40KHz Oscillator Doesn't

[Phil Cross]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the feedback
path of an op amp.

I used a CA3140 (4.5MHz BiMOS), 12V supply, biassed the + input to 6V
by means of a decoupled voltage divider, and fed back the output to
the - input via a 4.7M resistor. This set the DC conditions OK -
output sitting nicely at 6V.

I then connected the 40KHz transducer in parallel with the 4.7M
resistor, reasoning (probably naively) that at 40KHz resonance I would
get a 180 degrees phase shift, and the system would oscillate.
However, looking at the output voltage on my scope, it does not
oscillate - it stays stubbornly at 6V, the DC condition.

I am anxious to learn. Please could someone more experienced than I
please explain where I am going wrong. I would be very grateful.

Regards,
Phil.

 

[Jan Panteltje]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the feedback
path of an op amp.

I used a CA3140 (4.5MHz BiMOS), 12V supply, biassed the + input to 6V
by means of a decoupled voltage divider, and fed back the output to
the - input via a 4.7M resistor. This set the DC conditions OK -
output sitting nicely at 6V.

I then connected the 40KHz transducer in parallel with the 4.7M
resistor, reasoning (probably naively) that at 40KHz resonance I would
get a 180 degrees phase shift, and the system would oscillate.
However, looking at the output voltage on my scope, it does not
oscillate - it stays stubbornly at 6V, the DC condition.

I am anxious to learn. Please could someone more experienced than I
please explain where I am going wrong. I would be very grateful.

Regards,
Phil.

Those resonators are mostly capacitive, so you would get at most 90 degrees,
and a huge attenuation with 4M7.
I have used these resonators in a LC circuit setup.
You can also drive these from a low impedance power amp that makes it to
that frequency.

 

[Eeyore]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the feedback
path of an op amp.

I used a CA3140 (4.5MHz BiMOS), 12V supply, biassed the + input to 6V
by means of a decoupled voltage divider, and fed back the output to
the - input via a 4.7M resistor. This set the DC conditions OK -
output sitting nicely at 6V.

I then connected the 40KHz transducer in parallel with the 4.7M
resistor, reasoning (probably naively) that at 40KHz resonance I would
get a 180 degrees phase shift, and the system would oscillate.
However, looking at the output voltage on my scope, it does not
oscillate - it stays stubbornly at 6V, the DC condition.

I am anxious to learn. Please could someone more experienced than I
please explain where I am going wrong. I would be very grateful.

Your mistake would be to assume that the transducer would create a 180 degree
phase shift.

Graham

 

[colin]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the feedback
path of an op amp.

I used a CA3140 (4.5MHz BiMOS), 12V supply, biassed the + input to 6V
by means of a decoupled voltage divider, and fed back the output to
the - input via a 4.7M resistor. This set the DC conditions OK -
output sitting nicely at 6V.

I then connected the 40KHz transducer in parallel with the 4.7M
resistor, reasoning (probably naively) that at 40KHz resonance I would
get a 180 degrees phase shift, and the system would oscillate.
However, looking at the output voltage on my scope, it does not
oscillate - it stays stubbornly at 6V, the DC condition.

I am anxious to learn. Please could someone more experienced than I
please explain where I am going wrong. I would be very grateful.

You probably need to make the op amp circuit more unstable,
so that it almost oscillates on its own at the frequency you want,
as said in other replies you cant assume the transducer behaves in the way
you want,
but it most likly has a a dip in its impedance at resonance,
my gues is it doesnt make for a very good inductor or capacitor at
resonance that you get from a ceramic or quartz device.
so putting it in the negative feedback path would have the wrong effect,
you need positive feedback.

A possible circuit might be to have positive and negative feedback with the
transducer in the lower part of the negative feedback divider, with the
negative feedback canceling the positive feedback the circuit is stable
untill the transducer robs the circuit of negative feedback at resonance
therefore leaving more positive feedback.

Colin =^.^=

 

[Tony Williams]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the
feedback path of an op amp.

http://www.ee.latrobe.edu.au/~djc/UltraSonics/Ultrasonics.htm

That site looks to be a useful primer, and the author
uses LTspice for simulation.

 

[J.A. Legris]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the feedback
path of an op amp.

I used a CA3140 (4.5MHz BiMOS), 12V supply, biassed the + input to 6V
by means of a decoupled voltage divider, and fed back the output to
the - input via a 4.7M resistor. This set the DC conditions OK -
output sitting nicely at 6V.

I then connected the 40KHz transducer in parallel with the 4.7M
resistor, reasoning (probably naively) that at 40KHz resonance I would
get a 180 degrees phase shift, and the system would oscillate.
However, looking at the output voltage on my scope, it does not
oscillate - it stays stubbornly at 6V, the DC condition.

