Source-follower driven by floating opamp only needs low slew rate? - diyAudio
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Old 9th June 2002, 11:53 AM   #1
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Question Source-follower driven by floating opamp only needs low slew rate?

Lets say we have a source follower driven by an opamp as per the diagram. Normally the power supply rails of the opamp are referenced to earth so if we want the follower output to swing 40v then the opamp output stage must (neglecting fet threshold voltage) swing the full 40v plus say 1 volt extra because the gate-source voltage must increase that 1volt to turn the fet on harder.

In this circuit though - and I may just be re-inventing the wheel here - the ps rails of the opamp are referenced to the fet drain and as a result the only voltage the opamp output has to swing is the 1 volt necessary to turn the fet on harder. The bottom of the opamp is effectively bootstrapped by the main output voltage.

The voltage swing and the slew rate requirements of the opamp are greatly reduced. In fact the effective operating range of the circuit is not limited by the opamp at all unless the gate is driven to +15 volts by the opamp and this is still not enough to make the fet pull the load up. That of course means you have other problems.

Normally using an opamp to drive amplifier output devices is a big no-no but in this cct, seeing so llittle is demanded of it, maybe it would have a better chance of working well, using the entire opamp gain to linearise the output stage.

Has anyone else ever tried this cct?

GP.
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Old 9th June 2002, 12:30 PM   #2
djk is offline djk
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Audio Amateur had some articles by William Chatter that were this way. Denon made some receivers like this in the mid 80s (BJT). Velleman offers a kit like this.
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Old 9th June 2002, 12:40 PM   #3
Geoff is offline Geoff  United Kingdom
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GP

The following articles may be of interest to you:

"Bootstrapping your op amp yields wide voltage swings"

at http://www.e-insite.net/ednmag/

"Extending op amp supplies to get more output voltage"

Linear Technology Application Note 67 pages 58 & 59

Geoff
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Old 10th June 2002, 07:11 AM   #4
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Default Bootstraped OP AMP

The Audio Analog Puccini uses that tecnology but with bipolars darlingtons in the output stage...you can see it at...
http://www.pi.infn.it/~federico/puccini.htm
Regards
Jorge
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Old 12th June 2002, 12:13 AM   #5
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I would want to see some protection to keep the input to the op amp between the rails. Some op amps have this built in, in which case you only need a current-limiting resistor on the input.
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Old 12th June 2002, 02:06 PM   #6
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Where is the return for the floating supplies? THis almost looks like the high-side drive for a switching circuit. I dont think this will work if you have you floating reference connected to system GND.
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