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spec out low power bjt

Discussion in 'Electronic Design' started by panfilero, May 9, 2013.

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  1. panfilero

    panfilero Guest

  2. panfilero

    panfilero Guest

    thank you sir,

    so on my circuit, looks like my only current flow will be through R3, whichif I pull it up to 3.3V and use a 10k gives me about 330uA going through it, where do I start to push my limits here, if I have a 100k I end up at 33uA, is this pushing it... I'm just wondering how to evaluate this... 10k isprobally fine for what I'm doing but I'm just curious about how low I could bring my current... does the high beta take care of this?

    Is the UniShot non-retriggable?
     
  3. Bill Sloman

    Bill Sloman Guest

    Years ago (July 2008), we had a discussion here on making a low power oscillator -"micro power square wave oscillator". The base-emitter capacitance of the transistor involved turned out to be the feature that determined the current drawn at any sensible frequency, and the palm went to wide-band transistors with Ft's of 5GHZ and higher.

    I think I posted an LTSpice circuit using a pair of BFR92 transistors to create a 100kHz oscillator. Jeorg went one better with BFP620's (which are more expensive and even more prone to oscillate, but faster). You need a "base-stopper" - something of the order of 33R in series with the base connection - to stop this kind of part from oscillating at few GHz, and you need toremember that base-emitter break-down voltage is only 2V in this kind of transistor.
     
  4. panfilero

    panfilero Guest

    thank you sir, I'll look into these
     
  5. Jasen Betts

    Jasen Betts Guest


    It forms a low pass filter with the base capacitance


    Another way to fix it is to increase the collector resistors a bit.
    if the transistors aren't saturating you've got a high pass filter
    with unconstrained gain if they are you have sub-unity AC gain,
    and thus should be safe.


    I had to reduce the collector resistors in LTSpice else I didn't get
    the glitches you were seeing
     
  6. Jasen Betts

    Jasen Betts Guest

    I'm not familiar with that term ( googling "bipolar suicide bias" was
    no help :^) )

    I'm seeing a failure mode where the voltage source beeing is pushing a pulse
    through tee B-C junctions of the transistors when it goes high, adding
    emitter resistors but leaving the base connected to the capacitor will
    likely make that glitch worse.

    I'm seeing it run slightly faster. with increased input impedance of
    each stage the collector waveform output of the intermediate stages
    is squarer. that's about all.
    10ns made no diference. neither did 1ns, except for increasing the run
    time.

    I dont see the glitches like in your PDF until I reduce the collector
    resistor to about 1K



