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Log opamp, with negative Linear side?

W

Warren

Jan 1, 1970
0
This is a homebrew pet project that
I'm hoping there is a simple answer to.

I want to create a logarithmic opamp stage
that operates _linear_ during the
alternate (negative) half of the input wave.

Here is a textbook log amp with simple diode
in the feedback loop (display in fixed font):

Diode
+--------->|---+
| |
| |\ |
o-----R1----+---(-)\ |
| \ |
| >-----+---------o out
| /
----(+)/
| |/
|
_
V

The above suggests to me that the alternate
half of the wave will be linear BUT with open
loop gain. But I want to establish/lower the
gain to be about 3 or so.

The obvious thing would seem to be to add a
reverse biased diode in series with a
resistor in the feedback path (the resistor
to establish the gain). But this also suggests
to me that I will still have a _log_ function but
in the reverse. I'm trying to keep that linear.

I'm trying to avoid the need for precision
rectifier(s).

Warren
 
W

Warren

Jan 1, 1970
0
Warren expounded in 81.169.183.62:
This is a homebrew pet project that
I'm hoping there is a simple answer to.

I want to create a logarithmic opamp stage
that operates _linear_ during the
alternate (negative) half of the input wave.

Here is a textbook log amp with simple diode
in the feedback loop (display in fixed font):

Diode
+--------->|---+
| |
| |\ |
o-----R1----+---(-)\ |
| \ |
| >-----+---------o out
| /
----(+)/
| |/
|
_
V

Cancel that.. I'm going to design in a different
direction now.

Warren
 
J

Joe G \(Home\)

Jan 1, 1970
0
Warren said:
Warren expounded in 81.169.183.62:


Cancel that.. I'm going to design in a different
direction now.

Warren

Download for free LTSpice and you can spice model these circuits.

Joe
 
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