Why would you only include such a small part of the circuit?
Any way, no it can't ... it's not a rail to rail opamp and the effective 100 Ohm load would be very low for that particular opamp even if it was.
Output voltage swing vs output current is the one you want in that particular sheet ...
The problem is that I don't fully understand the circuit (having only a small part of it and no description doesn't help). It seems to be doing something tricky, the way it uses a linear optocoupler and the feedback diode.
Why are you so hell bend on saving a PNP transistor?
Hi,
To design around the IL300 you need to know the theory, or at least the calculations involved in setting up the various resistor sizes. The datasheet is one thing, but better still Vishay Application Note 50 is better. See page 5 or 6 of the PDF.
http://www.vishay.com/docs/83708/appn50.pdf
If you don't then although it may appear to work you will probably be using it outside of it's specified regions and introducing drift etc. Bear in mind there are two modes of operation of these things......the one NOT mentioned in the main datasheets is the better. If I remember rightly thats the one mentioned in the application note.
For what it's worth, I was using them on my home made PSU but found they tended to drift slightly with temperature......and I had designed around them by the book. It's just inherent to these types of optocouplers I'm afraid. I was looking for super accuracy though..............
Ian.
For what it's worth, I was using them on my home made PSU but found they tended to drift slightly with temperature......and I had designed around them by the book. It's just inherent to these types of optocouplers I'm afraid. I was looking for super accuracy though..............
Ian.
your drawing is wrong, the photodiode current goes into the op amp -input so that it is the servoed variable, do look in the Vishay app note 50 fig 18
the LED drive is just using the PNP as a unity gain current buffer for the op amp output V in the correct version of the circuit
since you are supposed to measure, servo the actual current from the photodiode the entire LED drive circuit is inside the feedback loop, there is next to zero consequence of the PNP buffer Vbe or hfe variations if the op amp gain open loop gain is high
if the op amp can provide the required current directly, allowing for the LED and current limit R V drops, the supply V and the op amp output Vsat, then you can lose the Q - see the other circuit examples in the app note
....if the op amp can provide the required current directly, allowing for the LED and current limit R V drops, the supply V and the op amp output Vsat, then you can lose the Q - see the other circuit examples in the app note
Please refer to application note 55 , figure 6 by Vishay . There is the part schematic of interest. I want to omit the PNP because it will include its thermal drift , or am I thinking too much.
that may be the general intent, reason they make these parts - but look, really look at the circuit fragment - tell me where IP1 is going, compare it to all of the actual correctly connected circuits in the DS and AN
don't you think there is a mistake?
can you tell me how the op amp knows where to level off at a linear current?
in all of the standard circuits the current information comes from measuring the IP1 diode current by connecting it to the op amp INPUT
Please refer to application note 55 , figure 6 by Vishay . There is the part schematic of interest. I want to omit the PNP because it will include its thermal drift , or am I thinking too much.
The PNP emitter follower does two useful things:
1. It unloads the operational amplifier output. Otherwise the heavy load would compromise its open loop gain. Most precision amplifiers prefer high impedance loads.
2. If the output stage of the operational amplifier drives a heavy load, then the power dissipation of the output transistors will cause thermal shifts in the input stage.
The transistor is inside of the feedback loop so its Vbe shift with current and temperature is irrelevant. The OP-07 is widely second sourced making it the least expensive low input offset voltage and low input current operational amplifier available although that may not have been the case when the application note was written.
I yet again encourage careful inspection of your original post drawing, comparison with an55 fig 6
your drawing in the 1st post is incorrect - what is so hard about actually taking a close look, reading my comment carefully
perhaps other thread contributors can see this error and will pipe up?
Please refer to application note 55 , figure 6 by Vishay . There is the part schematic of interest. I want to omit the PNP because it will include its thermal drift , or am I thinking too much.
The PNP emitter follower does two useful things:
1. It unloads the operational amplifier output. Otherwise the heavy load would compromise its open loop gain. Most precision amplifiers prefer high impedance loads.
2. If the output stage of the operational amplifier drives a heavy load, then the power dissipation of the output transistors will cause thermal shifts in the input stage.
The transistor is inside of the feedback loop so its Vbe shift with current and temperature is irrelevant. The OP-07 is widely second sourced making it the least expensive low input offset voltage and low input current operational amplifier available although that may not have been the case when the application note was written.
Thanks for the clarification. I will also try to replace OP-07 with a better opamp.
Hi,
To design around the IL300 you need to know the theory, or at least the calculations involved in setting up the various resistor sizes. The datasheet is one thing, but better still Vishay Application Note 50 is better. See page 5 or 6 of the PDF.
http://www.vishay.com/docs/83708/appn50.pdf
If you don't then although it may appear to work you will probably be using it outside of it's specified regions and introducing drift etc. Bear in mind there are two modes of operation of these things......the one NOT mentioned in the main datasheets is the better. If I remember rightly thats the one mentioned in the application note.
For what it's worth, I was using them on my home made PSU but found they tended to drift slightly with temperature......and I had designed around them by the book. It's just inherent to these types of optocouplers I'm afraid. I was looking for super accuracy though..............
Ian.
Just having a look at this app note now and looks like Fig 18 on page 9 describes the use of the PNP and Fig 21 is where the circuit snippet actually comes from and may be the differential amp referred to earlier.
The PNP (if selected and biased properly) compensates for the temperature drift of the IL300 forward diode. There are explanations of using a diode in the emitter for this reason (Page 9), and a full explanation of the circuit is on page 11.