Butler front end?

[mikep]

Im looking for information about the mysterious circuit that is used in the first stage of the OP275. Google isnt doing it for me on this one. who is this butler guy (or girl)? does this concept predate the Analog Devices implementation? I'm interested in trying this arrangement in a simple amp using discretes. can someone point me to a schematic that shows the basic topology? EDIT: found a simplified schematic in the OP176 datasheet. hmm...

thanks,
mike p

 

[bcarso]

US 5101126, James Butler and Douglas Smith, assigned to Analog Devices, for whom he worked at the time IIRC. Issued March 31, 1992.

See patent.

 

[ciminosound]

From Analog Devices

The OP-285 is a precision high speed amplifier featuring the Butler Amplifier front-end. This new front-end design combines the accuracy and low noise performance of bipolar transistors with the speed of JFETs. This yeilds an amplifier with high slew rates, low offset and good noise performance at low supply currents. Bias currents are also low compared to bipolar designs.

The OP-285 offers the slew rate and low power of a JFET amplifier combined with the precision, low noise and low drift of a bipolar amplifier. Input offset voltage is laser-trimmed and guaranteed less than 250 µV. This makes the OP-285 useful in dc coupled or summing applications without the need for special selections or the added noise of additional offset adjustment circuitry. Slew rates of 22V/µs and a bandwidth of 9MHz make the OP-285 one of the most accurate medium speed amplifiers available.

 

[PRR]

This link should work for a few days:
http://www.pat2pdf.org/patents/pat5101126.pdf

It is a refined "Slew Enhanced" input. It is old news to backup a diff-pair with a Class C booster so that when the main pair gives up, the booster slams the slew around. Butler's plan is more gentle, and uses the difference of curve between BJT and FET to divide the work. The FET does add something at small signal.

 

[mikep]

[quote author="PRR"]This link should work for a few days:
http://www.pat2pdf.org/patents/pat5101126.pdf[/quote]

thanks. more like, will keep me busy for a few days!

does this idea of paralleling fets with bipolars make sense in circuits other than the diff pair? Based on a simulation I just did, it seems to work in general, I just don't know if it is beneficial yet.

m