Help with diode model in multisim

[smk037]

I am trying to build a simple laser diode driver
problem is, I built the circuit without taking the diode into consideration, and am getting the right current outputs (45mA for 0 bit and 90mA for a 1 bit)
but every time I insert an LED just to kind of simulate the laser diode, I get some hideous output current

any idea what the problem could be?
I attached my circuit to this post

 

[smk037]

I should add, that voltage source has a 6.75 volt DC bias on it

 

[vk6kro]

You may not have enough supply voltage.

If you work out the voltage drops across the 50 ohm and 111 ohm resistors with 90 mA flowing, you get 4.5 volts and 10 volts.

With a 15 volt supply, that only leaves 0.5 volts for the transistor and the LED. The LED alone will need about 1.5 volts minimum.

Try it with a 25 volt supply.

 

[smk037]

You may not have enough supply voltage.

If you work out the voltage drops across the 50 ohm and 111 ohm resistors with 90 mA flowing, you get 4.5 volts and 10 volts.

With a 15 volt supply, that only leaves 0.5 volts for the transistor and the LED. The LED alone will need about 1.5 volts minimum.

Try it with a 25 volt supply.

I only have a 20 volt supply to work with, but thanks a lot, that did the trick

 

[smk037]

so, after measuring the photodiode characteristics, I built the circuit
At first, we had some real bad oscillation problems
we thought this was because there was some phase difference between the input voltage and the voltage coming from the feedback loop
to compensate, we just added a potentiometer in series with the feedback resistor and changed it until the oscillations were gone

we got this to work, but we also had to turn the voltage all the way down to 250mV with a 350mV DC offset

I talked to one of the fiber optics professors about this and he said my input voltage was too low for it to be a useful communication system driver (I didn't really understand his explanation, I'm guessing because the signal would just be too noisy at that point?)

The thing is, I can't seem to up the voltage without adding a lot of oscillation (because I would also have to up the feedback resistor if I did this)

I am thinking that I could reduce the phase shift between the input and voltage by using an LM311 as the comparator, and an LT1014 as the second operational amplifier in the circuit

I would normally just try and build this to see what happens, but I need to order the parts

Do any of you have any idea if this would work? Or if its not a phase change that's producing the oscillations, any idea what it might be? I was also thinking that the gain of the feedback loop is too high, but I'm not sure what I can do about that without having to design a completely new circuit

I am currently using LM741s for both the comparator and the amplifier stage

Thanks for the help.

Here is a picture of my circuit
The current ratio is actually .018, the feedback resistor is 800 ohms now, and the input voltage is 250mV with a 350mV offset