Spice Oddity

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Syd Rumpo 10 years ago

Hi

I have a line driver circuit in LTspice simulation. It's a bridge circuit with op-amps and bipolars with a neat well-defined quiescent current. Sadly, I can't show the design, but I'm sure it's not original. The frequency range is 10kHz to 80kHz.

So, I do a transient sim with a sinusoidal source stepping from 10kHz to

80kHz and all is good. Not flat, but within a dB. Then I do an AC analysis and find that 80kHz is apparently >16db down on 10kHz, which is not good. I check for clipping etc on the transient sim, but none found and the output waveforms look nice and clean.

I'll build this soon, and I'm inclined to believe the transient sim, (probably because I want to) but any insight as to why there should be such a disparity? is there a well-known gotcha?

As the man with the steering wheel on his penis said, it's driving me nuts.

Cheers

Syd

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John Larkin 10 years ago

What do you see when the transient analysis is run with really small signal levels?

The AC analysis assumes the bipolars are biased at their quiescent current, which (?) might be nearly off.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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Jim Thompson 10 years ago

Simulators often fail on .AC analyses when part or all of the signal path had non-linearities.

LTspice particularly fails with some of my TANH stuff, linearizing absurdly.

Choosing "Alternate Solver" fixes some of it.... if you're lucky.

In general .TRAN trumps .AC when there's a non-linear signal path.

"Why should we hang, Dick did the shooting" >:-} ...Jim Thompson

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Syd Rumpo 10 years ago

Using 14 times smaller input signal, I see the same frequency response, and the transient is 14 times smaller.

Cheers

Syd

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Syd Rumpo 10 years ago

Alternate Solver don't make no nevermind.

That's encouraging, thanks.

Cheers

Syd

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Bill Beaty 10 years ago

Strine?

Spice Ordditaey!

(b'thlight Davabaowea)

Graoncon trolla Myjoh Tome...

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DecadentLinuxUserNumeroUno 10 years ago

On Tue, 19 Jan 2016 19:24:04 -0800 (PST), Bill Beaty Gave us:

Yeah it's a bunch of monkeys (Monkees) standing around a big bright white monolith dancing to Saturday Night Fever in their birthday suits.

Or is that a stage act for a bunch of male strippers?

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Spehro Pefhany 10 years ago

So this isn't an Antipodean tribute to the late David Bowie?

--sp

-- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition:

formatting link

Microchip link for 2015 Masters in Phoenix:

formatting link

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Syd Rumpo 10 years ago

Not really, though LTspice does have a ground control on the toolbar, and it's often the case that my circuit's dead, there's something wrong.

Cheers

Syd

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Jim Thompson 10 years ago

Consider transfer functions:

____ ____/

and / / __/ / / /

...Jim Thompson

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DecadentLinuxUserNumeroUno 10 years ago

On Wed, 20 Jan 2016 15:21:35 +0000, Syd Rumpo Gave us:

And of course everything is made from Ziggy's stardust.

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DecadentLinuxUserNumeroUno 10 years ago

On Wed, 20 Jan 2016 08:25:58 -0700, Jim Thompson Gave us:

Hit the hole, not the bowl.

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John Larkin 10 years ago

It would be interesting to poke around various nodes and see where the difference originates.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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DecadentLinuxUserNumeroUno 10 years ago

On Wed, 20 Jan 2016 10:43:19 -0800, John Larkin Gave us:

The difference is between Bowie and Kubrik.

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Bill Beaty 10 years ago

His circuit's dead! There's something wrong!

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