Logarithmic Amplifier Design

[dan_thompson87]

Hello people!

I already posted a question similar to this back in January, however now I have more understanding on the subject I think my question might be a bit clearer!

I'm currently designing a sidechain to feed a VCA which will act as a compressor for an audio input.

The problem I have is that the sidechain signal is linear, and the VCA attenuates at 33mV/dB which is obviously logarithmic. I've done all of the maths and graphs in MS Excel and calculated that I need a logarithmic amplifier that does the following function:

Vout = ln(Vin)*0.393

Where Vin = the linear input signal, and 0.393 is the gain required to make 33mV per step. (I do also realise that I could simply use a log amp followed by a standard amp of gain 0.393 if that is simpler!)

I've done plenty of research on logarithmic amplifiers, and although there is an abundance of information on the theory behind them, there is no information on how to implement this theory and design one.

To sum up, my question is: how do I design an amplifier that has a logarithmic output and if possible, a gain of 0.393?

Thanks in advance!

[peranders]

Have you tried Google using the key words "logarithmic amplifier"?

[dan_thompson87]

Yes, I have. As well as actually making the effort to read numerous text books on operational amplifiers. Hence:

I've done plenty of research on logarithmic amplifiers, and although there is an abundance of information on the theory behind them, there is no information on how to implement this theory and design one.

Thanks anyway...

[sawreyrw]

Please explain what a mV/db is.

[scott wurcer]

You could search for Dave Blackmer's DBX patents. They (IIRC) had pretty clear disclosure of all the circuit blocks for this family of VCA's.

[dan_thompson87]

Originally Posted by sawreyrw

Please explain what a mV/db is.

Sorry this isn't a unit - it's 33mV per dB. I.e when 33mV is applied to the control pin on the VCA, the audio input is attenuated by 1dB.

You could search for Dave Blackmer's DBX patents. They (IIIRC) had pretty clear disclosure of all the circuit blocks for this family of VCA's.

Thanks, I'll look into that

[peranders]

Why can't you simply adjust the gain?

LOG104 from TI has 500mV/ per decade current. Put a V/I converter in front of the LOG104. A decade is 20 dB.

[Mooly]

Many moons ago I came up with this as an alternative to a special function IC.

This used the CA3046 matched transistor array.
The function I needed was V1 divided by 1/V2 for Cyril Batemans "Tan Theta meter" but the log amps may be of interest.

It was all a long time ago.

[wakibaki]

There's a logarithmic converter (amplifier) on p212 of my copy (1994) of Horovitz & Hill with values. As constituted it offers -1V/decade of input current. There's loads of good stuff in fact. ...read up to about p214.

w

[infinia]

Are you designing your own chip?

see log detectors here and some problems with discrete designs.


"The AD640 uses five cascaded limiting amplifiers to approximate
a logarithmic response to an input signal of wide dynamic
range and wide bandwidth. This type of logarithmic amplifier
has traditionally been assembled from several small scale ICs
and numerous external components. The performance of these
semidiscrete circuits is often unsatisfactory. In particular, the

logarithmic slope and intercept"

more reading here