Minimum load resistor for 2.5 Voltage Reference

I have the following circuit:

https://www.dropbox.com/s/0pguwnpvqjfx07r/Photo%20Feb%2009%2C%2011%2047%2016%20AM.jpg

It's a 2.5V voltage reference.

I need to determine the proper load resistor (-+10%) for it. How would I go about doing this?

Thanks in advance.

That's easy: 0Ω

Why? Well, first off that is _not_ a load resistor. It will, more likely, function as a series current limit resistor [unless the open end of the resistor is connected to ground -- then it could be called a _load resistor_.

The op-amp is functioning as a unity-gain buffer. The two 1K resistors serves as a voltage divider that cuts the 5V supply in half (i.e. to 2.5V). The op-amp, basically, reduces the impedance so it can supply that 2.5 volts to most loads without a voltage drop. Add a resistor in series with the [op-amp] output defeats the purpose (IF the purpose is, indeed, to supply a 2.5V reference ) A series resistence (your '?' resistor) will drop voltage whenever current passes through it (like when a load is applied) -- this would drop the voltage, supplied to the load, to lower than 2.5V

The op-amp circuit that you have posted is of a unity gain buffer, that means whatever value of input voltage (less than supply voltage Vcc & Vee) appears at the output. This kind of configuration is used to solve impedance issues since the input impedance is very high & the output impedance is very low. So it avoids problem of loading, coming back to the point this circuit outputs 2.5v (ideally) but the value of R? depends on the output current sourcing capacity of op-amp (refer datasheet). This stage is used to amplify current (keeping voltage constant) & matching impedances.

Correct not a true reference. Lets's ay the amp can source/sink 100mA, and your tolerance is +/-10 or +/- 250mV, the the resistor would be:


0.250V/0.100A, which is 2.5Ohms. Now this doesn't really make sense if it is to be used as an I-Limit device. If the device can supply 100mA, then the true value should be 2.5V/0.100A = 25Ohms. Of course the regulation current is now only 10mA, for +/- 10 regulation.

What are your requirements in terms of voltage and current for the 2.5V?