Compensated Exp generators and VCO's
sscprick at horus.sara.nl
Wed May 31 15:16:08 CEST 1995
(This has become quite long, sorry :-)
About exponential generators
For the Moog filter I used an exp generator built with the
LM3046 transistor array, and this works quite alright. For
a VCO the specs of the 3046 are questionable. The matching
between the transistors is ok-ish but not more than that.
The current gain (hfe) is 50-100, not more. About the log
conformity I have no data, nor on the bulk resistance of
the transistors. (Anyone for this value?)
The Formant VCO's and some other machines have used the
Fairchild uA726 in the past, which was a precise dual
transistor with built-in on-chip temperature
stabilization. Alas, this has become an unobtainable part.
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. Harris does have a CA3246, which looks like an
improved 3046, but it is not available here in The
Netherlands. It has a bit better specs for hfe, but again
no data on bulk resistance or log conformity.
A new VCO
A new project is a temperature stabilized VCO. The past
week I've done some literature research on log and antilog
(exp) generators, and I've come up with something I'd like
to discuss. A first schematic is available in my WWW page:
It features a precise and very log conformant dual
transistor pair (National Semiconductor LM394), which is
kept at a constant temperature with a heater circuit, as
one of the biggest sources of error is the change in
ambient temperature. This can be compensated in two ways:
- by using a tempco resistor, the infamous Q81's
- by keeping the temperature stable.
As for the Q81's: I cannot find them here in the regular
shops, and thus I have abandonned this route and opt for a
heated exp generator.
The heater consists of a piece of aluminium profile with a
powertransistor and a temperature sensor attached to it,
enclosed in a small plastic container with the dual
transistor. The container is something like a small
medicinal drugs package. Attach the cap to the circuit
board and screw on the container. This way airflow has no
influence. Of course further insulation with styrofoam or
something is a good idea.
As always there's also a downside to the heater solution:
it draws quite some power, especially at startup time. So
I will probably build a separate heater power supply.
Further errors in exp generators are introduced by the
bulk resistance in the transistors. This can be
compensated too,with the diode/4.3M resistor as shown in
the schematic. The bulk resistance is about 0.5 ohm for
By using a precise current source further log errors are
The LM394 is a rather popular part in the world, and thus
quite well available.
In the sawtooth generator the 2.2n capacitor is charged
untill the LM311 voltage comparator's treshold is reached.
Then a MOSFET switch discharges the capacitor. When the
treshold voltage is reached the output of the comparator
doesn't change back right away, it's held high for a short
period, depending on the 22p capacitor and 10k resistor.
This ensures the 2.2n is fully discharged through the
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.
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. The cost is that
sawtooth amplitude gets a little bit lower at higher f.
- Design with Operational Amplifiers and Analog Integrated Circuits
by S. Franco
- The Art of Electronics by Horowitz&Hill
- Opamp Cookbook by W.G. Jung
- Elektuur nr 266, December 1985 pg. 38 'Kristal-oven' about a
In the schematic there is also a part devoted to triangle
waves, but that part is in serious error. Comments and
further discussions are of course very welcome!
Some questions bothering me right now:
- anyone know a value for the bulk resistance for LM3046
- anyone know another 4 or 5-transistor array with better
specs than the LM3046?
- What did you do about temperature stabilization?
- Did you compensate for the discharge time in the
sawtooth generator and bulk resistance in the exp
- Do you convert the sawtooth waves to triangle or simply
use separate triangle-wave generation?
rick at sara.nl http://www.sara.nl/Rick.Jansen
S&H's a module and s&h's looking good
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