AW: Compensated Exp generators and VCO's

Haible_Juergen#Tel2743 HJ2743 at
Thu Jun 1 02:11:00 CEST 1995


thanks for the good description.
I'd like to add a few minor points to it, if that is ok.

> I wish there was a transistor array which did have great
> specs for the individual transistors, as substrate
> temperature stabilization is very easily done (one
> transistor a heater, one a temperature sensor, one a clamp
> and two the differential pair). Alas I'm not aware of such
> a part.
> - anyone know another 4 or 5-transistor array with better
> specs than the LM3046?

There are quad NPN and PNP arrays available from Maplin, UK.
I think the manufacturer is Elantec (?). The transistors are insulated
from each other, so You don't have to care for substrate voltage.
I don't know the part number now, sorry, but I have a few of them
at home. (Didn't use them so far, 'cause I went 3340 instead.)

> The discharge time is kept very short here by using a
> HC4066 analog switch instead of a discrete (MOS)FET. FETs
> with a low Rds,on are hard to find, while the switches in
> the common HC4066 have an Rds,on of only 25 ohm! Normal
> MOSFETs have a Rds,on of ca 100 ohm. On the downside: a 5V
> supply is needed now.

I used to use a bipolar transistor instead of a FET. Might be faster
and cheaper than a FET. I also didn't discharge the cap to a zero
asymptode, but to a different one, and I stopped the discharge when
0 was reached (2nd comparator + Flipflop). This gives a more linear
and thus faster discharge slope, I think. (But I never measured it, I
admit, so please correct me if it's wrong!)
Another hint: If You use a standard CMOS (CD4066) instead of the
the HCMOS type (74HC4066), 15 Volts should not be a problem!

> The discharge time of the capacitor introduces an error in
> the sawtooth. As the discharge time is always the same it
> gets increasingly important at higher frequencies. This
> can be compensated for by decreasing the treshold voltage
> at higher frequencies, so the charge time is a bit
> shorter, and thus linearity is ensured.
> - Did you compensate for the discharge time in the
> sawtooth generator and bulk resistance in the exp
> generator?

A common way to compensate that effect is a simple resistor
(or trimpot) in series to the capacitor. If You have a discharge
time T and a capacitance C, then R=T/C should compensate to 100%.
Make sure that the comparator input is connected to the current-
source/trimpot node and not directly to the capacitor.

Another thought: As the CEM3340 are said to be available soon again,
I doubt the *need* to go discrete. (If You just want to do it for the
challenge, that's another thing!).
Good transistor arrays plus all the circuitry for heating aren't much
cheaper than a 3340, and they consume much more PCB space and
routing skill.
I know I said different VCO circuits sound different - but if You *really*
want the sound of a "converted" triangle instead of a directly created
one, nobody keeps You from routing a 3340's saw wave into a discrete
triangle converter, for example.


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