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High Gain Phase Inverter - MOSFET Follies 8-)

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  • High Gain Phase Inverter - MOSFET Follies 8-)

    I did some work with some Zetex high voltage MOSFETs. These are a bit unique in that they are TO-92/E-line packages, same size as your garden variety small plastic signal transistor. They are 0.7W, 450V devices, and their input capacitance (Ciss) is only 55pF, unusually low for a high voltage MOSFET.

    That's low enough in fact that a 69K rp 12AX7 plate will drive it flat to over 100kHz.

    So I messed with this thing as a LTP. I put 100K plate loads on each side, used a 22K common source resistor and 1M bias resistors to a 68V zener bias point.

    The result? It's beautifully balanced, produces high drive, and more importantly, has enough output that you could shape the "clipping" nature of the thing to be as tube like as you want to mess with. This thing will swing 200V peak to peak on each side of the LTP with a 350V supply.

    And did I mention it had high gain?

    It was so high that I had to put a monster divider in front of it to get it down to a signal level on the output tube plates that was not just all square waves all the time.

    I modelled it up and the sim says that it has an open loop gain of a bit over 80db... That's enough to just plug your guitar right into the phase inverter and still overdrive the output tubes.

    So I put some source degeneration into the LTP, 1K on each source before the 22K source resistor. That got it back down to only 46db.

    For anyone wanting to play with the MOSFET Heresies, these devices are good fodder. The Ciss is low enough to let you get full audio band response from them in an environment which 12AX7 tubes work in. They're too linear to actually be tubes, of course, and they need enhancement biasing instead of depletion biasing, but other than that, they're a really useful adjunct to tube stages.

    I've messed with the cascode setup a bit - depletion mode JFET running into the source of the MOSFET - and that's interesting as well, but I don't have any firm circuits yet.

    Another thing I have to go mess with is current buffering a 12AX7. I have a couple of ideas on how to make a ZVN0540A multiply the current out of a 12AX7 by a fixed amount. That would let us use a "12AX7" as an output tube.

    Sigh. So many circuits and so little time...
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

  • #2
    R.G.,

    80dB gain! No worries implementing FX returns, reverb, etc. with this thing, eh? And what a killer input stage for a 'hybrid Zen-amp push-pull' kinda thing - two gain blocks and done.

    I wonder how two depletion-mode devices would work in a 9-pin replacement 'super-PI module'? I could see people being interested in such a thing... and also a 12AX7 output stage!

    Ray

    Comment


    • #3
      I don't know of any depletion mode devices that will drop right into the circuit.

      These devices have an unusually high transconductance, low Ciss, and handy packaging.

      The biasing is easy, but just not depletion mode. You have to tie the gates up at some voltage that then sets the right current through the source resistor and drain resistors, and it needs to be large compared to the variation in Vth.

      I could probably have gotten away with maybe 25 or 30V for a zener bias point.
      Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

      Oh, wait! That sounds familiar, somehow.

      Comment


      • #4
        That's pretty cool R.G.! I like the "Too linear to be a tube" part especially What's the part number of the device you used?

        BTW, I noticed you mentined current multiplying. I've been puzzling for years about how to use a solid-state output stage to boost the power of a tube amp, while still letting the tubes react to the speaker impedance. I wrote this years ago

        http://www.scopeboy.com/boots.html

        and I finally figured out about 2 weeks ago how to make a suitable output stage with unity gain, low distortion, and soft clipping. So now I can have some real MOSFET heresies like a 400W bass amp powered by a single OTL 6V6. I ain't telling how though! :P
        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

        Comment


        • #5
          I think he mentioned ZVN0540A. Unfortunately either not available or available only in bulk.

          Comment


          • #6
            Hmm, you're right, the ZVN0540 seems quite hard to get. The usual suspects like DigiKey, Farnell or Mouser don't seem to carry it. Where did you get yours RG?

            Hey, I just thought, if I can't get any can I substitute a 12AX7?
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

            Comment


            • #7
              ZVN0545A availability

              Steve, I've just seen them listed on the Farnell site for £1.54 p/no 9525424

              TDS

              Comment


              • #8
                Originally posted by Steve Conner View Post
                That's pretty cool R.G.! I like the "Too linear to be a tube" part especially What's the part number of the device you used?

                BTW, I noticed you mentined current multiplying. I've been puzzling for years about how to use a solid-state output stage to boost the power of a tube amp, while still letting the tubes react to the speaker impedance. I wrote this years ago

                http://www.scopeboy.com/boots.html

                and I finally figured out about 2 weeks ago how to make a suitable output stage with unity gain, low distortion, and soft clipping. So now I can have some real MOSFET heresies like a 400W bass amp powered by a single OTL 6V6. I ain't telling how though! :P
                Current dumping? OTL to drive it - what about "the sound is in the iron" aspect?
                Aleksander Niemand
                Zagray! amp- PG review Aug 2011
                Without the freedom to criticize, there is no true praise. -Pierre Beaumarchais, playwright (1732-1799)

                Comment


                • #9
                  Well, think of it as an experiment to see whether the sound is "in the iron" or not. I could get more ferric goodness by using a choke in the plate circuit of the tube output stage. If I wanted to boost a PP amp though, I can't see any way round using an output transformer.

                  I'm not telling whether it uses the Quad current dumping principle or not. :P
                  "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

                  Comment


                  • #10
                    Try a ZVN0545 in the TO-92 package. They're quite similar. Digikey has them here for USD$1.50 each.
                    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                    Oh, wait! That sounds familiar, somehow.

                    Comment


                    • #11
                      Originally posted by Steve Conner View Post
                      I'm not telling whether it uses the Quad current dumping principle or not. :P
                      I'm not telling either, your secret is safe
                      Aleksander Niemand
                      Zagray! amp- PG review Aug 2011
                      Without the freedom to criticize, there is no true praise. -Pierre Beaumarchais, playwright (1732-1799)

                      Comment


                      • #12
                        Sooo... from the dead this thread comes. Is this an accurate transcription of RGs LTP circuit? I have some of these MOSFETs coming from mouser and wouldn't mind playing with this idea.

                        Will this circuit allow global NFB? The addition of the zener is throwing me off, but I think it should work.
                        Attached Files
                        -Mike

                        Comment


                        • #13
                          Sooo... from the dead this thread comes.
                          I was also waiting for some updates.

                          Comment


                          • #14
                            Originally posted by defaced View Post
                            Will this circuit allow global NFB? The addition of the zener is throwing me off, but I think it should work.
                            Unfortunately, that circuit won't work as shown. the ZVN0545 MOSFETs are enhancement mode MOSFETs. You have to do something to their gate to get them to conduct at all. This is unlike tubes, which are depletion mode devices. Depletion mode devices conduct heavily all the time unless you do something to the gates/grids to turn them off. This circuit has the gates tied through 1M resistors to the sources, and so there is no forward bias at all, and hence no conduction at all.

                            What you'd have to do is to tie the gates to some source of positive voltage, and let the sources drop below that voltage by the Vth of the MOSFET, about 1-3V for the ZVN0545 acccording to the datasheet. The gate voltage sets the operating point, and the resistor from the source to ground sets the operating current.

                            Yes, you probably can put negative feedback into it.

                            Sorry I haven't updated this - I get pulled in a lot of directions, and don't have time to complete a lot of things I'd like to.
                            Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                            Oh, wait! That sounds familiar, somehow.

                            Comment


                            • #15
                              Does the high gain necessitate zener regulated bias? Or would a simple divider off the B+ suffice?

                              Comment

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