Best Transistor for a Gyrator???

[Bill Mountain]

I'm working on a project and need a gyrator.  I have already used both 1/2's of a dual opamp and I still need something to make a gyrator.  I plan on using a transistor instead of an additional opamp (unless I come up with a need for a quad opamp -  maybe buffered Vref?) and I was wondering what parameters I should look for in a gyrator transistor.

The only thing I could think of was using a darlington transistor (or other high HFE transistors) so it has a higher input impedance and would be a more accurate buffer.  But I doubt it really even matters.

Thoughts?

[R.G.]

You're right about high hfe (the AC small signal version of the DC "HFE"). High gain single or Darlington, although darlington probably isn't needed. A lot of Boss pedals use NPN bipolars at about 200-400 hfe for gyrators in voicing fuzz pedals. Keen's Second Law still stands - when in doubt, use a 2N5088.

[amptramp]

Darlingtons may have high gain and input impedance, but when the input signal moves towards cutoff, there is a substantial time taken for the charges in the output transistor base to recombine and turn the transistor off.  This leads to a difference in high frequency response between turning on and turning off.  Some people use a darlington with a resistor from the first emitter to the second emitter just to have a path for base current when it shuts off, but this reduces the input impedance.

http://www.geofex.com/

has an equalizer buried in "Vintage Effects Schematics" under Electro-Harmonix that shows op amp gyrators.  Transistors can't beat op amps at their own game and the op amps may be cheaper.  But as R.G. says, a single transistor with a high gain may be good enough.

[Bill Mountain]

Thanks guys.

I'll make sure to try it both ways before I commit.

Thanks!

[Bill Mountain]

Also...should I adjust the emitter resistor to get 4.5V on the collector?

[PRR]

> get 4.5V on the collector?

? ? ? Surely you want an Emitter Follower? You have not said what supply, but I'd ass-ume the collector is nailed to +9V.

[Bill Mountain]

> get 4.5V on the collector?

? ? ? Surely you want an Emitter Follower? You have not said what supply, but I'd ass-ume the collector is nailed to +9V.

Yes of course.  I meant to say 4.5V on the emitter.  I plan on using a 9V supply.  I'm tailoring the previous stage and so the eq stage with the gyrator doesn't clip at max boost.

[PRR]

> a higher input impedance and would be a more accurate buffer.

But you "want" resistance. Otherwise the Q tends to infinity, and that is almost never what we want in audio.

Mostly we use quite low Q. Like 1. So the input need only be a bit higher than the circuit, the output just a bit lower. Current gain of 100 limits us to Q=10(?). So any darn transistor is good enough for audio curves. '5088 is the go-to part. Lesser parts will do if you need to shave a penny.

[Bill Mountain]

Thanks for the help guys.  I don't pretend to understand.  I just try to make things work.