I would appreciate suggestions or best practices for designing a stable amplifier. My goal is to design a 94 dB gain low-noise op-amp amplifier for a 100 kHz signal with a bandwidth of 10kHz.
At this point I have made several designs but, in the end, the circuit does not work consistently. It sometimes shows the expected gain (even when only 47 dB is used), but then, after a few seconds, the gain drops significantly or the noise floor increases. Find my circuit below using the LTC6226 (1nV/√Hz 420MHz GBW):
Some of the things I have tried already are (note: the circuit was built on a breadboard):
- Very short wire connections.
- Supply capacitor directly to terminals. Big supply (680 μF) capacitors.
- Single or double op-amp.
- Separate boards and supplies for each op-amp.
- Increase gain to reduce BW.
- Use lower GBW opamp.
- Match input and output to 50 Ω.
One of the things I have not clear yet is the influence of the input bias current on the gain and stability of the amplifier. I thought the GBW was the primary factor. I thought the input bias current was mainly related to the minimum detectable signal.
Any suggestions on these topics would be great.