Hi all,

In Horowitz and Hill's "Art or Electronics", I see several illustrations of
very clever little "linear" amplifier configurations using CMOS
inverters,especially the 74HC04. I have an application (which DOES use a
PIC,as a freq. counter) where I need to take in a 100mV or so signal from
about 100Hz or so up to 50MHz and convert it into a 0-5V square wave. There
is a circuit in H&H which is similar to this and I have been using it as a
guide,but I am having problems. The basis of the circuit is simply two
inverters,the output of one connected to the input of the other. The first
inverter also has a 1Meg ohm feedback resistor across it,and the second one
doesn't. All other inverters on the chip have their inputs tied high or low.

The behavior I'm seeing is incessant oscillation. Sometimes, I'm able to
get the input to overpower the oscillation and I get a correct output.
However,more often than not,all I get is a 75MHz or so signal out of the
circuit. Even if I connect the input directly to GND using a 0.1uF cap,I
still get an output signal which is quite strong,but at a lower frequency.

Yes,I am breadboarding this,and that may be part of the problem,but I have
taken special care since this is such a high-bandwidth circuit. I have a
bypass cap (O.1uF) directly from Vss to Vdd,and the actual Vss and Vdd pins
are connected directly to the power supply.

I also tried connecting a potentiometer to the circuit thru a 100k ohm
resistor which would seem,theorically to give an amplifier with a gain of
-10. However,what I'm actually seeing is this: if the pot is adjusted away
from 2.5V, I see the correct behavior (the output is DC near one of the
rails). If I adjust it within a couple of 100mV of 2.5V, I get an
oscillation on the output,centered roughly around the expected DC output
value.

It almost seems to me that the inverter is acting as if it has a small
amount of hysteresis or that I am exceeding its phase margin due to
parasitic (or added in the test case of the 0.1uF cap above) capacitance. I
don't see how this is avoidable.

Can someone please tell me what I am doing wrong and how to get this to work?

Thanks very much,

Sean


|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
mailto:shb7@cornell.edu ICQ #: 3329174