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I am working on developing RF wave generation from a DDS IC (AD9834-Analog device). The voltage we require as an RF signal is 6 times (3V peak to peak, sinusoidal, DDS output~=0.5V with supply to the board 3.3V from LiPo battery (3.7V) ). Initially I started with opamp (LMV794, dual opamp from TI, GBW Product=88Mhz). One of the opamps is used as difference amplifier (DDS has two output, 180 phase difference only): technically doubles the voltage and then fed to a non-inverting configuration for further amplification(Gain=3) with the other opamp. It turned out that the amplification I was gaining was some kind of unstable. It generated a lot of noise. And sometimes, final output (output of non-inverting one) of OPAMP (LMV794) is around 1V with a HF noise (8 Mhz or 10.93MHz)even when DDS is not feeding any voltage. I replaced the OPAMP several times with new ones thinking that the OPAMP might be damaged. But since the final output did not satisfy me, I could not move on to proceed with the PCB. enter image description here

I switched to 2 stage class A amplifier built with NPN transistors(BC33740TA) on bread board, the input is fed from DDS on PCB. The output is quite satisfying, the class A amplifier proves to respond well with different RF frequency (5-15 MHz). I am planning to redesign the PCB with transistor (class A amplifier) instead of OPMAMP based amplifier. But, I need to know what I was missing with OPMAMP based amplifier. Can anybody explain?

-Thanks in advance
edited: added the schematic and formatted the question into two paragraphs

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    \$\begingroup\$ Consider adding paragraph breaks to make your question more readable. \$\endgroup\$
    – The Photon
    May 25 '17 at 0:58
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    \$\begingroup\$ Show the schematics and how bypassed and how the GROUND plane is organized. \$\endgroup\$ May 25 '17 at 2:26
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    \$\begingroup\$ You have some type of op-amp stability problem due to capacitive loading or poor layout, or you are attempting to exceed the gain bandwidth product of the op-amps. \$\endgroup\$
    – mkeith
    May 25 '17 at 2:49
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    \$\begingroup\$ Did you try to breadboard the op-amps? This is very difficult to do. They can be very sensitive to layout. However, if you succeeded with a BJT circuit, congratulations! Good job. \$\endgroup\$
    – mkeith
    May 25 '17 at 2:53
  • \$\begingroup\$ @mkeith: Thank you for your comment. I still consider myself a newbie: if you please explain GBW scenario for this design. The gain of first opamp is 1, second opamp is 3 with maximum frequency tried 15Mhz. I don't understand why I should blame Gain Bandwidth Product since LMV794 has GBW of 88Mhz \$\endgroup\$
    – sbz
    May 25 '17 at 15:48
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The lmv794 is not internally compensated to be unity-gain stable. You must compensate it yourself or operate at the minimum closed-loop stable gain of 10x (20dB).

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  • \$\begingroup\$ Thank you for the comment. You answer gave me an new insight to the problem. \$\endgroup\$
    – sbz
    May 25 '17 at 16:57
  • \$\begingroup\$ Unless your really need that amplifier, pick an amplifier that still has positive phase at the 0dB crossing (phase margin). Most datasheets will provide an open-loop gain & phase plot. \$\endgroup\$
    – sstobbe
    May 25 '17 at 17:01
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OP Amps have the benefit of low noise, , low supply current, low voltage , low output impedance at low gain and very high input impedance.

Without specs on load impedance, and gain vs BW you obviously made a poor selection of OA.

Video Amps are far superior for high gain but higher output impedance, and obviously a selection with much higher GBW with better phase margin is desireable in your specs with a Zout spec.

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