1N5817 schottky

At low current, it is going to just act like a pretty big capacitor. About 250 pF. See figure 4 on:

Ask for a 1N5711.

p.

"> I'm about to make a wideband rf sniffer using an op amp "

What RF frequencies are you looking for? What OPamp are you using. Your typical opamp is not going to work at RF frequencies (10 - 100 MHz.) Minicircuits makes some nice RF amplifiers that might work for you... these cost a bit more than op-amps but for $60-$100 you can do alright. George

Yeah, thanks.

I can use any op amp within reason. The input to the op amp will be dc. I want as wide a rf (Hz) range as is possible. Anyhow I think it's sorted now. Thanks for your help.

I don't know what you have in mind for a design lay out how ever, I can say that bipolar op-amps tends to miss behave around R.F. Their basic diode structure them self's tend to act as detectors and if subjected to R.F. near by, they could give you side effects you were not planning on.

Assuming that you intend to have this DIODE close to the op-amp, I thought you may want to know this when selecting your op-amp, shielding and lay out.

Yeah, ok, thanks. I have made at least 10 of them over the last 15 years and I've found a couple of problems with them, but like you said, shielding and another one, temperature compensation can be considered.

p.

Probably not the best idea, the 1N5817 is intended for LARGE currents, and is accordingly going to take a lot of RF current before it reacts.

A transistor makes a good detector, just capacitor-couple the RF to its base, use the op amp to average and amplify the emitter current. You can get slightly fancy, use two transistors and amplify the current difference with same DC bias on each. Differencing two transistors gets rid of thermal drifts. If the RF source is low impedance (like, a loop antenna?) you might want to couple (direct or inductively) to the emitter instead of the base, and sense the collector current.

It might be amusing to connect a speaker to the amplifier output, and see what AM station is strongest in your area...

Adding a tuned circuit will get you halfway to building a grid dip meter...

No, I am building a wide band rf sniffer - up to as high a frequency as I can get. Also a transistor has to be biased otherwise the forward voltage drop is too much (0.6) and the bias currents are too high for my application. I am going to use a micropower op amp which can be left switched on all the time, detecting, so to speak.

If I want to find out which am station is the strongest I'll build a crystal set/whatever and if I want a grid-dip or a wave meter I'll build one of those too.