Hi,
This is the first LAB amplifier I have built in one of the beautiful Fischer boxes.
My starting point was to make a LAB amplifier that would give 20dB gain and that would suppress as much as possible the
230V powergrid 50HZ frequency that we have here in the Netherlands.
I would like to have these specificationsInput 100K or higher
Bandwith about 1Hz to 1MHZ at -1dB
Power: 2x 9V battery
Some gain switch posibility
Notch attunation >50dB
Protection of +-60V on the in and the output connectors
Reasonable low noise
No squarewave abberations
this amplifier is designed te drive max. 1M 50 Ohm coax cable and do not terminate it!
And with these requirements, this has become the schematic for this notch amplifier.
Sorry, i still have to make the front for this amplifier, making it will not take much time now, because I have gained a lot of experience with the other box.
But it is also good to show here, how much impact it has if you make the front nice and tidy.
For this amplifier I chose the beautiful OPA140 amplifier series of TI, these have a low noise, Fet input, good square wave reproduction and quite large bandwidth.
The first opamp is a single version and the opamps used for the Notchfilter are the dual version, that is the OPA2140.
Adjustment of the amplification and attenuation all happen around the first opamp. This is actually nothing special, a textbook circuit.
And then we get the notchfilter, this type was developed by Henry P. Hall and below a link to explanation of this Notchfilter by: Kenneth Kuhn.
http://www.bramcam.nl/NA/NA-Noise-Measuring-Amp/Kenneth-Kuhn_the_hall_network.pdfFor people who live in the area where one has 60Hz as mains frequency, scale the capacitors down in the notch filter, that is the easiest.
I have chosen the Q of the filter in such a way that it is easy to adjust and remains stable enough.
I also selected the capacitors so that they are as equal as possible.
That's a bit more important than exactly 150nF, 3x 148.5nF is also good.
Because there is also a frequency trimpot available.
Some pictures...
The frequency response around 1MHz.
Red shows the C9, 35pF trimmer cap respons.
Wiring around the BNC connectors and switches.
Where necessary, the components are floating in order to keep paracitic capacities low.
At the bottom right you can see the 8-pin IC socket, which is for the input amplifier, the OPA140.
Also visible is that I regularly use the space in the IC socket for disconnecting capacitors or as here one of the resistors needed for the OPA140.
In the middle is the OPA2140 visible which is used for the Notch filter.
Here i use the nice Hameg HMF2525 as a 50Hz source for trimming the 50Hz Notchfilter.
And the Fluke 8920A (A beautiful instrument) tels the Notch dept, more than 51dB.
Just for fun!
Measuring the freecuency respons with my Rigol-815 SA, the rippel around marker-1
is due to the fact that the SA was not yet properly heated, so this is not due to the amplifier to be measured.
This is the noise from the amplifier when the input is terminated with 50 Ohms, at a 22Khz bandwidth.
The gain here is 20dB, so the noise at the input is about 1.3uV in this bandwidth.
And the last noise measurements, lower than 10Hz and if I have it right,
something of -3dB at 750Khz from the Audio Precision Analyser, 93.3uV and that is 9.33uV at the input.
Doing input protection testing, 60V power supply en a box with some dendering relais, simple and effective
Simple setup of the "distroyer" box
This is how its build.
Of course I tried to draw the Notch on graph paper, but I didn't quite succeed...
And this is the Notch displayed on the Audio Precision, it is difficult to make a goed picture of the display, sorry for that.
Shoot at it!
Kind regards,
Bram