Electronics Forum / Basics / April 2007

Open loop gain of what?

It doesn't, necessarily.

John

It is not necessarily so.  This effect (gain decreasing
linearly as frequency rises) is caused by a single pole (one
energy storage element with the energy supplied through a
linear source, like a resistor) of a low pass filter.  Many
circuits have more than one energy storage element in them,
and some may be in a high pass configuration, or in a
resonant configuration.  Those circuits do not have the gain
decreasing linearly and in proportion to rising frequency.

But if an amplifier (such as an opamp) is being designed to
have stable negative feedback programmed gain over a wide
range of gain choices, it is very helpful to this end if the
gain roll off with rising frequency, is dominated by a
single pole, since this keeps the input to output phase
shift near 90 degrees till the gain falls below 1.

Excessive phase shift around a closed loop can convert
negative feedback to positive feedback.  If this happens
while there is still effective gain around the loop, the
closed loop configuration is unstable and will sustain
oscillations (a self reinforcing echo that creates copies of
itself, that travel around the loop, instead of damping out).

Because that's where the gain bandwidth figure product come from !

Eg if an op-amp has a gain bandwidth product of 1 MHz, the gain is 1 at 1 MHz,
10 at 100 kHz ,100 at 10kHz and so on.

There will be a maximum gain at low frequencies where it stops increasing
though.

Look up dominant pole compensation for more details.

Graham