The gain-bandwidth product is the figure of merit
where is the midband gain and the bandwidth (see Amplifier frequency response). For a given device this product is roughly constant: whatever you do to the design, gain times bandwidth stays about the same. This constancy follows from a single dominant-pole roll-off — with one pole the gain falls at a fixed dB/decade, so the gain–frequency product is fixed; with several comparable poles GB is only an approximation.
What it means: gain and bandwidth are a zero-sum trade
If GB is fixed, then and are inversely linked. Want more gain? You lose bandwidth in proportion. Want more bandwidth? You give up gain. You cannot have both for free — the device only has so much “gain-bandwidth” to spend, and the design choice is just how to allocate it. This is the high-frequency counterpart of the gain-versus-headroom trade-off that already showed up in Overdrive voltage and MOSFET transconductance: amplifier design is a sequence of these “you can’t have everything” exchanges, and recognising them is half the skill.
Concretely, deliberately throwing gain away with Source degeneration or Negative feedback does not just buy linearity and predictability — it also buys bandwidth, because the GB you free up by lowering reappears as a larger . The lost gain is not wasted; it is converted.
Reused for the op-amp
The same idea carries straight into the Operational amplifier. An op-amp’s huge Open-loop gain falls off with frequency such that its gain-bandwidth product is a constant, quoted on datasheets as the unity-gain frequency — the frequency at which the open-loop gain has dropped to 1. [Background from general knowledge, not the source PDF: the explicit identification of as the op-amp’s GB and the unity-gain frequency; the PDF introduces GB for the transistor amplifier and notes it is reused for the op-amp.] For a closed-loop configuration the bandwidth is then divided by the closed-loop gain — exactly the same GB-is-constant trade, now applied to a feedback amplifier. So GB is one number that characterises the gain-bandwidth budget of any amplifier, transistor stage or op-amp alike. GB bounds the small-signal bandwidth; it is distinct from the large-signal Full-power bandwidth, which is limited by Slew rate rather than by GB. An amplifier can sit well within its GB-limited bandwidth and still be slew-rate-limited for large output swings.