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SillyAmp

Discussion in 'Electronic Design' started by Winfield Hill, Mar 22, 2007.

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  1. Jim started a cute thread on a.b.s.e. with a pdf post. Many
    if not most of you can't read binary groups, and the messages
    are not archived by Google so they soon disappear. I feel like
    a bit of ASCII art, so I'll try translating the thread here.
    ---------------------------------------------------------------

    Newsgroup: alt.binaries.schematics.electronic
    Title: SillyAmp - SillyAmp.pdf
    Author: Jim Thompson
    Date: Thu, 15 Mar 2007 15:20:19 -0700
    Message-ID: <>

    Just a silly playing around. [pdf image]
    Has 9nV/rt-Hz input-referred noise... not as good as compound
    NPN/PNP. But has 0.03% Distortion with no overall feedback loop.

    .. ________________________ +17
    .. | | ,---- 6.8k ----,
    .. 2.2k 2.2k | __ |
    .. | | | | \ gnd
    .. | +----------+---|+ \
    .. | | | >-----+----
    .. +------------- | ---------+---|- / | OUT
    .. IN | | | |__/ TL071 |
    .. --------- | ------------ | ----, | |
    .. | | | '---- 6.8k ----'
    .. \_|_ TL431 (2) \_|_ |
    .. ---+-----/ \ / \----+
    .. | --- --- |
    .. 3k | 470u | 3k
    .. | +-- 12 ---|(---+ |
    .. gnd | | gnd
    .. 6.8k 6.8k
    .. | |
    .. ____|______________|____ -17


    ed: http://focus.ti.com/docs/prod/folders/print/tl431a.html


    Author: John Larkin
    Date: Mon, 19 Mar 2007 15:46:04 -0700

    Hey, I can do silly amps, too. [jpeg image]

    .. +200
    .. +12 |
    .. | |
    .. R |
    .. | |/
    .. _V_ --> | npn
    .. | |\e
    .. IN | | OUT
    .. --------+ +---------
    .. | |
    .. | |/e
    .. _V_ --> | pnp
    .. | |\
    .. R |
    .. | |
    .. -12 |
    .. -200


    Author: Jim Thompson

    Ooooooh! That's gross ;-)


    Author: John Larkin

    How else are you gonna get 400 volts p-p swing with two active
    parts?


    Author: Tim Williams
    Date: Tue, 20 Mar 2007 14:15:43 -0600

    Easy, you do it with one.

    Warning: input is at -400V [gif image]

    .. +400
    .. |
    .. RL
    .. |
    .. +---- OUT
    .. |
    .. IN |-'
    .. ----+---|| 1500V
    .. | |-, MOSFET
    .. 100k |
    .. | Rs
    .. __|______|_____
    .. -400


    Author: John Larkin

    Fine, if your input doesn't mind floating on -400.


    Author: Fred Bartoli
    Date: Wed, 21 Mar 2007 23:45:45 +0100

    That one is 800Vpp, and 400Vpp isn't necessary symmetrical.
    And you're having a bit of crossover distortion with the optos.

    Speaking of dreadful amplifier: [ascii "image"]

    ..
    .. .-------+-----< +400
    .. | |
    .. .-. |
    .. | | |
    .. | | |
    .. '-' |
    .. | |
    .. .-------+----||-+
    .. | | ||->
    .. | | ||-+
    .. | | |
    .. --- --- |
    .. --- --- |
    .. | | |
    .. | | |
    .. >-------+ +-------+----+---->
    .. | | | |
    .. | | | .-.
    .. | | | | |
    .. --- --- | | |
    .. --- --- | '-'
    .. | | | |
    .. | | | |
    .. | | ||-+ ===
    .. | | ||<- GND
    .. '-------+----||-+
    .. | |
    .. .-. |
    .. | | |
    .. | | |
    .. '-' |
    .. | |
    .. '-------+----< -400


    Author: John Larkin

    That's the famous KTFG (kiss the fets goodbye) circuit.


    Author: Jim Thompson

    Yep, Just prior to the KYAG ritual ;-)
     
  2. Author: John Larkin
    One could ac couple, unless it's a DC amplifier, then
    back to two parts is better. The pnp is an mpsa92.

    +275
    IN |
    --47k--, RL
    | |
    |/e +---- OUT
    ,---| |
    | |\ |
    gnd | |-'
    +---|| 600V
    | |-, MOSFET
    100k |
    | Rs
    __|______|_____
    -275
     
  3. Fred Bartoli

    Fred Bartoli Guest

    Winfield Hill a écrit :
    That one has a highly optimized 2nd order distortion.
    Should sound good and satisfy the most exigent tube aficionados :)
     
  4. Jim Thompson

    Jim Thompson Guest

    How about a feedback resistor from "OUT" to PNP(e) ??

    But there's probably not enough loop gain to reduce the distortion by
    much.

    ...Jim Thompson
     
  5. Fred Bartoli

    Fred Bartoli Guest

    Jim Thompson a écrit :

    Before doing that, you'll need to properly bias the input stage.
    Exercise left to the student :)
     
  6. Jim Thompson

    Jim Thompson Guest

    Are there any "students" here? They certainly don't step forward and
    attempt to learn :-(

    ...Jim Thompson
     
  7. John  Larkin

    John Larkin Guest

    Mini-nits: both my optocouplers were the same, npn's, and my
    input-side supplies were +-2, to keep idle current down:
    Is not! You're just jealous that you don't have 400 volt optocouplers
    in any of your ic libraries.

