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Low-z Wide Band Amplifier
Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
For those interested in this project, I just wanted to mention that I
updated the schematic with a 600pF capacitor on the +input of the op amp.
The op amp is a little under damped and this cap vastly improves the
flatness of the frequency response and stability. You make have to adjust
the value a little depending on your layout. It works much like the
compensation adjustment on a scope probe. Select the capacitor value that
gives the best square wave response. However, 600pF (+-100pF) is pretty close.
Often in TC measurement work, we want to feed a coil a signal from a very
low impedance signal source. While my new signal generator can make
wonderful signals, it is a 50 ohm output device. So I dug out some old
plans and whipped together a low impedance high frequency amplifier. It is
basically just the mighty LM12CLK op-amp with a power supply. It runs DC
up to 200kHz at full amplitude and up to about 700kHz small signal. At
300kHz at 4 volts, I shorted the output with a jumper and the voltage
dropped to 3 volts! So I guess the output impedance is pretty low :-)) It
was just driving the small inductance of the wire.
I did a 130kHz to 140kHz frequency sweep of my big coil with a Tek 5100 HV
Probe on the top of the coil to get the following scope picture:
As the coil passes through resonance the input voltage stayed rock solid.
Note how channel two is directly reading the coil's Q in this case (I
pushed a lot of button's :-)). A schematic and a few pics are at:
It is pretty straight forward without critical parts but read the data
sheet and keep the feedback paths really short per good RF practice.
Everything is over designed (I am not sure about the current in the 10uF
caps) so it should be pretty tough. The output voltage rises at higher
frequencies but not bad in typical TC ranges. The voltage could probably
be evened out with a little thought. I used mostly parts I already had so
I am not sure about the cost, but about $50-$100. It seems to work very well!
I hope to try that NST resonance thing shortly.
PS: I am still working on the Coil tuner project too. I need to wait until
I get the 1.8nF caps to work out the calibration issues but it is going