I am anxious to learn. Please could someone more experienced than I
please explain where I am going wrong. I would be very grateful.

Regards,
Phil.

Here's an application note with lots of info and several example
circuits.

http://www.senscomp.com/specs/piezo application note.pdf

 

[Fred Bartoli]

colin a écrit :

You probably need to make the op amp circuit more unstable,
so that it almost oscillates on its own at the frequency you want,
as said in other replies you cant assume the transducer behaves in the way
you want,
but it most likly has a a dip in its impedance at resonance,
my gues is it doesnt make for a very good inductor or capacitor at
resonance that you get from a ceramic or quartz device.
so putting it in the negative feedback path would have the wrong effect,
you need positive feedback.

A possible circuit might be to have positive and negative feedback with the
transducer in the lower part of the negative feedback divider, with the
negative feedback canceling the positive feedback the circuit is stable
untill the transducer robs the circuit of negative feedback at resonance
therefore leaving more positive feedback.

Well, how do you expect to have a different loop gain between both
circuits? There's no low side impedance on the feedback network/divider.

Hint: opamps have (almost) infinite input impedance and your transducer
has a finite one.

 

[Ancient_Hacker]

Phil Cross wrote:

I then connected the 40KHz transducer in parallel with the 4.7M
resistor,

There's your problem. The op-amp has a very high input impedance, and
the resonator has a low impedance and lots of stray capacitance. Try
putting a 47K resistor from input to ground so the resonator works into
a more reasonable impedance.

Also you may not be getting the phase-shift you expect. You may have
to a dd a RC phase shift network to get the phase right.

 

[Fred Bartoli]

Ancient_Hacker a écrit :

Phil Cross wrote:



There's your problem. The op-amp has a very high input impedance, and
the resonator has a low impedance and lots of stray capacitance. Try
putting a 47K resistor from input to ground so the resonator works into
a more reasonable impedance.

Also you may not be getting the phase-shift you expect. You may have
to a dd a RC phase shift network to get the phase right.

47K is probably still way too much. To be effective it has to be of the
transducer impedance order of magnitude, which surely is much lower than
47K.

 

[neon]

you cannot put any active components in the feedback loop of any amplifiers not unless you can predict the results by body analysis. that transducer belongs in the output not on any part of the feedback. stay away from hi value of R f/b the offset current can offset enough to saturate the output. and finaly get a lm555 running at 37kc pushing a 100 ohm speaker and another speaker as detector into an amp that what i did [cheaper] it works.

 

[Jamie]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the feedback
path of an op amp.

I used a CA3140 (4.5MHz BiMOS), 12V supply, biassed the + input to 6V
by means of a decoupled voltage divider, and fed back the output to
the - input via a 4.7M resistor. This set the DC conditions OK -
output sitting nicely at 6V.

I then connected the 40KHz transducer in parallel with the 4.7M
resistor, reasoning (probably naively) that at 40KHz resonance I would
get a 180 degrees phase shift, and the system would oscillate.
However, looking at the output voltage on my scope, it does not
oscillate - it stays stubbornly at 6V, the DC condition.

I am anxious to learn. Please could someone more experienced than I
please explain where I am going wrong. I would be very grateful.

Regards,
Phil.

hbt feed back to the +

 

[Phil Cross]

I thought I could make a 40KHz oscillator, and hence ultrasonic
transmitter, by putting a 40KHz ultrasonic transducer in the feedback
path of an op amp.

I used a CA3140 (4.5MHz BiMOS), 12V supply, biassed the + input to 6V
by means of a decoupled voltage divider, and fed back the output to
the - input via a 4.7M resistor. This set the DC conditions OK -
output sitting nicely at 6V.

I then connected the 40KHz transducer in parallel with the 4.7M
resistor, reasoning (probably naively) that at 40KHz resonance I would
get a 180 degrees phase shift, and the system would oscillate.
However, looking at the output voltage on my scope, it does not
oscillate - it stays stubbornly at 6V, the DC condition.

I am anxious to learn. Please could someone more experienced than I
please explain where I am going wrong. I would be very grateful.

Regards,
Phil.

Thank you for all the responses. I am now going to put a wet towel
around my head, study them and the links in them, and see what I come
up with.

Thanks again,
Phil.

 

[Rich Grise]

Thank you for all the responses. I am now going to put a wet towel
around my head, study them and the links in them, and see what I come
up with.

Tell me - does the wet towel work as good as foil? ;-)

(anybody else see "Total Recall"?) ;-)

Cheers!
RIch