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10M ohm resistors in this circuit. Why so high? Can I use 4.7M ohm instead?

Started by Philtre, June 09, 2018, 07:03:42 AM

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Philtre

I'm building the Giant Hogweed fuzz circuit. R19 and R20 are both 10M ohm. I'm wondering why the value is so high and could I use 4.7M ohm instead?


Scruffie

Should be fine I'd think, sets the input impedance of T3 and forms a filter with C9.
Works at Lectric-FX

EBRAddict

It's fine. It's just setting a bias point or reference voltage. The extra 450nA isn't going to hurt anything or cause your power bill to skyrocket.

Philtre


EBRAddict

Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Philtre

Quote from: EBRAddict on June 09, 2018, 05:55:37 PM
Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

Adam_DIY

Quote from: Philtre on June 09, 2018, 09:07:41 PM
Quote from: EBRAddict on June 09, 2018, 05:55:37 PM
Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

10 Billion Ohms


Philtre

Quote from: Adam_MD on June 09, 2018, 09:11:33 PM
Quote from: Philtre on June 09, 2018, 09:07:41 PM
Quote from: EBRAddict on June 09, 2018, 05:55:37 PM
Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

10 Billion Ohms



LOL!  :D

reddesert

10 Mohms is about the largest value you are likely to find in a resistor assortment bag or commonly stocked in small web stores, although large distributors may have larger values.

If you want to build the circuit as intended, just teepee two 4.7M resistors together in series for each 10M resistor. I do this frequently to approximate oddball values.

somnif

Quote from: Adam_MD on June 09, 2018, 09:11:33 PM
Quote from: Philtre on June 09, 2018, 09:07:41 PM
Quote from: EBRAddict on June 09, 2018, 05:55:37 PM
Copied from some random web site about MOSFET amplifiers, the second part is most applicable here:

"it is desirable to make the values of these two resistors as large as possible to reduce their I2R power loss and increase the mosfet amplifiers input resistance."

Interesting, thanks for that! Wonder what "large as possible" means? 10, 20, 30 M ohm?

10 Billion Ohms



https://www.mouser.com/ProductDetail/Ohmite/MOX1125231008F?qs=sGAEpiMZZMtlubZbdhIBIJUr%2fIAV1Mb2feZPNMSd1M0%3d

Easy enough, bit pricey, but readily available.  ;D

Philtre

Thanks, all.

Tayda have 10M ohm 1% in stock. I'll get a couple on my next order.




somnif

I have absolutely no idea what a teraohm resistor would be used for.

BUT, I can guess it would probably be way overkill for setting the input impedance on a MOSFET.  ;D