Is a resonant PI possible?

[Unregistered]

JC1

Information Only!!!

Not saying this amp will work for front end amp.

though it might.

i would put it downstream.

Or even another brand maybe.

thought i better clear that up.

[Unregistered]

JC1

Duh,

looking at your signals the presence of metal will produce a negative going signal, but unless very big very close will not produce a half volt signal.


Positive going signal.

Getting old sucks.

[Tinkerer]

Hybrid amplifier

Here are some better pictures. The 0 of channel B is offset by one V so the full decay curve is visible. Settings are 2uS and 10uS – 1V div. The gain is not linear but not exactly logarithmic either. I will have to do some plotting to see how it behaves. I get 200 mili V deflection from a 0.6 gram gold nugget on the coil. Tinkerer

[Unregistered]

JC1

Hi Tinkerer,

Eric is exactly right. To make this into a log amp you will need to replace the npn transistor with a pnp transistor.
In your configuration. You will also need to add an additional 2.2 meg ohm resistor from the - input of the op amp to the wiper of a pot 10 - 200 k ohm, which is hooked between your -5 volt and ground. With about -0.5 volts on the wiper the output of your amp should be at +0.5 volts and the transistor (pnp) will be barely biased on.

This should give log amplification.

Realize that with the transistor off, your gain is 2,200.
Once the transistor starts doing its log thing, the gain will reduced by the conduction of the transistor and the gain will be less, but more or less logarithmic.

[Tinkerer]

Resonant coil, cable and preamp

Oh by the way, your damping looks just fine.

My bad, again!

I assume by resonant you mean the coil inductance and capacitance form a resonant circuit at what? 10KHz.

Interesting, remember a talk about that somewhere once.

thanks JC1, There seems to be a resonant spot of the coil/cable assembly that makes the decay curve at the output of the preamp fall just right. It is somewhat sensitive to minor variation of the damping resistor, meaning as little as 4 Ohm difference are significant. this is how I can get a sample before 10us with 2,200 gain at the preamp. There are other factors involved, like the schottky diodes, that do not seem to show their high capacitance. I dont fully understand what is happening, but I think it is worth my time to try to understand it because the results seem significant. Tinkerer

[Unregistered]

JC1

Hi Tinkerer,

It appears that the transistor in the configuration you have could be acting as a clamp with an emitter resistor that is equal to the input resistor 1k, for a gain of 1 roughly with the xsistor on full. If true, then the removal of it could mess up some of the fast response time.

so you may need two transistors in the feedback, the other for your log amp.

as far as resonanting things by the formation of a peak in the magnitude (gain) response, this might help speed things up, but keep in mind that movement of the coil closer and further from the ground will result in a change coil response.