# Simulating an RL circuit

J

#### John Larkin

Jan 1, 1970
0
John, Are you just discovering that trick?

In general: R*C = L/R

I was using that to make inductive drivers for modems ~1977.

But it dates MANY years back to GenRad bridge designs.

...Jim Thompson

I think I'd seen it a long time ago, and forgot. I started thinking
about the step-response currents in each leg and slapped meself upside

We just rediscovered it in the context of split-path amplifiers. As a
practical matter, to make it really precise, one would probably have
to use a little Coilcraft slug-tuned inductor as part of L, to tweak
things up. There will be capacitive parasitics in L, too, a more
serious issue if this is to be truly wideband, like DC to 1 GHz for
starters.

John

J

#### Jim Thompson

Jan 1, 1970
0
I think I'd seen it a long time ago, and forgot. I started thinking
about the step-response currents in each leg and slapped meself upside

We just rediscovered it in the context of split-path amplifiers. As a
practical matter, to make it really precise, one would probably have
to use a little Coilcraft slug-tuned inductor as part of L, to tweak
things up. There will be capacitive parasitics in L, too, a more
serious issue if this is to be truly wideband, like DC to 1 GHz for
starters.

John

Yep. Although the GenRad bridges used precision wire-wound resistors
paralleled with carbon resistor plus a cap.

I think it's actually fairly good even at frequency extremes and
slight mismatches.

...Jim Thompson

P

#### Phil Hobbs

Jan 1, 1970
0
John said:
You said it. Put me right off my feed, it did.

Hey, if

R = 50

C = 1 uF

L = 2.5 mH

---------+---------R-----------C--------+---------
| |
| |
| |
+---------R-----------L--------+

then this looks like a 50 ohm resistor at all frequencies.

That has some serious possibilities.

John

That's a two-pole RC diplexer, viewed from the source's perspective.
Those are commonly used in receivers to ensure that the mixer sees a
constant 50-ohm load, which is very important in maintaining mixer
linearity.

Cheers,

Phil Hobbs

J

#### John Larkin

Jan 1, 1970
0
That's a two-pole RC diplexer, viewed from the source's perspective.
Those are commonly used in receivers to ensure that the mixer sees a
constant 50-ohm load, which is very important in maintaining mixer
linearity.

Cheers,

Phil Hobbs

So now we can put an amplifier in each leg. It gets trickier if the AC
path doesn't have a simple single-pole rolloff, but we can probably
work around that.

John

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