Opamp+BJT filter: How is it called ?

[frank_p]

How is it called ?
Where can I find infos on how it works ?
Ref: DOD Grunge.

Thanks !

[gritz]

the transistor and components surrounding it simulate an inductor with one end grounded - it's called a gyrator. In series with the 27n cap it forms a notch filter which (basically) sucks signal at a particular frequency out of the feedback loop of the opamp, so the opamp boosts signal at that frequency. The gyrator is very often used in graphic equalisers and can be made around an emitter follower (like in your example) or round a non inverting opamp buffer.

There's an online calculator here: freecircuitdiagram.com/2010/09/17/bandpass-filters-gyrator-frequency-calculator/

The wikipedia entry on gyrators is (as usual) overly complicated and has obviously been copied from a text book by someone who doesn't have a clue. No change there then.

[Earthscum]

http://www.muzique.com/lab/gyrator.htm

Just happen to have that page open. AMZ has less stuff going on in the web page, just a faster load is all.

[frank_p]

the transistor and components surrounding it simulate an inductor with one end grounded - it's called a gyrator. In series with the 27n cap it forms a notch filter which (basically) sucks signal at a particular frequency out of the feedback loop of the opamp, so the opamp boosts signal at that frequency. The gyrator is very often used in graphic equalisers and can be made around an emitter follower (like in your example) or round a non inverting opamp buffer.

Thanks Jonathan and David.  Ohh...  I thought that it was the whole block that was called the gyrator...   
Good simple explanation.  Thanks !!

[sault]

If I may suggest my own gyrator calculator. It's been over a year since I wrote it, now, and I've never gotten around to fixing the #()!@$ capacitor value. Until I remember to do it you'll have to do the values mentally... ie, if you've got 0.027 uF, that's actually 27 nF, that kind of thing. On the other hand, it does let you see how the frequency response changes in real time as you adjust the values.

http://awasteofsalt.com/gyrator

Dang. I need to update how I plot the graphics, too. Ach, well, if it helps, it helps...

[frank_p]

If I may suggest my own gyrator calculator. It's been over a year since I wrote it, now, and I've never gotten around to fixing the #()!@$ capacitor value. Until I remember to do it you'll have to do the values mentally... ie, if you've got 0.027 uF, that's actually 27 nF, that kind of thing. On the other hand, it does let you see how the frequency response changes in real time as you adjust the values.

http://awasteofsalt.com/gyrator

Dang. I need to update how I plot the graphics, too. Ach, well, if it helps, it helps...

Yes, I've seen it.  Thanks !
I like to do my own calculations.