Math test help: (real input is 43K || 470K || hfe x 390 ohms)

[petemoore]

  And I can't pass my own test.
   (real input is 43K || 470K || hfe x 390 ohms)
  Gus provided this equation in regards to a 2n5088 with a 43k/470k bias string and 390ohm resistor.
   I understand how it works functionally, and how the bias string resistance can 'adjust the math' on the input impedance, but when working the ohms math I lose the decimal point !
  Lets call the impedance 1000hfe [just for a round number], and make the emitter resistor on the Bipolar transistor 100ohms.   
  1000 x 100 = 100,000...seems high
  that number can be practically disregarded when it's to be paralleled with a 43k.
  I understand the ll = parallel,
  ..and 43k ll 470k would be less than 43k by a 'bit'...I can grab a 43k and parallel a 470k and get a DMM read...or guess say about mid 30's-K ish, close enough to decide whether more suitable resistors should be sought out to adjust input impedance.
  I would like to work through the math and apply it.
   

[stm]

Pete,

100,000 ohms compared to 43,000 ohms is not insignificant!  It reduces impedance down to 30,000 ohms.

Also, hfe for a 2N5088 is closer to 500 rather than 1000.  Then, for the original problem you have a 500*390 = 195,000 base impedance.

Making the total calculation: Req = 1 / ( 1/43 + 1/195 + 1/470 ) = 32.77 kohms.



Hfe would be 500 rather than 1000

[petemoore]

100,000 ohms compared to 43,000 ohms is not insignificant!  It reduces impedance down to 30,000 ohms.
  That's great, my ballparking ability went right out the window when I started looking harder at the math...
  Also, hfe for a 2N5088 is closer to 500 rather than 1000.  Then, for the original problem you have a 500*390 = 195,000 base impedance.
  Making the total calculation: Req = 1 / ( 1/43 + 1/195 + 1/470 ) = 32.77 kohms.
   My math is terrible, I'm now confused by that ^. Oh man, I don't know where to start asking now.
  'R' means resistor
  'E' means emitter
  so 'Re' means resistance of the emitter resistor and emitter?
  'q' .... hmmm....hmmm
  '=' means equals...
  '1' again hmmm.
  '/' means divided by
  '[  ]..in parenthesis' means work these numbers and signs first.
  '+' means plus or add.
   Hfe would be 500 rather than 1000 yes, 500 is another good round number and closer to the actual Hfe you'd find in a typical 5088.
 

[johngreene]

100,000 ohms compared to 43,000 ohms is not insignificant!  It reduces impedance down to 30,000 ohms.
  That's great, my ballparking ability went right out the window when I started looking harder at the math...
  Also, hfe for a 2N5088 is closer to 500 rather than 1000.  Then, for the original problem you have a 500*390 = 195,000 base impedance.
  Making the total calculation: Req = 1 / ( 1/43 + 1/195 + 1/470 ) = 32.77 kohms.
   My math is terrible, I'm now confused by that ^. Oh man, I don't know where to start asking now.
  'R' means resistor
  'E' means emitter
  so 'Re' means resistance of the emitter resistor and emitter?
  'q' .... hmmm....hmmm
  '=' means equals...
  '1' again hmmm.
  '/' means divided by
  '[  ]..in parenthesis' means work these numbers and signs first.
  '+' means plus or add.
   Hfe would be 500 rather than 1000 yes, 500 is another good round number and closer to the actual Hfe you'd find in a typical 5088.
 

There's no smiley face so I'm not sure if you are kidding or not. Req means R(equivalent). Meaning the total resistance or input impedance.

'^' means raised to the power of. i.e. 5^2 is 5 squared.

[petemoore]

There's no smiley face so I'm not sure if you are kidding or not. Req means R(equivalent).
   
  But seriously, possibly stumped, perhaps just more of a "Doh" type thing though.
  I really think I 'get' it.. generally and basically..the numbers just aren't clicking'.
  '^' means raised to the power of. i.e. 5^2 is 5 squared.
  I was using it as an 'up' pointer, to the line of text above '^'.
   Req = 1 / ( 1/43 + 1/195 + 1/470 ) = 32.77 kohms.
  1/43 = .02325xxxx
  1/195 = .00512xxxxetc.
  1/470 = .0021xxx
  .0235 + .0051 + .0021 = .2386
  1/.2386 = 4.19111
  Here I've worked the parenthesis equation first using '/' on the calculator where /'s are..., then worked that sum as 1/x and got 4.19111xxxxxxxxxxxxxxxxxxx
  Of course that doesn't look like it's even in the same city as the ballpark..
 
  . 
 
 

[johngreene]

There's no smiley face so I'm not sure if you are kidding or not. Req means R(equivalent).
   
  But seriously, possibly stumped, perhaps just more of a "Doh" type thing though.
  I really think I 'get' it.. generally and basically..the numbers just aren't clicking'.
  '^' means raised to the power of. i.e. 5^2 is 5 squared.
  I was using it as an 'up' pointer, to the line of text above '^'.
   Req = 1 / ( 1/43 + 1/195 + 1/470 ) = 32.77 kohms.
  1/43 = .02325xxxx
  1/195 = .00512xxxxetc.
  1/470 = .0021xxx
  .0235 + .0051 + .0021 = .2386
  1/.2386 = 4.19111
  Here I've worked the parenthesis equation first using '/' on the calculator where /'s are..., then worked that sum as 1/x and got 4.19111xxxxxxxxxxxxxxxxxxx
  Of course that doesn't look like it's even in the same city as the ballpark..

I understand now! doh.

um, I don't think .0235 + .0051 + .0021 = .2386.  I get 0.0305..  and if you take 1/0.0305... you get 32.77...

The only way I can get .2386 is if I leave a '0' out of .0235 and transpose the 1 and 5 in .0051. So .235 + .0015 + .0021 = .2386 

--john