>> whereas OPA2277 looks like bipolar input. That means the later one needs larger input bias current, which could cause too large voltage offset at the output of your amplifier.> So basically, don't use it.
No, basically you don't rely over-much on free advice found on forums, even from a sharp dude like kvant.... you look it up and figure it out yerself.
OPA277 datasheet http://focus.ti.com/lit/ds/symlink/opa227... shows input bias current of 3 nanoAmps. One microAmp in 1 MegOhm is 1V which is a lot. But 3nA in 1Meg is 0.003V which is small. Most audio circuits can use bias resistors of 100K or less, now we are down to 0.000,3V or 0.3mV which is negligible. To this we must add up to 0.1mV of offset voltage error... still negligible.
In fact DC performance of OPA277 is frikkin great for most audio use.
What I see is modest high frequency performance. GBW is 1MHz, slew rate is 0.8V/uS. Same-as a '741. True, we used to use bag-fulls of '741 op-amps. For low gain and low voltage levels, they are still inoffensive. Might be "OK" in a headphone amp. But far better chips can be bought for just a few bucks.
Output drive is another question. OPA277 is rated 35mA into a short, not specified to work good for loads under 2K. (Again, it seems to be a '741 but with fabulous input DC performance.)
The '2277 would be a dandy DC servo amp.
FWIW: the old "FET vs BiPolar" line is fading. FETs can have startlingly high gate current at high temperature where BJT tends to decline (maybe not relevant for personal audio). The '177/'277 have a low intrinsic bias current (similar to '741, which echoes its '741-like AC performance) then adds a bias-current compensation technique which cancels 99%-101% of that bias current. (Some trim is done by lasering the die.) '741 bias current was rarely any issue in audio, and this is 100 times better. At the input... the output is still '741-like and we now prefer better devices for most audio.
OPA2132 has even lower bias current, but the '277 is already "plenty good". Bandwidth and slew at like 10 times better... if '741 was almost good enough for audio, these are fine audio devices. Performance is not specified for loads under 600 ohms, and max current is only 40mA: this is not an authoritative 32-ohm driver, though many people are perfectly happy.
FWIW: "FET input" '2131 has higher bias current than "BJT input" '277... when temperature is 100 deg C or above. Yes, we could not touch the knob while the amp was at boiling water temperature.
'2131 has amazingly lower THD specs than '277. The '277 numbers do not stink, and the '2131 zero-zero-zero specs look like a typo (but are probably genuine, under specific conditions). THD is a poor predictor of audio accuracy or pleasure.
posted 02-23-2008 12:10 AM CST (US) 
