At 02:06 AM 9/1/2003 -0500, you wrote:


>In the second circuit, I would think that the emitter serves/acts as a diode
>when the transistor is on, so that another diode at the emitter is not
>required.  I do realize that the proper diode behaviour is achieved in the
>B-E junction of the transistor, but thought that the C-E junction is somewhat
>unidirectional as well.

This is a bad design as you're counting on the reverse beta of the
transistor to be very low, which it may not be. BJTs work similarly
when E and C are reversed, just with low beta (and low "collector"-base
breakdown voltage, and high base-"emitter" breakdown). The base current,
multiplied by the reverse beta forms a current sink shunting the switch,
so it's quite likely marginal, depending on the part values and
so on.

Put the diode in- the voltage drop across the BJT will only be 100mV
or less if it's properly saturated, so you're still laughing like a
hyena compared to the guaranteed TTL input levels with Vdd=5V.

With that addition I don't see anything wrong with the circuit.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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