    John
     
  8. Actually, it's not so bad with a significant Rs. That's
    because the power high-voltage MOSFET will be operating
    in its sub-threshold region, where its gm is nearly as
    high as a BJT at the same current. With the high supply
    voltage and a modest ratio RL/Rs = 50, for 500V out with
    10V in, Rs > 1/gm will keep the distortion under control.
    Actually, as mentioned above, with Rs = 0, the loop
    gain could be sizeable, and rightly, done it could work
    reasonable well. The pullup capabillty is rather wimpy
    and there are several DC offset sources.

    +325
    ZERO |
    -- Rx --, RL = 150k
    IN | | 5W
    -- 20k --+-- 1M --+
    -/+5V | |
    |/e +---- OUT
    ,---| | +/- 250V
    | |\ | G = -50
    gnd | |--'
    +----|| 600V
    | |--, MOSFET
    2.2M |
    __|________|_____
    -275

    A real circuit would be more complicated, like some
    I've posted before, including an opamp, a totem-pole
    pullup transistor, current limiting, and compensation.
     
  9. Fred Bartoli

    Fred Bartoli Guest

    Winfield Hill a écrit :
    I was just referring to the absence of bias for the PNP, assuming the
    input signal was symmetric :)

    I'd probably reduce the 2.2M to 22k or such, which is about 200uA and
    60mW for the PNP, for a better gate positive slew rate and GBW product.
     
  10. Yep, but be careful, that 200uA would have to come from
    the PNP summing junction, making a high output offset to
    correct. Maybe 2.2M is too high, but consider, Crss is
    only 3pF for an FQP1n60 above Vds = 25 volts, and with
    Vgs about 3 volts at Id = 2mA, we get a rising slew rate
    of S = I/C = Vgs/RC = 0.5V/us. 220k would give us 5V/us.
    If this stage is placed inside an opamp feedback loop,
    then we can begin to get aggressive about the currents.
    Unless a pullup transistor is added, external capacitive
    loads will seriously slow down this wannabe beast anyway.

    +300
    ZERO | QUICK-n-DIRTY
    -- Rx --, RL = 150k HV AMPLIFIER
    IN | | 5W
    -- 20k --+-+-- 1M --+
    -/+5V | |
    |/e +---- OUT
    ,---| | +/- 250V
    | |\ MPSA92 | G = -50
    gnd | |--'
    +------|| 600V MOSFET
    | |--, FQP1N60
    220k |
    __|__________|_____
    -260
     
  11. John Larkin

    John Larkin Guest


    I'd feel better if there was a gate zener, and maybe a resistor in
    series with the PNP collector; I hate all-silicon paths from -260 to
    ground. Then go ahead and add a source resistor, to get some current
    limiting and improve linearity.

    If you want silly, how about an optocoupler/mosfet cascode? Or better,
    ie sillier, an opto+depletion fet cascode!

    Actually, that's getting interesting.

    John
     
  12. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    I thought about that last one as a replacement to the HV optos when you
    first 'published' your idea here a couple of years ago.
    But now that's 4 active parts. Well six, including the leds.
     
  13. The zener is mandatory. Diode protection for the PNP,
    yep. Safety current limiting is wise, although after
    making hundreds of such amps with mpsa92 trannies, I
    had only one failure and it blew the mpsa92 wide open.
    Lots of energy stored in the supply caps after all.

    +300
    ZERO | QUICK-n-DIRTY
    -- Rx --, RL = 150k HV AMPLIFIER
    -/+5V IN | | 5W
    -- 20k -----+--+-- 1M --+
    | |
    ,--|>|--+ |
    | | |
    | |/e +-- 1k --- OUT
    +-----| | +/- 250V
    | |\ MPSA92 | G = -50
    gnd | |
    47k |
    | |--'
    +----+--|| 600V MOSFET
    | \|_ |--, FQP1N60
    220k /_\ |
    | | 12V |
    __|____|______|_____
    -260
     
  14. John Larkin

    John Larkin Guest

    v+
    |
    |
    r
    |
    +-----------out
    |
    |
    d
    in---------+ g ---+
    | s |
    | | |
    a c |
    k ---> b |
    | opto e |
    | | |
    gnd +-----+----- v-




    3 parts total! And you can get low-voltage optos as duals, so you can
    make it full totem-pole, class B, 500 volts p-p out, with three parts!

    Apex, cry your heart out.


    John
     
  15. v+
    |
    |
    r
    |
    +-----------out
    |
    | Supertex
    d depletion-mode
    in--/\/\---, g ---,
    | s |
    | | |
    a c |
    k ---> b |
    | opto e |
    | | |
    gnd +-----+----- v-
    Awesome. Don't forget a resistor for the opto.
     
  16. Jim Thompson

    Jim Thompson Guest

    Another silly KYAG circuit ;-)

    ...Jim Thompson
     
  17. Jim Thompson <wrote:
    ?? Really? KYAG? How so? ?? That looks like a relatively-well
    behaved circuit, with excessive gain. But it does cry out for a
    feedback loop.
     
  18. Jim Thompson

    Jim Thompson Guest

    Concern for opto NPN punch thru during power-up... I could go for a
    zener or some-such in there.
    I've been musing really retro... maybe a mixed toob/silicon amplifier
    stage ;-)

    ...Jim Thompson
     
  19. John  Larkin

    John Larkin Guest


    You want silly? How about a parametric amplifier using the
    nonlinearity of ceramic caps!

    John
     
  20. Tim Williams

    Tim Williams Guest

    Bring it on! I'd love to see anything about parametric amplifers-- the only
    thing I can find on them is "they work by varying some part of the
    circuit"...drrrrr...okay...

    Ceramic caps would be a very accessible way to demonstrate one.

    Tim
     
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