Log amplifier schematics

[Sergey Kubushin]

Hi, everybody!

I'm looking for a schematic for a logarythmic amplifier. It should work in
audio range (20 Hz - 20 KHz) and has 50-80 dB range. It doesn't have to be
very precise, it's intended use is audio amplifier's output indication with
a linear neon tube, but it must be reasonably stable. The simplistic opamp
with two germanium diodes won't do. Monolythic IC is preferrable if its
cheap enough (the cheapest I can find from AD are $10 apiece that is kinda
high) and is not packed into a case unvisible without a microscope (SO-xx
are fine and preferrable). Relatively simple opamp schematic will also do.

All the information is highly appreciated.

P.S. I don't have "The art of electronics" on my shelf here in the US and
I'm not going to buy it for a one schematic. I do have my Russian edition
that I bought when living out there but unfortunately its still collecting
dust out there, I haven't brought all my treasures to the US so far...

 

[Robert Baer]

Hi, everybody!

I'm looking for a schematic for a logarythmic amplifier. It should work in
audio range (20 Hz - 20 KHz) and has 50-80 dB range. It doesn't have to be
very precise, it's intended use is audio amplifier's output indication with
a linear neon tube, but it must be reasonably stable. The simplistic opamp
with two germanium diodes won't do. Monolythic IC is preferrable if its
cheap enough (the cheapest I can find from AD are $10 apiece that is kinda
high) and is not packed into a case unvisible without a microscope (SO-xx
are fine and preferrable). Relatively simple opamp schematic will also do.

All the information is highly appreciated.

P.S. I don't have "The art of electronics" on my shelf here in the US and
I'm not going to buy it for a one schematic. I do have my Russian edition
that I bought when living out there but unfortunately its still collecting
dust out there, I haven't brought all my treasures to the US so far...

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Back in the old daze of CP/M, Solid State Music made a number of these
under the Exar brand; the part numbers are (and sitll appear to be)
SSMxxxx.

 

[Ban]

Hi, everybody!

I'm looking for a schematic for a logarythmic amplifier. It should
work in audio range (20 Hz - 20 KHz) and has 50-80 dB range. It
doesn't have to be very precise, it's intended use is audio
amplifier's output indication with a linear neon tube, but it must be
reasonably stable. The simplistic opamp with two germanium diodes
won't do. Monolythic IC is preferrable if its cheap enough (the
cheapest I can find from AD are $10 apiece that is kinda high) and is
not packed into a case unvisible without a microscope (SO-xx are fine
and preferrable). Relatively simple opamp schematic will also do.

All the information is highly appreciated.

Sergei,
this is a nice and temperature compensated log amp working in the first
quadrant, so both Vin and Vref have to be positive.



Vout = -Ut(R3+R4)/R4 ln((R2*Vin)/(R1*Vref)

+--------------->-+ +-<------------------+
| | | |
| Ic2| |Ic1 |
| ||C1 10p | | |
+--||--+ | | |
| || | | | |
100k | | | | C2|| |
___ | |\ | T2 | | T1 +--||--+
Vino-|___|-+-|-\ | |/ \| | || |
R1 | >-+ GND-| dual NPN|--+ | 100p | 100k
GND-|+/ | |> <| | 33k | /| | ___
|/ | | | | ___ | /-|-+-|___|-oVref
| +-----+----)--|___|-+-< | R2
| | R5 \+|-GND
| ___ | \|
+-----|___|---+-------+V1
| R3 |
| .-.
o | |+0,3%/K
Vout | |R4
'-'
|
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

ciao Ban

 

[Robert Baer]

Sergei,
this is a nice and temperature compensated log amp working in the first
quadrant, so both Vin and Vref have to be positive.

Vout = -Ut(R3+R4)/R4 ln((R2*Vin)/(R1*Vref)

+--------------->-+ +-<------------------+
| | | |
| Ic2| |Ic1 |
| ||C1 10p | | |
+--||--+ | | |
| || | | | |
100k | | | | C2|| |
___ | |\ | T2 | | T1 +--||--+
Vino-|___|-+-|-\ | |/ \| | || |
R1 | >-+ GND-| dual NPN|--+ | 100p | 100k
GND-|+/ | |> <| | 33k | /| | ___
|/ | | | | ___ | /-|-+-|___|-oVref
| +-----+----)--|___|-+-< | R2
| | R5 \+|-GND
| ___ | \|
+-----|___|---+-------+V1
| R3 |
| .-.
o | |+0,3%/K
Vout | |R4
'-'
|
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

ciao Ban

...and National has an IC which is a very well matched pair that
should be an excellent choice...

 

[Sergey Kubushin]

...and National has an IC which is a very well matched pair that
should be an excellent choice...

Thanks a lot. Can you tell me that part number?

 

[Ben Bradley]

Thanks a lot. Can you tell me that part number?

He surely means the LM394, but it's not cheap ($5 or so at
digikey), and surely overkill for your app. I'd use the 'matched'
transistors in a LM3046/CA3046 transistor array, and use the savings
to buy lunch.

 

[Robert Baer]

He surely means the LM394, but it's not cheap ($5 or so at
digikey), and surely overkill for your app. I'd use the 'matched'
transistors in a LM3046/CA3046 transistor array, and use the savings
to buy lunch.

Except that they have an excessive bandwidth and have a poor log
range.