EMRFD Message Archive 4265

Message Date From Subject
4265 2010-02-15 09:04:56 ajparent1 figure 6-110 bidirectional amp question
Figure 6-110, page 6.61 first version of EMRFD.

I was looking at the circuit and had a question as there was a
comment about operating voltages.

In actual testing it was not easily possible to get to more than
4-5V across the bases of the devices risking causing avalanche. Seems the 680-1000-680 series resistors create a voltage divider
that limits the voltage between the bases for the cases of open
emitters (tx and RX lines via 100 ohm not ground or VCC). The
alternate case (actual use) when in actual biased mode (both
of the 100 ohms grounded or to Vcc) the votage at the collectors
is about .4V (collector to emitter) or less making the possible voltage from base to base hard to exceed 4V for any case under

Using low DC beta (hand picked worst case) 2n3904 and 2n3906
I never hit a voltage that put the base of the reverse biased
device at more that 3.2V at Vcc of 9V. since the emitter to
base break down for either device is more than 4V and the
worst case breakdown voltage is for the '06 at 5V those
allows plenty of room for operating at higher voltages.

I do understand what can happen if the device avalanche but the
operating conditions are self limiting and favor it not happening.

The reason for this is I'm playing on the bench with a compact
monoband SSB trx and 8V is already there and 6V requires another
regulator. If I don't need the regulator it's fewer parts needed
when not needed. That and a few mA.

Anyone else have thoughts on this?