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The applications of base connection in 4N25 opto-coupler

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Robert12

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Hi,

Some opto-couplers, like 4N25, have a terminal for the base of the output device, what are the advantages of having this terminal? Looking for an answer, I ran into this. I can somehow understand that the base connection can increase the speed but I don't understand how this terminal can be used to increase the noise immunity.
Moreover, the above link suggests that the base connection can help to have matched devices, in what application the mentioned matching is important?

Any comment is appreciated!
 

ronsimpson

Well-Known Member
Most Helpful Member
matching is important?
If you have several data line with a clock. They all have 4N25s to isolate. To get maximum data rates you need the delay through all the 4N25s to be the same. If the clock is too fast or slow you can get errors.

I did not read your link; Light causes current to enter the Base of the transistor. We can think of the Base current as the same (from light or from current from a voltage source). For turn on; if the current goes from 0 to 100uA in 1nS the transistor will turn on fast. For turn off; the current goes from 100uA to 0uA in 1nS, but there are capacitors inside the transistor. (I am thinking of the B-E capacitor) It will take a long time for the charge to run down. Add a B-E resistor will cause the charge to run down faster.

Adding a B-E resistor will speed up the turn off time (delay) but will reduce the gain.
 

schmitt trigger

Well-Known Member
The link that you have provided is very clear, I have myself used this technique:

Impulse noise immunity (or reduction) at output: This is required when input current suffers brief spikes or sharp extraneous rise / fall, such as due to poor power regulation. A capacitor is connected between base and emitter of the phototransistor. This acts in effect like a low pass filter, adding some smoothing to the input signal, and bypassing sharp spikes. It does reduce signal sensitivity and introduce a delay, though.
For a quick and dirty value, use a 0.1 nF capacitor, though it is worth trying higher and lower capacitances, depending on adverse effects if any.
 

Robert12

New Member
Thanks for replying!

The link that you have provided is very clear
Yes, you're right, now it is almost clear to me too. But I was a little bit confused before, by seeing some datasheets mentioning that the base lead has not been made available to make the device immune to noise! while some others like 4N25 has the base available to increase the immunity to noise components.
Apparently, making the base lead available gives us the opportunity of attenuating some noise components from the input of the opto-coupler but also makes the output vulnerable to other noise sources, am I right?
 

Mosaic

Well-Known Member
I have found a 39K across the Base-Emitter of the 4n35 significantly improves the switching times by several uSec.
 

crutschow

Well-Known Member
Most Helpful Member
I have found a 39K across the Base-Emitter of the 4n35 significantly improves the switching times by several uSec.
That will also significantly reduces its transfer gain sensitivity.
 

Robert12

New Member
Thank you all for replying!

Has anybody seen a solid-state relay with base connection available. (an SSR with BJT switching device which has a base lead)? What would be the applications of this terminal being available?

Thanks in advance!
 

Robert12

New Member
Thanks for replying Mosaic.

I have seen a brief discussion of these devices in "solid state relay handbook with applications" but the book was written a long time ago and apparently there is no such device now.
 

crutschow

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Has anybody seen a solid-state relay with base connection available. (an SSR with BJT switching device which has a base lead)? What would be the applications of this terminal being available?
I don't think there is such a device because there would be no practical reason for having that terminal available in an SSR.
 

Robert12

New Member
no practical reason for having that terminal available in an SSR.
The book explains that this terminal can be used to increase the collector current and achieve a smaller voltage drop when the switch is turned on. Since the book is Crydom's, I looked at the company's DC SSRs with BJT, but there was not such device.
 

Beau Schwabe

Active Member
Another use for having the base connection available is to set it up in a current mirror configuration with a second (or more) opto-isolator. Good for differential signaling of small variations on the input of the opto
 

Robert12

New Member
Another use for having the base connection available is to set it up in a current mirror configuration with a second (or more) opto-isolator. Good for differential signaling of small variations on the input of the opto
Thanks for replying Beau Schwabe.
I'm interested to know more about this. You mean the method acts as a single to differential conversion? How? Is there any reference describing this technique?
 

Beau Schwabe

Active Member
No, not a single to differential conversion, (more like differential to single conversion) just simply a current mirror amplifier (or Differential Amplifier) where you can drive both inputs optically rather than with a conventional direct (wired) connection. I've never really found much in the way of an application note for using the base connection of the output transistor of an opto-coupler, but I've designed plenty of circuits using current mirrors where this can be applied. This is especially useful when the LED to an Opto-coupler's input may be "ON" but not completely saturated and you have an input signal capable of slightly modulating the Opto-coupler's LED.

With the reference below, IN1 and IN2 are essentially your optical isolated inputs to the Opto-coupler. Q1 and Q2 can be replaced with resistors 100 Ohms or so. (experimentation may be required) Q5 and Q6 are the transistor outputs of the Opto-coupler utilizing the base connections.

Reference:
http://www.play-hookey.com/analog/diff_amps/diff_amp_mirror_load.html
 

Robert12

New Member
With the reference below, IN1 and IN2 are essentially your optical isolated inputs to the Opto-coupler. Q1 and Q2 can be replaced with resistors 100 Ohms or so. (experimentation may be required) Q5 and Q6 are the transistor outputs of the Opto-coupler utilizing the base connections.

Reference:
http://www.play-hookey.com/analog/diff_amps/diff_amp_mirror_load.html
I think the main advantage of this circuit will be the differential-to-single conversion, am I right? Can you think of any other advantage?

Opto-couplers with the base connection remind me of the body of PMOS transistors in integrated circuits. There the body could give us some flexibility in the design as if we had two gates for a single transistor. For example, I remember we could use this to introduce positive feedback in op-amp design.
 
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