# Op-amp dominant pole bandwidth

I have been playing around with an op-amp design using TINA from Texas Instruments. I have selected the LM301A. TINA says that this op-amp has a dominant pole at 5 Hz. I have put the circuit below. VG1 is feeding in a 2 Hz sine wave at 50 mA. The supply is $\pm 5\text{ V}$. When I simulate this circuit the -3 dB point seems to be around 38 kHz. I am obviously doing something wrong, I just don't see it. Can anyone offer any advice?

The dominant pole can be seen only in the open loop configuration. When you add negative feedback, gain is reduced and the BW is enhanced (for the closed loop system)

Op amps which use dominant pole compensation have a constant gain-bandwidth product. You've cited a 5Hz dominant pole, and the LM301A datasheet gives a typical open loop voltage gain of $160\text{V}/\text{mV} = 160,000$. This gives a gain-bandwidth product of $160,000 \times 5\text{ Hz} = 800,000\text{ Hz}$. You have configured this op amp with a closed loop gain of $|A_v| = 20$ so the expected bandwidth is $800,000\text{ Hz}/20 = 40,000\text{ Hz}$, which is close to your observed -3dB point of 38kHz.

• Thanks for the reply! It's good to see the maths behind it. – o.fithcheallaigh Jun 2 '16 at 14:49