[sdiy] LM13700/LM13600-based filters

[Harry Bissell]

I beg to differ...  ;^)

with the OTA cut off, there isn't a feedback loop anymore.

The integrator is tied to the input of the darlington buffer, and
will drift with the bias current it sees. If you use an inverting
opamp intergrator, the same thing happens buut its a lot less severe
than the internal buffers.

Its hard to adjust the OTA to exactly the point where it will not cut off completely

H^) harry


----- Original Message -----
From: Antti Huovilainen <ajhuovil at cc.hut.fi>
To: Harry Bissell <harrybissell at wowway.com>
Cc: Tom Wiltshire <tom at electricdruid.net>, synth-diy diy <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 26 Apr 2010 11:29:31 -0400 (EDT)
Subject: Re: [sdiy] LM13700/LM13600-based filters

On Mon, 26 Apr 2010, Harry Bissell wrote:

> There are some major issues. One is that if you use the LM13700, the 
> bias of the buffers is preset and may limit performance. In the LM13600 
> the bias is variable with the Gm input, and might feed through to the 
> output.
>
> In all cases, there will be an offset voltage because of the
> voltage drop of the darlington pair. This may not be a problem

Actually, no, there won't be. In a lowpass filter the buffer is inside the 
feedback loop that operates at below cutoff, and thus the only offset is 
due to the OTA input.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova

-- 
Harry Bissell & Nora Abdullah 4eva