Switching 800mA at 10Mhz?

Probably the biggest problem is that you're letting the transistor saturate, which will severely limit your speed. Try using a resistor in the emitter to set the current. By controlling the base voltage, you can set the emitter current via the emitter resistor. Then, you can select a collector resistor to ensure that the transistor doesn't saturate. This should get you a lot closer...

You're gonna be wanting to disclose your minimum requirements for: risetime falltime edge linearity delay rise delay fall symmetry transient response voltage compliance load impedance A description of what you're driving might be helpful.

What are you using to measure the current waveform? Taking a wild guess that you're not measuring the current at all, but rather measuring the voltage across the resistor. How are you hooking that up. Probing technique can mess up your measurement. What's the bandwidth of your measurement system? mike

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Thanks for the replies. Here are some more details. I intend to modulate a "bright" LED which draws about 600-800mA at 10Mhz. (At some point I will convert an ethernet signal too OOK and use this as a transmitter). Conventional devices for laser diodes usually move less that 50mA. This LED has a fast response time of about 10nS so we are comfortable that this will not be a limiting factor. The vendors of course don't give a lot of information for these devices. The forward voltage is

3.7V. Dynamic resistance is 0.8ohms. (I presume this is the slope of the forward voltage). See: At 10Mhz I am assuming that I can transmit a reasonable square wave if I can pass frequencies of 100Mhz thus I am looking at rise/fall times of about 10ns. These comply with some of the FETs and BJTs out there. I am measuring the waveform with a 10x probe via the voltage accross the resistor. Also picking it up from a PIN diode set up as a reciever-they seem to comply. Since I get a clean square wave using the same probe at the sig-gen and base I assume that this is not the issue? At any rate I expect it will be easier to operate the resistor or a diode first as a starting point. I presume driving 800mA at these frequencies is not uncommon and probably there are some circuit designs already available as a starting point. I can't find much in my library

-Horowitz & Hill alludes to some things but I would prefer a complete circuit to start with. I think I have tweaked the most from my present simple design-a Darlington driven by a buffer-it gives a much better waveform than a simple common emitter transistor circuit.

Thanks, Fritz

Try a 2N7000 (or 7002) n-channel fet. Drive from an HC-family gate running at 5 or even 6 volts, maybe a couple sections in parallel. 800 mA is close to the edge here, so maybe use two fets in parallel, with a couple HC gate sections driving each. Should be plenty fast.

Whole thing (one hex HC gate, two fets) should cost about 80 cents.

Heck, two 74AC04's with all 12 sections in parallel would do it all by itself.

John

The secret is not to let the voltage move. Bias the LED on with the most current you can tolerate for the minimum intensity. Then drive the thing with the collector or drain of a transistor. The voltage will only have to move far enough to drive the dynamic resistance. The base/gate drive you need depends on the reverse transfer capacitance of the device. For bipolars, I'd look into the improved versions of the 2N3866. Don't remember the numbers of the newer parts. Low Cob is what you want. You might find a cascode connection beneficial depending on the turnoff edge. I've achieved an amp/nanosecond slew rate with 4 IRF530s in parallel and 100 ohm gate drive resistance for a power supply dynamic tester.

mike

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