I've been working with the following very basic opamp circuit:


simulate this circuit – Schematic created using CircuitLab

I've built this Voltage amp a few times for a couple of low-frequency opamps, and it never let me down. I now need to amplify a high-frequency signal for the first time, however, so I switched to the OPA699 in the hope of amplifying a ±10 MHz signal.

The above image shows the exact circuit I built. I built it "dead bug style", meaning the opamp has been glued upside down onto a copper plate, and all resistors have been soldered together mid-air. The copper plate functions as a ground plane. The opamp is directly connected to a power supply, which is specifically designed to supply an extremely stable (there's some huge capacitors in there) ±5V voltage. The input signal is a 100 Hz sine, supplied by a TABOR 8020 function generator through a small cable, while the output is readout using a BNC bus connected to my oscilloscope.

Now my problem: when I build this circuit with my low-frequency opamps, both on a breadboard and dead bug style, all is well. But with this high-frequency opamp, the circuit doesn't work. When I don't apply any voltage to the input whatsoever, the opamp still clips. Applying a small DC input voltage doesn't change a thing.

All that appears to generate any output whatsoever is applying a signal that is sure to overload; a 5Vpp sine wave for instance gives a very noisy clipped sine wave as output. The output is then riddled with 12 MHz noise.

When I attempt to apply an offset to the opamp, I again get MHz-range noise.

I'm starting to believe that either the OPA699 isn't built for such a voltage-amplifier circuit, or that I'm making a huge mistake in my circuit. I know that at high frequencies, opamps can start oscillating (which is why I opted for dead-bug-style), which might invalidate my circuit. Is there any way I can get this to work?

TL;DR: I built the above circuit. When I give a sine input, the output either clips completely or consists mostly of MHz-range noise. I'm thinking that either I'm misusing the OPA699, or perhaps my circuit is not meant to be used in high-frequency applications.

OPA699 datasheet: http://www.ti.com/cn/lit/ds/symlink/opa699.pdf

  • \$\begingroup\$ What are you applying to the VH and VL pins? \$\endgroup\$ – Hearth Apr 27 at 2:14
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    \$\begingroup\$ Where are your bypass caps on power supply rails? \$\endgroup\$ – Ale..chenski Apr 27 at 2:16
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    \$\begingroup\$ Perhaps the layout and stray positive feedback is making it unstable. Can you show photos? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 27 at 2:23
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    \$\begingroup\$ "directly connected to power supply with huge capacitors in it" is not necessarily as good (or even: sufficient) as "small decoupling caps very close to the Opamp"; wires have non-negligible inductance, and that negates the stability of your supply. \$\endgroup\$ – Marcus Müller Apr 27 at 8:42
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    \$\begingroup\$ Oh wow, I feel very stupid now--I now see I've completely botched the circuit at the VH and VL pins so it's no wonder the whole thing clips. I've become so used to low-frequency amplification that I think way too lightly of opamp-circuitry. Before I get back to ruining the relatively expensive OPA699, I'd like to get some more experience in making this type of circuits. Do you have any recommendations? The OPA698 is already 1EUR cheaper. I want the cheapest opamp that provides the same challenges as the OPA699, so that I won't mess up when I return to the 699. \$\endgroup\$ – Heatherfield Apr 27 at 9:19

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