passive rc-bandpass with 0dB attenuation in the passband

How close to 0dB? What sort of Q (bw/f0)? What's the load?

Key words: passive rc-bandpass filter... an oxymoron, statement and poster :-) ...Jim Thompson

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It depends on how loose your definition is of "bandpass filter".

|| ___ Vin o-----||---o--|___|---o-----o Vout || | | | | .-. --- | | --- | | | '-' | | | | | === === GND GND (created by AACircuit v1.28.6 beta 04/19/05

There's a passive RC bandpass filter for you. It's not a _resonant_ bandpass filter, by any means, but it has a magnitude response that's zero at f = 0, rises to some maximum, then falls to zero as the frequency approaches infinity.

Ask your prof? Review your class notes?

There's some passive RC filter with not too many components that actually has a bit of voltage gain. It's usefulness is pretty much limited to winning bets in University bars, but it's out there. I can't even remember if it's bandpass, but if it isn't it may have a bandpass cousin.

Can I buy a vowel? Er, make that an inductor....

exactly 0dB, unloaded, Q to be determined from the other specifications

no inductors allowed here

Q is almost meaningless in this case -- any passive RC bandpass filter is going to have a damping ratio greater than 1, and the various definitions of Q only converge for damping ratios much less than one.

Tim Wescott wrote in news:SPednY_R0pRPY8vRnZ2dnUVZ snipped-for-privacy@web-ster.com:

LTSpice fans, here are 4 RC's with a peak "gain" of 1.57dB at 113Hz:

Version 4 SHEET 1 884 680 WIRE 96 224 -48 224 WIRE 208 224 176 224 WIRE 320 224 208 224 WIRE 208 240 208 224 WIRE -48 320 -48 224 WIRE 96 320 -48 320 WIRE 208 320 208 304 WIRE 208 320 176 320 WIRE -48 336 -48 320 WIRE 208 336 208 320 WIRE -48 432 -48 416 WIRE 208 432 208 400 FLAG -48 432 0 FLAG 208 432 0 FLAG 320 224 Output SYMBOL res 192 208 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res 192 304 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL cap 192 240 R0 SYMATTR InstName C1 SYMATTR Value 0.1µ SYMBOL cap 192 336 R0 SYMATTR InstName C2 SYMATTR Value 1µ SYMBOL voltage -48 320 R0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC 1 SYMATTR InstName V1 SYMATTR Value SINE(0 1 261) TEXT -24 480 Left 0 !.ac oct 10 10 1k TEXT 256 336 Left 0 ;Voltage Gain > 1\nFrom Epstein, "Synthesis of Passive Networks\nWith Gains Greater than Unity," Proc. IRE,\nJuly 1951

Mark Freeman

Mark Freeman wrote in news:hcKdnUV7nO1sOsrRnZ2dnUVZ snipped-for-privacy@cablespeedmi.com:

Excuse me, I meant 1.157dB.

Mark Freeman

can you prove this or refer to a proof of this statement?

thanks in advance

nukey

snipped-for-privacy@web-ster.com:

That's the low pass version of the delayed-recovery filter patented by G.A. Philbrick, I wouldn't call this a bandpass filter Do you think this can be transformed into a bandpass-filter? (you will probably end with more than 4 components?)

thanks for sharing your time

nukey

Ok, so I have used a more generalized definition of Q (bw/f0). Fortunately, definitions rarely have convergence problems. Applying them in specific instances is another matter ;)

The simpleminded 4-component RC filter (the obvious serial-parallel arrangement) won't get to exactly 0dB... would only approach it for truly wide bandwidths, even with no load.

Yes, I cited that circuit to contradict Jim's statement that "you can't make a passive RC bandpass circuit", not to answer the OP's question.

As I told the OP, I do dimly remember a passive RC circuit that had voltage (but certainly not power!) gain at some frequency. But I don't think it was a bandpass, and I don't know if one could build a bandpass version of it. If you could design a bandpass, passive, RC circuit with gain, then it would be a snap to design one with a 0dB peak. Realizing exactly a 0dB peak with component tolerances and all would be more of a trick, I'm sure.

With horrible skirts. "Band-pass" usually implies skirt-rate relative to bandwidth.

...Jim Thompson

Yes. It's more of a "scholar's bandpass" than anything you might want to use in real life. It's not a bad mental tool to use when cooking up an active filter, because an active bandpass can be made by "sharpening up" a passive one, ditto with an active lowpass and (with due respect for stability) an active highpass.

ions

er

Dear Tim, You probably did not see my question in the other message, therefore I'd like to ask it here once more: Can you prove the following statement or point to a proof for it:

any passive RC bandpass filter is going to have a damping ratio greater than 1,

regards, nukey

No, I can't. I'm not sure if that was ever even proved to me -- it's just one of those "obvious truths" that get presented early in one's educational career, and is never questioned thereafter.

I'm certain that it _is_ true -- if it weren't I'd have seen a circuit that took advantage of a resonant RC network. Further, I'm certain that some clever network theorist has proved it. I just don't know where to look for such a proof.