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Old 23rd August 2012, 07:34 PM   #101
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Originally Posted by RNMarsh View Post
Scott - you have the floor/ball..... take it and run. Are you going to do a discrete version of the 627 with us? Or? -RNM

I have something simple to start with as my discrete 627 is complicated, but I need to do it from home tonight.
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Old 23rd August 2012, 07:38 PM   #102
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Since virtually all of the likely designs to pour forth are going to want an output buffer, I dredged up and dusted off the attached.

It can be scaled back to lower rails without too much trouble. The bandwidth per se from a 1k source R is about 35MHz. There is the advantage of reduced thermal distortion when used with about a 515 ohm load, since the output devices are equidissipational*, that is, their dissipations roughly track. But when, as we anticipate, this is inside of an overall global loop with plenty of loop gain, the rest of the works will easily correct for dissipation-induced shifts, as the shifts should be fairly slow, and thus the thermal concerns should be minor.

The bootstrapping of the input devices raises the input Z, but also requires some lumped C to reduce peaking. Since such C is constant, the distortion at high frequencies at the input is reduced.

Short-circuit protection is left as the proverbial exercise to the reader.

Brad

*the approximate condition for this is that the peak output current into the load at just before onset of clipping is twice the quiescent current. Things are slightly more complicated by the emitter ballasting and the fact that the input devices draw their operating current from the output devices due to the bootstrap arrangement.
What I thought was meant by "opamp" was that it had both + and - inputs that were balanced to ground and basically required a differential input stage. There's absolutely nothing wrong with this circuit, but I assumed we were limited to a certain kind of topology. Did I make a wrong assumption?
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Old 23rd August 2012, 07:42 PM   #103
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Originally Posted by bcarso View Post

Short-circuit protection is left as the proverbial exercise to the reader.
It is limited by beta of output transistors and currents supplied by sources.
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Old 23rd August 2012, 07:44 PM   #104
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Originally Posted by dirkwright View Post
What I thought was meant by "opamp" was that it had both + and - inputs that were balanced to ground and basically required a differential input stage. There's absolutely nothing wrong with this circuit, but I assumed we were limited to a certain kind of topology. Did I make a wrong assumption?
This circuit is an output buffer that Brad proposed.

I have another version, can't find the picture...
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Old 23rd August 2012, 08:03 PM   #105
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Originally Posted by Wavebourn View Post
This circuit is an output buffer that Brad proposed.

I have another version, can't find the picture...
Oh, my mistake. Sorry Brad!
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Old 23rd August 2012, 08:05 PM   #106
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If we are not counting current sources, what about current mirrors? Do we not count them also?
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Old 23rd August 2012, 08:24 PM   #107
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You should quit 'counting' and start designing. These arbitrary rules are self limiting.
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Old 23rd August 2012, 08:25 PM   #108
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Quote:
Originally Posted by dirkwright View Post
What I thought was meant by "opamp" was that it had both + and - inputs that were balanced to ground and basically required a differential input stage. There's absolutely nothing wrong with this circuit, but I assumed we were limited to a certain kind of topology. Did I make a wrong assumption?
This is presented as a PART of a final opamp design. Note the triangles in the various Groner-presented topologies? Those are where this would go --- they are ~unity-gain buffers.

EDIT: sorry read and replied before I saw Wave's reply.
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Old 23rd August 2012, 08:25 PM   #109
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In terms of buffers I like Chris Paul's buffer design... not sure where to find the schematic though... a similar on was suggested in a diy thread a year or two ago and I piped in with the requisite moving of the input devices to bootstrap them to the outputs... maybe I can find that if I search enough...

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Old 23rd August 2012, 08:28 PM   #110
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Originally Posted by Wavebourn View Post
It is limited by beta of output transistors and currents supplied by sources.
It might survive a momentary short, like a brush with a clumsy test lead ground, but it's not likely bulletproof for magic smoke releasing probers
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