OPA172: Open loop gain variation at 1Khz over operating temperature

Hello Shibab,

I did some investigation into your question about how the OPA172 open-loop gain (Aol) will vary with the 23 mH + 310 ohm load, over temperature. Here's my conclusions:

1. Open-loop gain accomplished simulations with and without the 23 mH + 310 ohm resistor in place show virtually no difference in the nominal value (80 dB) at 1 kHz. There is a difference in the unloaded Aol vs loaded Aol of about 4.3 dB at 1 Hz, and 0.5 dB at 10 Hz, both frequencies well below 1 kHz. The OPA172 model includes the complex open-loop output impedance (Zo) which is an important factor in such simulations.
2. Our years of experience with a wide variety of different operational amplifiers has allowed us to use establish and apply a general approximation for the variance in a parameter such as gain-bandwidth product (GBW) over temperature, supply range and process variations. The gain in that term is the Aol gain. The approximation that we apply is the GBW will not fall below 0.5x typical on the low end, or exceed 1.5x typical on the high end. Thus, for the OPA172 that has a typical GBW of 10 MHz, its full range is 5 MHZ to 15 MHz. However, it is more likely to be closer to the typical 10 MHz.
3. Applying the typical 10 MHz GBW at 1 kHz, the Aol = GBW / f = 10e7 / 1e3 = 1e4 (80 dB)
4. Applying the minimum 5 MHz GBW at 1 kHz, Aol(MIN) = 5e6 / 1e3 = 5e3 (74 dB)
5. Applying the maximum 15 MHz GBW at 1 kHz, Aol(MAX) = 15e6 / 1e3 = 1.5e4 (83.5 dB)

Using this range of Aol variation would allow the closed-loop gain (Acl) to be calculated over temperature, etc.

The Op Amp Bandwidth sessions of the TI Precision Labs series may be referenced if additional information on the subject of GBW is needed:

www.ti.com/precisionlabs

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

Thanks you for the reply.

From you post what I understand is over the operating temperature -55 to +125C, the estimate for Aol variation at 1Khz is 74dB to 83dB. Typical is 80dB.

Can we consider the Aol value at -55C will only be in increasing order than the Aol at 25C?

Also the Aol at +125C will only be in decreasing order than the Aol at 25C?

I thought a tolerance analysis will be more meaningful if we can consider the actual behavior of Aol at -55 and 100C, than considering the above specified Aol variations at both temperature extremes.

Also, can you please provide any white paper you have on the subject "Opamp Aol variation over temperature"?.

Thanks,

Shihab.