Slew rate and Bandwidth for 5 MHz signal

Question

I'm trying to amplify a sine wave signal with opamps of high bandwidth and slew rate. It works fine up to about 700 kHz, then the gain decreases and about 1 MHz it's only 1, when it should be about 5.

I'm testing with the circuit below and two different opamps, with similar results: LMH6658 (bandwidth 270 MHz, slew rate 700 V/μs) and AD8066 (bandwidth 145 MHz, slew rate 180 V/μs).

LMH6658: https://www.ti.com/lit/ds/symlink/lmh6658.pdf?ts=1711450406602

AD8066: https://www.analog.com/media/en/technical-documentation/data-sheets/ad8065_8066.pdf

I don't know if I miss something. Should I expect to be able to get a gain of 5 with these opamps up to 5 MHz?
I'm testing in a breadboard. The input signal is about 600 mV, and the sine wave is clean and not distorted in the output, even above 1 MHz.

Could be that the opamps are fake/relabeled, but before returning and ordering them in Mouser I wanted to make sure. I don't know if there could be other problems. I understand that the opamps with that bandwidth and slew rate should be enough for these frequencies, but I'm not sure. Thanks!

Edit: I add some concrete numbers. Checking it better I see the performance of both opamps is different (after many tests I had a mix of results in my head). LMH6658 is much better, but anyway the gain starts to decrease below 500KHz.

LMH6658 (input signal ampl -> output signal):
    100KHz: 680mV -> 3.32V
    400KHz: 680mV -> 3.24V
    600KHz: 680mV -> 3.16V
    800KHz: 640mV -> 3.00V
    1MHz:   600mV -> 2.84V
    2MHz:   480mV -> 2.16V

AD8066:
    100KHz: 680mV -> 2.56V
    600KHz: 680mV -> 1.30V
    800KHz: 640mV -> 1.00V
    1MHz:   600mV -> 760mV
    2MHz:   480mV -> 360mV

Answer 1

On a breadboard you can expect lots of capacitance between each pair of rows of connectors, perhaps 10pF. With your feedback resistance of 10kΩ, combined with capacitances of that order, an estimate of cut-off frequency might reasonably be:

$$ f = \frac{1}{2\pi \times 10k \times 10p} = 1.6MHz $$

There will be similar attenuation anywhere where the signal path contains significant impedance. You could try reducing R1 and R2 by a factor of 2, or 5, for example, just to see if the situation improves. If it does, then you'll have to find a better way to prototype the circuit.

Answer 2

A couple of reasons.

First, the schematic shows no bypass caps on supply.

Second, you have built the circuit on a breadboard. You can't expect high performance analog or digital operation.

Third, it depends how you are measuring it. 1x scope probes may have around 5 MHz bandwidth anyway.

You said you were measuring with 1x probes. That is likely the problem that limits the bandwidth. As a rule of thumb, never use 10x/1x probes in 1x mode unless you are in a situation where you know you need to use it.

You said you are using electrolytic bypass caps. A general purpose electrolytic may be useless at 1 MHz. It also makes things worse if the electrolytic is on breadboard, with long wiring through breadboard and adapter to IC. A 100nF ceramic right on the adapter might make a difference, if there is no improvement from switching probes to 10x.