[Scott Dorsey]

In article , Paul wrote:
And it's interesting that they are using back-to-back Schottky
and Zener diodes for phantom fault protection and ESD. We used
Schottky for ESD protection on GSM/DCS cellular phone power amplifiers.

The problem with back to back diodes is that they are never totally off
so with a very high-Z input the leakage can be an issue. It still gives me
the willies even with a mike preamp, but I can live with it. 1N4148s to the
supply rails don't leak because the diodes are totally turned off, and they
don't act as parasitic varactors because they are totally turned off so the
capacitance is fixed.

So if you put the transformer in the front, can you safely
leave out the back-to-back protection diodes?

Think about it. You have a 1:5 step-up transformer in the front, so now you
can ditch the long-tailed pair and just have a single transistor for your
input stage. You get "free gain" from the better match with the step-up,
and now _all_ of the differential stuff is being done by the transformer.
Plus you get free low-pass from the transformer.

You don't need _any_ common-mode clamping on the input, because the
transformer is going to eat any common mode transient, even hundreds of
volts worth. You don't need to protect the transformer. So you can
put your protection on the secondary of the transformer and now the protection
is on a single lead. It doesn't need to be matched or balanced in any way,
it's just a single clamp now. And you can just clamp to the rails with
1N4148s, you don't need anything else.

Switch the phantom on or off, the transformer is going to eat it all.
Maybe you get a little peak on the output if the phantom resistors are
very badly mismatched.

Now... because of the step-up you do need clamps that can handle higher
voltage, but the job of the clamps becomes a lot easier.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."