Wrongly documented TI LM324 unity-gain bandwidth?

Question

I was reading the datasheet of the LM324 from TI (http://www.ti.com/lit/ds/symlink/lm324.pdf).

They specify a generic GBW of 1.2MHz. This is usually done with no output applied, from what I understand. Then there is the \$ A_{vd} \$ which is max 100K (100 V/mV). This is the open-loop gain measured with a load, so taking into account loading effects, if I get it right.

Is this gain really reducing by a factor of 10 with few \$ k \Omega \$ on the output?

To understand this better, I read the SLOA11 from TI. They mention \$ B_1 \$ to be the bandwidth at unity-gain with a load specified under the \$ A_{vd} \$ measurement, which should result in a BW of 100kHz. Making it different from the GBP measurement with no load (resulting in unity BW of 1.2 MHz). But in the LM324 datasheet, \$ B_1 = 1.2MHz\$ instead of \$ 100kHz \$. So I am not sure here what are the correct terms and how to use them.

Answer 1

I would reckon that the 100k figure is the gain at DC. Now if the dominant pole is at 12 Hz, then the GBW product above this frequency will be approximately 1.2 MHz. The output impedance of an opamp is normally of the order of 100 ohm so a load of a few k is not significant.

Answer 2

They specify a generic GBW of 1.2MHz. This is usually done with no output applied, from what I understand. Then there is the Avd which is max 100K (100 V/mV). This is the open-loop gain measured with a load, so taking into account loading effects, if I get it right

Having a GBW product of 1.2 MHz doesn't mean that there is any frequency where the gain is 1.2 million. Having an open-loop DC gain of 100 V/mV doesn't mean that the gain must drop to 0 dB at or below 100 kHz.

Assuming this is an old-fashioned dominant-pole-compensated design, you could imagine that the DC gain is 100 V/mV and the dominant pole in the open loop response is at about 3-4 Hz, so that the response falls of at 20 dB/decade to reach 0 dB at 1.2 MHz.