J
John Larkin
- Jan 1, 1970
- 0
John Larkin said:SDC wrote:
I need a single-supply positive peak detector with a gain of 2.
(Previously
I used an op-amp with a gain of 2 followed by a peak detector, but would
like to simplify the circuit.) The input voltage will range from 0V to
about
2.5V.
Will the following circuit work? (Please excuse my ignorance - I don't
have
much experience with op-amps.)
... Steve
IC=LM324
+-[10K]-+----[10K]----+----------+
| | | |
=== | | |\ |
GND | |\ +-- |-\ |
+--|-\ 1N4148 | >---+---o Vout = Vin(pk) x 2
| >-->|--+----|+/
Vin o-------|+/ | |/
|/ 0.01uF---
---
|
===
GND
looks good here except for the use of the diode
with that diode you have selected the Vfm (Max Forward voltage) is
aprox 0.65 like most diodes in that fam.
this means the first 0.60 or so won't be seen in the
circuit from the your (Vin) also, this will produce a
0.65 error in the calculation of the multiplier due to the
drop.
using a lower (Vfm) diode in my opinion would help things
greatly.
something like what a Germanium diode would do.(aprox 0.3 volts).
also there is the problem of linearity, a little negative feed
back via the same type of diode could counter act it.
just an observation.
--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
We've already covered this point in the earlier posts. The diode will not
introduce any error.
... Steve
You'll eventually need to discharge that cap, too. Some opamps, like
the 324, have positive input bias current so will charge the cap until
it hits the positive rail.
John
Yep, that's the biggest drawback. (We also covered this earlier). I've spent
the last hour or two searching for a suitable rail-to-rail input and output
FET-input quad op-amp. The AD8544 is looking good so far, with the AD844
running a close second.
... Steve
But you'll still have to discharge the cap once in a while.
Mathematically, the charge on a cap is an indefinite integral, namely
unknown if you don't know its full history. As a practical matter, the
question is "peak voltage since when?"
And a 1N4148 willy typically leak a lot more than a decent fet opamp.
John