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Cute amplifier - bootstrapped

Discussion in 'Electronic Design' started by [email protected], Jan 11, 2013.

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  1. Guest

    Adapted from something I saw on the web...

    I had to scratch my head until I understood C1: it bootstraps Q1's
    collector load (R2) impedance, turning it effectively into a current
    source. Gain is accordingly greatly increased. G = ~220 @ 40KHz.

    +3.3V
    ---
    |
    .-------+-------.
    | |
    5K R1 C1 |/ Q2
    | 100n .------| 2n5089
    +---||-|--. |>.
    | | | |
    15k R2 | '-----+------>
    | | |
    C2 +------' R3 820R
    10n |/ Q1 |
    | |>. | |
    | | | R4 470R
    | === | |
    | | ===
    '-----R5------'
    220k

    R3-R4 set the d.c. output level.

    Gain is component-sensitive, but the bootstrap is a cute technique.
    That with feedback could stabilize the gain.
     
  2. Guest

    That's another advantage.
     
  3. Guest

    You need to turn that around, it is the emitter follower that is being bootstrapped and not the CE. So the CE + CC composite looks like the classic very high gain single pole amplifier inside the feedback loop formed by the resistor shunt feedback divider, everything is simplified. See the textbook write-up here, section 1.17.2:
    http://books.google.com/books?id=1-...epage&q=bootstrapped emitter follower&f=false
     
  4. Guest

  5. Guest

    Huh? You bootstrap the follower to increase its input impedance, not lower its output impedance.
     
  6. Guest

    I remember that trick of yours. Fun.
    One of the neatest I've seen was a VLF JFET amplifier. It used
    transformers to bootstrap its input capacitance to nada.

    James
     
  7. Guest

    I made a cascode version, but that was getting silly. A dual op-amp
    makes more sense at that point with fewer parts.

    Talking about silly, a little positive feedback really makes it
    sing...

    +3.3V
    ---
    |
    .-------+-------.
    | |
    5K R1 C1 |/ Q2
    | 22n .------| 2n5089
    +---||-|--. |>.
    | | | |
    15k R2 | '-----+------>
    | | |
    C2 +------' R3 820R
    10n |/ Q1 |
    | |>. | |
    | | | R4 470R
    | | | |
    | +---------------'
    | | |
    | R6 3r3 |
    | | |
    | === |
    | |
    '-----R5------'
    220k

    G= 1050 @ 40KHz.

    There actually is some negative feedback via R5--that stabilizes the
    gain a little bit.
     
  8. Guest

    That's plenty cute--a three-transistor inverting amp, with feedback,
    then an emitter follower, plus the bias stuff. But is there a
    bootstrapped collector load in the signal chain? I didn't see it.
     
  9. Guest

    Okay, those are fine points, but in this case the designer is trying to eliminate the shunt effect of the emitter follower input bias resistor network, and the bootstrapping succeeds well there.
     
  10. Guest

    I believe that is how that circuit is put together. Whatever the application was, they knew they needed controlled gain with buffering for the loading.. So someone figured out they can use the super high midband impedance of the bootstrapped CC as a super high gain setting loading for the CE, all of which makes for an easily compensated relatively accurate feedback amplifier.
     
  11. Guest

    Yep, that's a classic way to raise a.c. input impedance.

    ISTM there's really no distinction compared to describe it as
    bootstrapping the collector load. The collector load R2, being in
    parallel with the Q2 input impedance, bootstrapping R2 raises QA2's
    input impedance, that's true. But, the key effect in this circuit is
    raising the first stage gain, so it seems most appropriate to look at
    it from that perspective.

    Either / or.
     
  12. Guest

    Okay, look at it under a microscope if you prefer it that way.
     
  13. Guest

    Yep. Jim William's description of the Philbrick varactor amp inspired
    R6 above. IRL Jim's throwing a handful of transistors at it makes
    more sense for an IC designer--less component-sensitive--but it's
    always fun to play.

    As mentioned before, I'll likely use a dual op-amp instead, cascaded
    inverting stages, a.c.-coupled, d.c. offset. That's one op-amp, two
    caps, six resistors--boring, reliable, and repeatable.
     
  14. Guest

    Q2's input resistance is gain(Q2) x (R3+R4) anyhow, which is on the
    order of half a megohm.

    So, Q2's un-bootstrapped input impedance doesn't limit Q1's gain, R2
    does. Boosting Q2's input impedance per se wouldn't help.
     
  15. Guest

    I've seen that ages ago, a faded memory, I think likely from that same
    G.E. databook. Yours is much linear-er than the circuit I posted.

    Hmm. Could even bootstrap the first collector load. Only drawback is
    possibly a limited output swing compared to the emitter-follower Q2
    version...

    I might goof around thinking about that.


    James
     
  16. Guest

    That's not even remotely close. That is the two transistor job with superstablized Q-point self-bias and series feedback.
     
  17. Guest

    Yep- really- and that circuit has come up as a topic before. You couldn't get to square one figuring out the DC bias then IIRC.
     
  18. Guest

    You're very welcome.
     
  19. Guest

  20. Guest

    If I'm remembering right, I first saw that in Microwave Journal, late
    '80's. Neat stuff. I never used it, but I found the VLF thing later
    (Burhan, Radio Electronics (of all places)) and was reminded of it.
     
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