s to r

danial stocks diode at hotmail.com
Mon Mar 13 12:26:07 CET 2000


this is the simplest sq to saw converter that I can think of which doesn't 
need access to CV etc..

1 - square wave feeds into an OTA set up as integrator. the o/p from this 
will be a tri wave, amplitude varies as invers of frq. altering the bias on 
the OTA will affect the amplitude of tri wave o/p. tri wave o/p is fed into 
a precision rectifier [1 op amp - see nat semi LM 3914 apps/ datasheet for a 
suitable cct] - this then provides a dc o/p = to the wave amplitude. this 
feeds into another op amp, with it's other i/p at a reference vtg. the o/p 
of this drives the bias on the OTA. thus we now have a sq - to tri conv with 
constant amplitude o/p using 1 OTA & 2 opamps

now here's the tricky bit.. tri wave feeds into an op amp - into both the + 
& - inputs at once. the i/p resistors however are 2 discrete resistors in 
series. the junction of these 2 res also connects to a mosfet to gnd.[ this 
is done on both i/ps]. one mosfet has its gate as i/p, this also goes to an 
inverter [transistor] which drives the other  mosfet gate. so.. when this 
i/p is high, one mosfet is on[ we'll say the one on the - i/p] thus the - 
i/p is pulled to gnd via a res, the + mosfet is off and so has no effect. 
thus with proper working out of vals the op has a gain = +1.. if the mosfet 
i/p is low, the +i/p fet is on, grounding it, and the -i/p fet is off, 
having no effect. thus the op has gain = -1 . this i/p connects to the sq 
wave i/p.. so when the sq wave is high, the we get a rising ramp from the 
[non inverting] integrator - and the op amp is +1 gain so we see a rising 
ramp @ the o/p. when the sq wave is low, int puts out a falling ramp, but 
the op is -1 gain, inverting it, so again a rising ramp @ the o/p.. this op 
amp gain polarity switcher has been built and used quite effectively in 
another project,. In the one I made, the - i/p has 100k res in, [mosfet,] 51 
k res to summing node. + in has 100k in, [mosfet] 200k to + in, 110k from + 
in to gnd.. for perfect saw o/p might have to put some trimpots in to 
calibrate both halves of the saw [total +/- gain matching wasn't necessary 
in the last use of this cct..].. jfets should work just as well..
so there it is,, 1 OTA, 3 op amps, 2 fets, 1 trans is all the active devices 
req'd for this
Cheers,
Dan
Dan
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