    Version 4
    SHEET 1 940 680
    WIRE 128 16 -32 16
    WIRE 224 16 128 16
    WIRE 320 16 224 16
    WIRE 416 16 320 16
    WIRE 512 16 416 16
    WIRE 608 16 512 16
    WIRE 704 16 608 16
    WIRE 800 16 704 16
    WIRE -32 32 -32 16
    WIRE 128 32 128 16
    WIRE 224 32 224 16
    WIRE 320 32 320 16
    WIRE 416 32 416 16
    WIRE 512 32 512 16
    WIRE 608 32 608 16
    WIRE 704 32 704 16
    WIRE 800 32 800 16
    WIRE 48 128 16 128
    WIRE 128 128 128 112
    WIRE 128 128 112 128
    WIRE 224 128 224 112
    WIRE 240 128 224 128
    WIRE 320 128 320 112
    WIRE 320 128 304 128
    WIRE 416 128 416 112
    WIRE 432 128 416 128
    WIRE 512 128 512 112
    WIRE 512 128 496 128
    WIRE 608 128 608 112
    WIRE 624 128 608 128
    WIRE 704 128 704 112
    WIRE 704 128 688 128
    WIRE 128 144 128 128
    WIRE 320 144 320 128
    WIRE 512 144 512 128
    WIRE 704 144 704 128
    WIRE 224 192 224 128
    WIRE 224 192 208 192
    WIRE 416 192 416 128
    WIRE 416 192 400 192
    WIRE 608 192 608 128
    WIRE 608 192 592 192
    WIRE 800 192 800 112
    WIRE 800 192 784 192
    WIRE 16 208 16 128
    WIRE 128 240 128 224
    WIRE 144 240 128 240
    WIRE 320 240 320 224
    WIRE 336 240 320 240
    WIRE 512 240 512 224
    WIRE 528 240 512 240
    WIRE 704 240 704 224
    WIRE 720 240 704 240
    WIRE 224 288 208 288
    WIRE 416 288 400 288
    WIRE 608 288 592 288
    WIRE 800 288 784 288
    WIRE -32 304 -32 112
    WIRE -16 304 -32 304
    WIRE 16 304 16 288
    WIRE 16 304 -16 304
    WIRE 224 304 224 288
    WIRE 224 304 16 304
    WIRE 416 304 416 288
    WIRE 416 304 224 304
    WIRE 608 304 608 288
    WIRE 608 304 416 304
    WIRE 800 304 800 288
    WIRE 800 304 608 304
    WIRE -16 320 -16 304
    FLAG 800 192 out
    FLAG -16 320 0
    SYMBOL npn 144 192 R0
    SYMATTR InstName Q1
    SYMATTR Value 2N3904
    SYMBOL res 208 16 R0
    WINDOW 0 41 29 Left 2
    WINDOW 3 37 55 Left 2
    SYMATTR InstName R1
    SYMATTR Value {R_c}
    SYMBOL res 112 16 R0
    WINDOW 3 35 76 Left 2
    SYMATTR InstName R2
    SYMATTR Value {R_b}
    SYMBOL cap 112 112 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C1
    SYMATTR Value 1n
    SYMBOL npn 336 192 R0
    SYMATTR InstName Q2
    SYMATTR Value 2N3904
    SYMBOL res 400 16 R0
    WINDOW 0 41 29 Left 2
    WINDOW 3 37 55 Left 2
    SYMATTR InstName R3
    SYMATTR Value {R_c}
    SYMBOL res 304 16 R0
    SYMATTR InstName R4
    SYMATTR Value {R_b}
    SYMBOL cap 304 112 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C2
    SYMATTR Value 1n
    SYMBOL npn 528 192 R0
    SYMATTR InstName Q3
    SYMATTR Value 2N3904
    SYMBOL res 592 16 R0
    WINDOW 0 41 29 Left 2
    WINDOW 3 37 55 Left 2
    SYMATTR InstName R5
    SYMATTR Value {R_c}
    SYMBOL res 496 16 R0
    SYMATTR InstName R6
    SYMATTR Value {R_b}
    SYMBOL cap 496 112 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C3
    SYMATTR Value 1n
    SYMBOL npn 720 192 R0
    SYMATTR InstName Q4
    SYMATTR Value 2N3904
    SYMBOL res 784 16 R0
    WINDOW 0 41 29 Left 2
    WINDOW 3 37 55 Left 2
    SYMATTR InstName R7
    SYMATTR Value {R_c}
    SYMBOL res 688 16 R0
    SYMATTR InstName R8
    SYMATTR Value {R_b}
    SYMBOL cap 688 112 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C4
    SYMATTR Value 1n
    SYMBOL voltage -32 16 R0
    WINDOW 123 0 0 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR InstName V1
    SYMATTR Value 5
    SYMBOL voltage 16 192 R0
    WINDOW 3 -10 135 Left 2
    WINDOW 123 0 0 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR InstName V2
    SYMATTR Value PULSE(0 5 100u 100n 100n 600u 1 1)
    SYMBOL res 144 240 R180
    WINDOW 0 -20 96 Left 2
    WINDOW 3 -44 63 Left 2
    SYMATTR InstName R9
    SYMATTR Value {R_f}
    SYMBOL res 336 240 R180
    WINDOW 0 -20 96 Left 2
    WINDOW 3 -44 63 Left 2
    SYMATTR InstName R10
    SYMATTR Value {R_f}
    SYMBOL res 528 240 R180
    WINDOW 0 -20 96 Left 2
    WINDOW 3 -44 63 Left 2
    SYMATTR InstName R11
    SYMATTR Value {R_f}
    SYMBOL res 720 240 R180
    WINDOW 0 -20 96 Left 2
    WINDOW 3 -44 63 Left 2
    SYMATTR InstName R12
    SYMATTR Value {R_f}
    TEXT -34 -8 Left 2 !.tran 0 2m 0 10n
    TEXT 160 -16 Left 2 !.PARAM R_b=100k
    TEXT 336 -16 Left 2 !.PARAM R_c=1k
    TEXT 488 -16 Left 2 !.step PARAM R_f 0.00001 10000.00001 10000
     
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