I built a non-inverting amplifier with a gain of 20. This was built in order to add 2 signals (a DC and an AC) while mostly isolating them from each other and then to drive an LED that is ON (from the DC) but slightly modulated (due to the AC). The circuit's output to an oscilloscope is clean and the data taken using a lock-in amplifier is good. The problem comes when I add a load, the max output voltage goes to ~2.5 V and it looks like the signal is hitting the rails, but my power supply voltages are +/- 18 V. For the LED, positive voltage will cap at 2.5 while negative can go to the actual rails (forward bias vs reverse bias).
I have tried putting a capacitor in series with the LED, in parallel, putting a resistor in series - nothing works. I tried using 2 different LED's, 1 is low power (1.4 V, 20 mA) the other is high power (3.5 V, 1.3 A) and in both cases I get this sort of problem. The low power one will turn on but the high power one is the one I'm interested in anyway. Max current drawn from supply is 20 mA.

 

A LED is much like a Zener Diode. The LED will not let the output of the amplifier get much above the turn on voltage of the LED. Add a resistor in series with the LED to limit the current.

 

LED stands for Light Emitting Diode, so it acts like low-impedance, forward biased diode with a voltage drop characteristic of the LED color.
They thus need some method to limit the current through them.

Add a resistor in series with the LEDs as Ron suggested.

 

Ok, thank you both. That helps explain some of the issue. In the case of my high power LED (which will conduct 1+ A), I never draw a high enough current through my amplifier, even with a series resistor (I have 2 resistors, a 1.5 ohm and a 0.47 ohm that can withstand 10 W of power). The LM675 spec sheet says it can output up to 3A of current. My output is much lower than that, tens of milliamps.

 

Another thing is your circuit can apply reverse voltage to the LED. This will damage it as the max reverse voltage LEDs can handle is often something like 5V, not 18V as it may be in your case.
LEDs are not lamps. They are diodes as mentioned, and are current devices, not voltage. You must limit the current and the reverse voltage.
Add the series resistor and a reverse diode across the LED. Try a 15R resistor, but it will have to be a 20W one.
Or, lower the amp supply voltage. 1R5 and 0R47 are way too low for a single LED.

 

Also, You are not showing the rest of the input circuitry.
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Ok, I see. Thank you very much @dendad.

 

Also, You are not showing the rest of the input circuitry.
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The rest of the input circuitry is an AC and a DC power supply with reference to ground.