[sdiy] important questions!

[Scott Bernardi]

The TL07x and TL08x series opamps are popular in synth-DIY work for a few reasons.
1) High slew rate, which is important to render large waveforms at audio frequencies
accurately. The TL opamps have a slew rate of about 13v/uSec, compared to about
0.5v/uSec for a 741 type.
2) Very low input bias current.  This is important to reduce errors when buffering
capacitors like in a sample and hold, or in an integrator like in a VCO. The integrator
and reference amplifier's input bias current will determine the lowest frequency (i.e.,
charging current) that a VCO will go to.
3) They are cheap and readily available in singles (TL071/81), duals (TL072/82) and
quads (TL074/84).

I'm not sure if I understand your second question.  The V-I characteristic of a JFET is
approximately parabolic. The V-I curve of a bipolar transistor is exponential. And
actually most amplifier circuits use a small portion of the V-I curve where it can be
assumed that it is approximately linear (small signal analysis).  The superposition
principle (where the output can be derived as the sum of the effects of multiple input
signals acted separately) applies to linear circuits.
As for fourier series in nonlinear circuits and sums and differences, are you perhaps
talking about a four quadrant multiplier circuit?  In a particular application of the
multiplier circuit called the balanced modulator (or ring modulator), two input signals
are applied, and the output is the sum and difference frequencies of the two inputs.
These results can be derived from fourier analysis. A balanced modulator is a popular
synthesizer module.

Somebody else will have to address the question of the history of the discovery, but it
probably predates even the vacuum tube.


Arun Bohm wrote:

> Hi
>    Why do so many DIY-ers seem to be into the TL082? What are it's
> characteristics and what is it's pin-out. In terms of studies I've only just
> got to the part where we discuss (in books) how the gain is affected by the
> frequency of the input.
>    Also, another question. What is it about non-linear devices which allows
> for the sums and differences across it's junctions? I understand that it has
> to do with some fourier interpretaion of the seemingly parrabolic curve
> which describes it's voltage current relationship. But it has been some time
> since I did a fourier transform, and even then only for homework.
>    Was this first discovered theoretically and then applied experimentally.
> Had to have been. Not anything like observing forces between current
> carrying wires, a totally experimental discovery.
> Love,
> Bohm.
>
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