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ash_dac 13th June 2006 07:06 PM

FET inputs or bipolar inputs on a power amp?

I have received a copy of Audio IC Op-amp applications (Walter G Jung) 1st edition , and I have been reading it all day. I feel it's a very well written book. FET inputs are recommended to overcome input bias limitations on slew rate.

Why is it most power amplifiers designs have bipolar inputs ? (often BC547 or similar)

I'm off to watch Brazil :)

Jeb-D. 13th June 2006 07:15 PM

Because most power amps suck! J/K;)

RetroAudio 13th June 2006 07:29 PM

probably cause most jfets need some sort of matching and dualies cost too much?

darkfenriz 13th June 2006 08:04 PM

For an op-amp input impedance of higher than 10 Mohm may be useful at some applications, especially with precise offset or high source impedance. For audio power amp 20kOhm is more than enough because of how signal sources are designed.
Moreover JFETs are not very happy with high Vds voltages, like 30-50V, so for higher power amplifiers they need to be cascoded.

traderbam 13th June 2006 08:26 PM

...and the transconductance of JFETs is not determined by their drain current. So parts need to be matched for both Vgs offset and gm. Very expensive for manufacturing.

...and the gm of JFETs is rather low compared to BJTs for typical LTP currents in power amps. An LTP with 1mA tail current will enjoy 40mS bipolar gm but typically less than 10mS JFET. This lower gm may necessitate an extra gain stage in some designs.

...and a well designed amp is not necessarily slew rate limited by the LTP transistor types.

AndrewT 14th June 2006 09:49 AM

Borbely uses FETs exclusively in his amps.
Have a look at Borbely's site and see the complexity he has incorporated into his front end.

Presumably, as the years have gone on, he has striven for more quality and this has progressively increased the number of front end devices he uses. Compare current to 1980 or so.

ash_dac 14th June 2006 06:49 PM


So it's possible but complex, and expensive! :(

I guess that leaves FET inputs for low level circuits (op amps, preamps,etc). I have looked at the AD823 that has a jfet front end.

"This new front-end design combines both bipolar
and JFET transistors to attain amplifiers with the accuracy and
low noise performance of bipolar transistors, and the speed and
sound quality of JFETs." (OP275 )

OP275 is on this site :

Is this just marketing (re-inforcing their market position) ?

How does a FET sound ? :confused:

AndrewT 14th June 2006 07:21 PM

OPA275 appealed to me, but the project I bought them for never got off the ground, so they still lie (about 10years) in stock. A variety of useful attributes, that potentially could sound nice.
Some day I'll get around to sockets and comparisons.

ash_dac 16th June 2006 08:49 PM

OPA604 datasheet - has another one of those 'FET notes' with a diagram. (see page 12)

"Many audio experts believe that the sound quality of a
high performance FET op amp is superior to that of
bipolar op amps.",P1356,D4280

FET distortion increases with frequency but the distortion is easy on the ear ? (according to the audio experts)

AndrewT 17th June 2006 09:04 AM


FET distortion increases with frequency
there's something else I didn't know.

Low pass filter with rising distortion in the pass band due to high source impedance is instructive!

More implications to be revealed by our experts presumably.

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