FET amplifier for transducer, follow-up

[John Ross]

Hi.

Thanks for the good responses. My newsreader is acting funny, so I
can't find and follow-up to the original thread. I miss my old school
UNIX text-based newsreader but I also digress.

To answer some questions:

- The reason I was looking at discreets was cost. Management wants to
add a feature, last minute (of course) and has given me a VERY small
budget to do it in. I figured that some FETs resistors and capacitors
would be cheaper than op-amp.

- I don't know the details of the sound transducer (someone else did
all that, including pre-conditioning, it might be a microphone
element?), just the sound pressure to voltage sensitivity ("gain") and
the frequency response, which drops off past 12 kHz or so.

- From all the advice, sounds like an op-amp is the fastest/cheapest,
I guess I'll go that route.

- I was avoiding the 324 because it's GBWP is low, but I know there a
bunch of single-supply op-amps I can look into.

Thanks.
J.

 

[Joerg]

Hello John,

- The reason I was looking at discreets was cost. Management wants to
add a feature, last minute (of course) and has given me a VERY small
budget to do it in. I figured that some FETs resistors and capacitors
would be cheaper than op-amp.

Usually yes, but not at this low frequency. Well, maybe if you have to
squeeze out the last cent but most likely not with FETs as they are
expensive.

- I don't know the details of the sound transducer (someone else did
all that, including pre-conditioning, it might be a microphone
element?), just the sound pressure to voltage sensitivity ("gain") and
the frequency response, which drops off past 12 kHz or so.

- From all the advice, sounds like an op-amp is the fastest/cheapest,
I guess I'll go that route.

- I was avoiding the 324 because it's GBWP is low, but I know there a
bunch of single-supply op-amps I can look into.

Low? It's 1MHz and you get four amps to one package which is a real
bargain. So you can cascade amplifiers. Its slew rate is a bit
borderline here but may be enough, depending on the output amplitude you
need.

Then there is ye olde TL084 at 3MHz unity GBW and an input resistance so
high you couldn't even measure it. About 10c more though :-(

If you really want to do it in discrete you need to use bipolar
transistors because FETs such as the BSS123 are around 5c a piece. You'd
have to find a transistor at around 1-2c but there are plenty.

If want the thrill of a real white-knuckle design you could use
unbuffered CD4000 series inverters and do an analog amp with these. But
you need lots of experience to do that and know how to "side bias" them.
It can be scary but has been done. The CD4007 is a nice CMOS building
block and about the only cost efficient FET solution I would see in this
case. If you use any CD series chip other than the 4007 in an analog
fashion be prepared to dodge flying tomatoes during the design review.

Regards, Joerg

http://www.analogconsultants.com