Announcement

Collapse
No announcement yet.

Bootstrapped Gain Stage Theory

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #61
    You wrote "driving the plate of the first triode" which I interpreted as using the plate as input.

    Did you consider using a LTPI, out-of-phase signals applied to opposite grids and using either of the plates as output?
    Are you familiar with differential amplifiers?
    - Own Opinions Only -

    Comment


    • #62
      Originally posted by SoulFetish View Post
      Yeah, instead of a grounded plate input stage. For instance... if you are mixing two out of phase signals -- one feeding the inverting input (grid), and the other feeding the non-inverting input (cathode), might it be better to take the output of the plate of that stage?
      Or maybe something like this? Just thinking aloud here, so everybody calm down if it sucks.
      [ATTACH=CONFIG]54915[/ATTACH]
      But one input is 5Meg and the other is 5K! Why not just use a differential amplifier? You'll get about 20db of gain too.

      Click image for larger version

Name:	6111 Diff amp.jpg
Views:	1
Size:	279.7 KB
ID:	854673


      But anyway. One signal is dry and the other is wet from the reverb right? The reverb signal won't be out of phase but the phase will vary wildly with frequency. There's no need to be concerned about the mixing phase.

      PS: Oh.. I see HH suggested the same idea. Simulpost I guess
      Last edited by nickb; 08-26-2019, 09:54 PM.
      Experience is something you get, just after you really needed it.

      Comment


      • #63
        Originally posted by nickb View Post
        But anyway. One signal is dry and the other is wet from the reverb right? The reverb signal won't be out of phase but the phase will vary wildly with frequency. There's no need to be concerned about the mixing phase.
        I can dig that.
        If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

        Comment


        • #64
          One signal is dry and the other is wet from the reverb right?
          If it's actually about mixing reverb and dry signals (didn't know up to now), these signals are not coherent (have no definite phase relation).
          - Own Opinions Only -

          Comment


          • #65
            Originally posted by nickb View Post
            Ah! Okay doke. Done like that Zin is 220 ohms.
            If you remove the second stage altogether, the Zin is 2.6K
            What I learned from this discussion and especially from the sim results (thanks Dave and Nick) is that bootstrapping the grid leak resistor of a CF not only increases grid input impedance but can also considerably increase cathode impedance. Such increase of output impedance may not be desirable. Resulting cathode impedance obviously depends on the grid circuit (source) impedance.

            Now I wonder what bootstrapping does to a common cathode circuit.
            Last edited by Helmholtz; 09-02-2019, 05:20 PM.
            - Own Opinions Only -

            Comment


            • #66
              Originally posted by Helmholtz View Post
              What I learned from this discussion and especially from the sim results (thanks Dave and Nick) is that bootstrapping the grid leak resistor of a CF not only increases grid input impedance but can also considerably increase cathode impedance. Such increase of output impedance may not be desirable. Resulting cathode impedance obviously depends on the grid circuit (source) impedance.

              Now I wonder what bootstrapping does to a common cathode circuit.

              Wait, didn’t we have to effectively short the grid to the “ground” point at the top of the bootstrap resistor to bootstrap the cathode? In a normal cathode biased cathode follower, where the grid is bootstrapped, how does that increase the internal cathode impedance in a meaningful way?
              If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

              Comment


              • #67
                Wait, didn’t we have to effectively short the grid to the “ground” point at the top of the bootstrap resistor to bootstrap the cathode?
                Other than with a LTP, the top of the tail resistor in a single CF stage is no "ground" point. It actually carries the major part of the cathode signal (Vtail = Rtail x Icathode) and that is what is required for effective bootstrapping of the "grid leak" resistor.

                The cathode impedance values for the CF alone (provided by Nick) show a variation of a factor 12 between open grid input (2.6k) and AC shorted grid input (220 Ohm). The latter value is about what I calculated for the CF without bootstrapping. For an intermediate grid circuit impedance in the 100k range I thus expect increased cathode impedance with bootstrapping.

                As CFs typically don't need grid bootstrapping, the results may not seem relevant.
                But while in most cases increased output impedance would be considered a drawback, it might be desirable to increase the low cathode output impedance of a split-load PI. With a 12AX7 a 10k cathode impedance might be feasible.
                - Own Opinions Only -

                Comment


                • #68
                  Originally posted by Helmholtz View Post
                  Other than with a LTP, the top of the tail resistor in a single CF stage is no "ground" point. It actually carries the major part of the cathode signal (Vtail = Rtail x Icathode) and that is what is required for effective bootstrapping of the "grid leak" resistor.
                  I suppose. But it serves as the negative voltage reference point in the same way any cathode biased common cathode stage does where the grid leak is tied to ground. This was the only comparison I was trying to make.

                  But while in most cases increased output impedance would be considered a drawback, it might be desirable to increase the low cathode output impedance of a split-load PI. With a 12AX7 a 10k cathode impedance might be feasible.
                  I absolutely agree! (and need to do some testing) But now I have to reconsider how I set the bias in the drivers of my DC coupled output stage

                  An CF output impedance of 2.6k sucks.
                  If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                  Comment


                  • #69
                    But it serves as the negative voltage reference point in the same way any cathode biased common cathode stage does where the grid leak is tied to ground.
                    Obviously it makes a significant difference if the far end of the grid leak is connected to ground or to a signal.

                    The interesting aspect to me is that bootstrapping seems to counteract cathode degeneration/negative feedback. Maybe not surprizing because bootstrapping is actually positive feedback.
                    - Own Opinions Only -

                    Comment


                    • #70
                      Originally posted by Helmholtz View Post
                      Obviously it makes a significant difference if the far end of the grid leak is connected to ground or to a signal.
                      Indeed
                      If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                      Comment


                      • #71
                        Maybe not surprizing because bootstrapping is actually positive feedback.
                        ...and the amount of positive feedback is influenced by the grid "load".
                        - Own Opinions Only -

                        Comment

                        Working...
                        X