low output capacitance buffer?

szechuan · 24th June 2011, 03:27 AM

so, over the past few days i've been looking for the ultimate discrete small signal buffer, and i've found that many of the more promising designs are composed of complementary components, such as a JFET source follower with a CCS or two different buffer circuits merged to bring out the best of each individual circuit. low input capacitance design considerations are made in most of the designs i've considered, but i've found ZERO designs that aim for low output capacitance while fully decoupling DC from the output. i'm thinking this could be accomplished with a trafo, any other thoughts?

oh, and heres a basic list of design constraints, just so i don't keep babbling
-very low C-out
-must be discrete (no opamps!)
-output Z less than 10k ohms
-may be active buffer circuit or passive add-on
-intended to drive cables that have high capacitance
-if active it has to run off 18 volts or less
-if active it must have no more than 2 active components (transistors)
-my design constraints aren't set in stone. if you think a minor exception will yield a significantly superior result, then thats ok, but NO OPAMPS!

thanks in advance

VladimirK · 24th June 2011, 06:31 AM

If one measures, by immittance meater, an output impedance (Zout modulus and phase angle) of any correctly designed and assembled schematics, at 1 kHz it is usually close to being active (pure resistance) with rather small phase angle. If the phase angle negative, then Zout has some capacitive component, if positive - then inductive component.
Measured Zout has nothing to do with presence/absence of the output cap/trafo in the schematics.

Samuel Groner · 24th June 2011, 11:30 AM

If you invent an output buffer with capacitive output impedance behaviour at high frequencies, please let me know. It would greatly simplify stability issues with amplifiers...

The output impedance of buffers becomes inductive at high frequencies. That's the real problem. Output capacitance is neither present in real world circuits, nor would it be a problem usually.

Samuel

szechuan · 24th June 2011, 09:21 PM

oh i see. i was hoping that a low output capacitance would help with cables that are designed with low inductance and resistance at the cost of high capacitance. i'll struggle towards the low reactance/inductance high frequency stability buffer design. fortunately ive got all summer to do so

24th June 2011, 03:27 AM	#1
szechuan diyAudio Member Join Date: Feb 2011 Location: Maryland	low output capacitance buffer? so, over the past few days i've been looking for the ultimate discrete small signal buffer, and i've found that many of the more promising designs are composed of complementary components, such as a JFET source follower with a CCS or two different buffer circuits merged to bring out the best of each individual circuit. low input capacitance design considerations are made in most of the designs i've considered, but i've found ZERO designs that aim for low output capacitance while fully decoupling DC from the output. i'm thinking this could be accomplished with a trafo, any other thoughts? oh, and heres a basic list of design constraints, just so i don't keep babbling -very low C-out -must be discrete (no opamps!) -output Z less than 10k ohms -may be active buffer circuit or passive add-on -intended to drive cables that have high capacitance -if active it has to run off 18 volts or less -if active it must have no more than 2 active components (transistors) -my design constraints aren't set in stone. if you think a minor exception will yield a significantly superior result, then thats ok, but NO OPAMPS! thanks in advance

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24th June 2011, 06:31 AM	#2
VladimirK diyAudio Member Join Date: Feb 2007 Location: Tashkent	If one measures, by immittance meater, an output impedance (Zout modulus and phase angle) of any correctly designed and assembled schematics, at 1 kHz it is usually close to being active (pure resistance) with rather small phase angle. If the phase angle negative, then Zout has some capacitive component, if positive - then inductive component. Measured Zout has nothing to do with presence/absence of the output cap/trafo in the schematics.

24th June 2011, 11:30 AM	#3
Samuel Groner diyAudio Member Join Date: Jun 2005 Location: Zürich	If you invent an output buffer with capacitive output impedance behaviour at high frequencies, please let me know. It would greatly simplify stability issues with amplifiers... The output impedance of buffers becomes inductive at high frequencies. That's the real problem. Output capacitance is neither present in real world circuits, nor would it be a problem usually. Samuel

24th June 2011, 09:21 PM	#4
szechuan diyAudio Member Join Date: Feb 2011 Location: Maryland	oh i see. i was hoping that a low output capacitance would help with cables that are designed with low inductance and resistance at the cost of high capacitance. i'll struggle towards the low reactance/inductance high frequency stability buffer design. fortunately ive got all summer to do so

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