Hex VC mixer

[Magnus Danielson]

From: bader <trip at blackstar.myip.org>
Subject: Re: Hex VC mixer
Date: Mon, 24 Jul 2000 13:22:16 -0500 (EST)

> 
> 
> While were on the subject of digital logic for analog functions, This
> reminds me of something i saw with a 4011 that made it function
> as a quad op-amp. Connect one of the inputs to vcc, and the extra input
> acts like the negative input on an op-amp. Add your resistors and
> feedback loop, there you go.
> 
> I haven't tried this myself, Same idea, maybe?

Not quite, but it is up along a similar path thougth. Digital logic is really
just digital since we keep pounding it into your poor heads, heck, there is
no digitial chips around just yeat, it's all very analog. It is however
convenient to say its behaves like digital logic. Anyway, this is how it works:

Take any CMOS inverter and sweep the input from rail to rail and you will see
a curve like this:

----
    |
    |
    |
    |
     ----

Now, in the steep slope in the middle the inverter is acting like a linear
amplifier with a negative gain. The trick to use this gain is to bias the
input so that the signal is operating in this linear section. You can do this
to a 4049 using a resistor between input and output and then a resistor to
either rail (no, it is not the VCA operation). Now, I haven't thougth of using
an 4011 as a quad op-amp, but looking carefully at it I see that you can use
the same biasing trick to get it going.

However, both inputs would be negative ;)

> On a sidenote, maybe you could use the input connected to vcc to toggle it
> between inverting an non inverting. Once again, i haven't tried this.

Hmm... I do not think so....

Now I just await for someone to propose how to use an Z80 or i80386 as a
compressor, just by setting it up correctly in analog operation mode ;)

Cheers,
Magnus