RE: [SI-LIST] : IBIS and bipolar emitter followers?

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From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Thu Nov 18 1999 - 16:33:21 PST


Dave,

I don't understand your problem statement. Even though I am not a
circuit designer I will try to sum up what I know about bi-polar
buffers in attempt to answer your question. Maybe someone out there
can help out if I write something that is incorrect.

Bi-polar buffers usually have a common-emitter configuration for the
pulldown, and emitter-follower for the pullup. The IV curve of the
bi-polar pulldown is fairly similar to the IV curve of a MOSFET
pulldown (except that it does not exactly go through the origin, and
there may be multiple deflection points in the curve going towards
saturation). Since the pulldown is an inverter, its base is high
(not necessarily Vcc) when driving the output low. The pullup, being
an emitter follower is non inverting, so it's base is low when the
output is driven low.

But note that the emitter follower pullup cannot "push" the signal down,
because its BE diode would get reverse biased. Falling edges are driven
by the pulldown transistor only. Therefore I don't see how an upward
reflection during a falling edge could cause problems, because the BE
junction is reverse biased anyway. A downward reflection may forward
bias the BE diode (if the base is not all the way to GND yet) but that
would only cause it to keep the reflection from going down, as if it was
pulling it up.

The SI problem that you may run into with bi-polar outputs is that after
a rising edge you can have bad ringing if the signal overshoots above
the voltage that the BE junction can pull to. Remember, above that
voltage the BE diode gets reverse biased, which is high impedance,
allowing for essentially undamped oscillations on the driver side.

Since this effect is adequately described by the shape of the IV curve,
IBIS simulators should be able to simulate it correctly.

By the way, N-channel (MOSFET) pullups do the same...

What you need to watch out for is that for both N-channel and bi-polar
pullups (emitter or source followers) the transients of the buffer are
best approximated by shifting the IV curves, while for open-drain/collector
pulldowns and P-channel (open drain) pullups the transients can be
approximated with scaling the IV curves. IBIS does not spell out how
to deal with the IV curves, it is up to the simulator to use the
appropriate algorithm. They may not all do this correctly. My guess
is that most of them scale the IV curves no matter what, and that is why
I said in my previous EMAIL that IBIS may work better for MOSFET than
bi-polar.

Anyway, I have to stop corresponding, since I have to get some "real work"
done.

Arpad Muranyi
Intel Corporation
=========================================================================

-----Original Message-----
From: David Chengson [mailto:chengson@juniper.net]
Sent: Thursday, November 18, 1999 3:03 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : IBIS and bipolar emitter followers?

If the circuit one is attempting to model has bipolar
transistors configured as emitter followers, can IBIS
models simulate the de-biasing of emitter followers?

A problem that can occur is during the
falling edge transition, a reflection from an impedance
discontinuity, if sufficiently large, may de-bias
the emitter followers (which are always suppose to be
biased in the linear active region).
The base of the transistor has already transitioned from
high to low, and the reflection at the emitter can
cause the base-emitter voltage to drop much less than
600mV, and the emitter current drops to small values
(leakage currents). Spice models this fairly well.

The last time I looked at IBIS models, they only had
voltage sources and resistors. But that was awhile back.

Dave Chengson
Juniper Networks

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