[sdiy] inversion, CMOS, etc.

René Schmitz uzs159 at uni-bonn.de
Fri Aug 2 23:35:03 CEST 2002

Hi Gavin!

>OK, here's a sort of "Grab Bag" message, I've had a lot of ideas lately and
>no budget so things are sort of floating around in this abstract space...
>1.  I need to invert a logic level pulse from positive to negative.  In
>other words the way the signal is now is 0V-+5V,  I need it to go from
>0V--5V.  Normally in this application I'd use an OpAmp to invert it BUT this
>is a very High Frequency application and So far the circuitry is all TTL so
>if I can get away with not building an additional bipolar supply to power Op
>Amps I'd like to.  In my understanding a CMOS inverter like the 4049 or a
>TTL inverter like the 7404 will just put the signal 180 degrees out of
>phase.  Or if I need to use an Op Amp has anyone used the 351 for RF type
>frequencies that can vouch for its slew rate at high frequencies?

A humble PNP transistor in base circuit could be your friend. Base to GND, 
and emitter via say 2k2 to your logic output, and collector to -5V via 2k2. 
However the output will sit at -5V for 0V input and at 0V for 5V input. So you 
might still need an inversion. But the base circuit is pretty fast.

>2.  I also need to take a signal oscillating between -5V and +5V and bring
>it up to 0V - +5V while keeping the waveform intact.  A comparator powered
>between GND and +5V will act like a schmitt trigger and a half have
>rectifier will preserve the upper peak but square off the lower.  Will
>mixing in a +5V DC signal through an opamp mixer bring the signal up to 0V -
>+10V?  Then I can reduce it with resistance to the appropriate amplitude.

Right. Even a passive summer will work here, since you have level to spare. A 
voltage divider (with equal resistors) from your signal to a +5V reference will 
do that.

>3.  Thinking about pattern generation and analog computers has led me to the
>idea of building a voltage controlled comparator.  A simple circuit, to be
>sure, has anyone seen or implemented something like this?  

Thats frequently seen in the PWM parts of VCOs.


uzs159 at uni-bonn.de


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