Stable 94 dB gain op-amp amplifier

Question

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.

I have also found some great articles 1 2 3 about this but my problem persists.

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.

Answer 1

Few know that GBW demands increase with the square of Q, thus you need a GBW = 10^2 * 100 kHz = 10 MHz at 0dB gain. With an overall closed loop gain of 94 dB ~ 2.5e4 you need more low gain stages with a total GBW = 2.5e4 * 10e6 = 2.5 e10 total.

Thus using inverting input for a BPF with 420 MHz GBW or 4.2e8 and a max gain of 420MHz/10MHz = 42 you need 2.5e4 /42 = 595 stages. Clearly using integrator internally compensated Damm Fast OA, is not the solution and rather you need video amps that are not unity-gain stable.

However reducing the filters to unity gain and using wideband high gain amps can reduce the \$Q^2\$ to a linear GBW issue. (Added see my pro bono cct design below).

With only 330 MHz non-inverting GBW, at 100kHz with have only a gain of 3.3e3 of open loop x 2 stages = 6.6e3 gain reduced in both Q and gain with load effects of 10 ohms relative to open loop output impedance to get far less gain of 47 dB or ~ 2e2 or reduced to 3% of avail BW limited by current.

Open loop Z, is approx Voh/Isc=5V/60 mA typ= 83 ohms which in 2 stages limits it by ~39 dB.

Using the same Op Amps but with metal foil shields (cans over PCB gnd plane), this might be stable and give 94 dB gain Bessel shaped Q=10 @ 100 kHz with GBW=330 MHz OA's and gain stage at front and back ends depending on interference sources to prevent saturation but problematic if cascaded raising sensitivity to <0.01 pF positive feedback.