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Auto Gain
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NuttyMonk



Joined: Jun 30, 2020
Posts: 62
Location: UK

PostPosted: Tue Jun 30, 2020 3:06 pm    Post subject: Auto Gain Reply with quote  Mark this post and the followings unread

Hi all,

i'm new to the site and to creating modular synthesizers in general although i am not new to music production and have made VST's in the past.

I have a question about automatically adjusting gain.

Often when i'm building out an oscillator schematic i see that the voltage doesn't remain constant over the whole frequency range. Also, when i am building an oscillator design that has several different outputs (square, triangle, sawtooth) they have different voltage levels from each other (see the CEM3340 for example). In some cases i would like to maintain a preset voltage level across the whole frequency range and in other cases i might want to save on parts and PCB space by using one opamp for multiple switchable sources (like the CEM3340 example).

I have been looking around for ways to adjust the gain automatically e.g. to maintain a preset level which is built into the design, such as +-5v (p-p).

I have found a schematic on Engineers Garage Engineers Garage - Automatic Gain Control which works really well when i simulate it in National Instruments Multisim but the main component (BFW10 Transistor) in the schematic is rare and isn't cheap either (£2.50 each). I am not sure if they are new/old stock too meaning they may run out eventually. Also the output voltage is determined by the input voltages of the opamp and i'd rather not have to set up a second set of voltages with another power supply. I am happy just using +-12v and don't want to have to set up 5v rails as well.

So my question is whether there are any other ways for me to achieve the same outcome. I have seen in this post on this site http://electro-music.com/forum/viewtopic.php?highlight=auto+gain&t=71912 that there are other ways possibly using a transconductance opamp but haven't delved into their workings yet. Also there is the possibility of using a FET in the feedback loop of the opamp.

Can anyone offer possible alternatives? Preferably with some example schematics? The simpler the better to begin with. As i said earlier i am still an electronics noob.

Any help would be appreciated.

Cheers

NM
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gabbagabi



Joined: Nov 29, 2008
Posts: 651
Location: Berlin by n8
Audio files: 23

PostPosted: Wed Jul 01, 2020 5:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello, and welcome to the party party!

i remember there was something about AGC in the LM13700 Datasheet, Page15

https://www.ti.com/lit/ds/symlink/lm13700.pdf

good luck!
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NuttyMonk



Joined: Jun 30, 2020
Posts: 62
Location: UK

PostPosted: Wed Jul 01, 2020 7:53 am    Post subject: Reply with quote  Mark this post and the followings unread

gabbagabi wrote:
Hello, and welcome to the party party!

i remember there was something about AGC in the LM13700 Datasheet, Page15

https://www.ti.com/lit/ds/symlink/lm13700.pdf

good luck!


Thanks gabbagabi.

I've had a look and oddly enough i think i understand how it works. Will simulate it in Multisim and see whats what.

These LM13700 OTA's look quite versatile. Lots of VCF and VCO schematics in there too. You've just given me weeks of tinkering to do. And they aren't too expensive Smile

Thanks
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gabbagabi



Joined: Nov 29, 2008
Posts: 651
Location: Berlin by n8
Audio files: 23

PostPosted: Thu Jul 02, 2020 12:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

let me add some sentences

keep in mind that the Iabc input is extremly sensitive on over-current, the limit is about 2mA. a protective measure in form of a 10k on that input is "a good idea"

The Iabc input use as reference negative supply voltage, means tied to lets say minus 12V ( if u aare using +/-12V split power supply) results in no amplification. the higher the voltage goes from there - the higher the amplification.

as iam heavily using multisim my self, i have encountered problems to reach "silence" means no amplification as i was trying to simulate a VCA circuit
means i guess the simulation results may not as accurate as one would expect from such a sophisticated software
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