signal, then using a standard opto-isolator in a circuit such as this, may work for you.
Yes, that will work perfectly thanks.

#### Janis59

Joined Aug 21, 2017
1,383
The only thing You must know it is needed max dV/dt. If it goes into near GHz, You are in trouble. When it is kHz, anything is good enough.
For example: need to design the smps to say 3 MHz. If be 1,5 MHz there be many candidates, but here one may wish this smps would be adjustable within D=5%-95%, thus You may multiply 2 parts x 3MHz / 0,05 = 120 MHz and if to remember that front of gate must be at least 0,05 of the pulse length it means 2400 MHz - what is sure having no ANY optos. However around 800 MHz one may try to find at least something, and for less than 1 MHz it costs just three cents a kilogram.

#### Bordodynov

Joined May 20, 2015
2,993
See

For example: need to design the smps to say 3 MHz. If be 1,5 MHz there be many candidates, but here one may wish this smps would be adjustable within D=5%-95%, thus You may multiply 2 parts x 3MHz / 0,05 = 120 MHz and if to remember that front of gate must be at least 0,05 of the pulse length it means 2400 MHz - what is sure having no ANY optos. However around 800 MHz one may try to find at least something, and for less than 1 MHz it costs just three cents a kilogram.