Using OPA861 in an optical communication module with LED

David;

It would help if you can mention the bandwidth of the signal with which you wish to modulate the LED module and the LED forward voltage at 700mA.

Dear Neil,

I intend to first try an unmodulated signal of ~10MHz. The LED's typical forward voltage @ 700mA is 3 V with a +-0.06V tolerance.

David;

I think you could probably dispense with the bias tee and the class AB driver by using a fast, high current op amp such as the OPA2670 in a voltage-controlled current source configuration. By setting the constant-current DC output to 350mA and modulating this with your 10MHz signal, you should be able to achieve 100% amplitude modulation. It will be necessary to build this on a copper ground plane both for good high frequency performance and for heat sinking. Use as low voltage for your power supply as possible to minimize the amplifier's power dissipation.

It would be far easier to design a circuit like this but TI has neglected to provide a TINA circuit or even a SPICE model of this device.

David, Neil,

The OPA2670 uses a fully differential architecture and cannot be used as two individual op amps. The OPA2673 is a very similar part but is not fully-differential meaning that the 2 op amps can be used separately. There is a Spice model for the OPA2673 available on the OPA2673 product page.

Thank you for the clarification, Kristoffer.

It is very odd that TI describes both the OPA2670 and OPA2673 as " Dual Wideband, High Output Current Operational Amplifier .." ---rather confusing. BTW, whoever created the OPA2673 TINA "Reference Circuit" did not distinguish himself with that minimal effort. No simulation results were shown, just a circuit.

Our "Circuit Examples" used to be far more informative and they were drawn with some artistic esthetics; an example of this can be found in the OPA627 "Reference Circuit" .ZIP file (these are not actually reference circuits at all-- they were only provided to show that the TINA macromodel worked properly and in a useful circuit besides) . Why TI provides these files in a .ZIP format is a mystery as the files are small and the additional unzipping task just wastes the user's time.

Here is that file (attached) to compare with TI's current efforts. If you were a customer, which would you prefer to see?

Dear Neil, Kristoffer

thank you for your suggestions.

Neil, do you mean I should feed the LED module from the OPA both with direct current for power as well as with the signal? Got kind of confused!

Regards

David