High impedance scope probe amplifier?

[edan]

Hi,
I have a low-end oscilloscope that isn't very sensitive. I'd like to boost the input by at least a gain of 10, ideally 100. I need to see frequencies up to 1Mhz. It's OK, in fact it's better, if the amplifier is an in-line battery powered device.

The first thing that came to mind is to find a 10MHz or 100MHz (unity gain) op-amp, wire it up as a non-inverting amplifier, make a resistor-divider virtual ground with ceramic bypass capacitors directly on the pins, and call it done. I haven't been able to find any fast enough opamps just yet, though I'm sure they're out there. (I live in Europe so going to Digi-Key means paying through the nose for shipping and import.)

Can anyone suggest a common op-amp that's fast enough? Or do you have a better idea?

 

[chuckey]

I used a wide band(10 MHZ) fixed gain chip amp ( gain= X 10, X100) , way back in 1969. I think OPAMP is leading you up the garden path, try searching on wideband amp, programmable amp, RF amp. Just re-read your post 1MHZ is VIDEO, so try video amp.
Frank

 

[edan]

Hey, what do you know -- some digging turned up the AD8055:
https://www.analog.com/static/imported-files/data_sheets/AD8055_8056.pdf

Still an opamp, of course. But reasonably cheap and available. The datasheet specifies a 100 ohm input (series) resistor in the application example for a low-noise preamp, any thoughts on why? (The amplifier is specified for a 10MOhm input impedance, which is great.)

Also, any thoughts on handling probe compensation? The scope I'm using does the compensation inside the scope, not at the probe... maybe I just need to get a different probe.

 

[godfreyl]

The opa355 is pretty fast too. Frequency response is -3dB @ 15MHz for a gain of 10. Supply voltage = 2.5V min, 5.5V max, so it could be powered by three 1.5V cells or four 1.2V cells. Datasheet is here: https://www.ti.com/lit/ds/symlink/opa355.pdf

---------- Post added at 04:15 ---------- Previous post was at 04:02 ----------

The datasheet specifies a 100 ohm input (series) resistor in the application example for a low-noise preamp, any thoughts on why?

Assuming you mean the circuit below:
They're not suggesting to add a resistor in series with the input. Rs is just the source resistance i.e. the resistance of the microphone or whatever is connected to the input.

 

[chuckey]

If you use a circuit such as the one above, put a pair of back to back diodes from your input to earth to protect the amp from overloads. If you use a 10 megohm probe that was designed for some other scope, use it and wire a 50pF trimmer from + i/p to earth in your amp. This should then be adjusted to get the best response from prob+amp. As the output impedance of the amp is low it should be able to drive the input of the scope without any further compensation.
Frank