The circuit linked below is intended to be a CCS amp with a gain of 2 and double pole LPF. The latter smooths an eight step, digitally simulated sinewave (8 Hz, 1.5V swing at pin 3). Load is a 200 Ohm air core solenoid.
The finished device will be a field calibration source for my Earth micropulsation sensing coils. It operates from 4 x AA cells.
This is as far as I have progessed, based upon a combination of existing circuits. I am having trouble finding the relevant information to proceed.
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It only outputs unity gain. With the LM358 I am hoping for at least
5V.
The 1.5V sinewave looks fine, until a load is connected. At 200R the upper right half the the waveform distorts. At 25R the output goes flat. As monitored across the load.
The FET is shown with source and drain interchanged. Even connected correctly, the 2N2700 has a typical gate cutoff voltage of 2.1 V, which means it needs to stay that far above the transistor base, and
2.8 V higher than the output on the coil. You would do better with a higher power supply voltage, like 9 or 12 V. (Unless you've selected a transistor with especially low cutoff voltage- the range is pretty wide.) That it works at all with a 200 ohm load probably means you've got a FET with low cutoff voltage.
The amplifier, besides being a filter, is also a unity-gain voltage follower below cutoff. It's a constant voltage source for the output on the top of the load coil, so there's no gain of 2. You would need a different circuit configuration to get a constant current source.
The filter has a cutoff frequency of about 4 Hz, a little low for an 8 Hz input signal.
With a load of only 25 ohms and 3 V p-p output, you might be pushing the 2N2222 a little hard without a heat sink.
If I can suggest something, you might split up the functions- filter, gain amplifier, constant current source- into separate stages until everything is working, and then condense it if possible. It's easier to troubleshoot things piecemeal.
I simulated the circuit in LTspice, which you can download for free (just Google it). The .asc file, which I'll send in another post, can be loaded and will display the schematic, and you can simulate it. You can get an undistorted output with either a 200 ohm or 25 ohm load by changing the power supply voltage from 6 to 12V, or by changing the FET to an FDS6574A, which has a low (0.65 V) cutoff voltage. You can change this by right-clicking the FET symbol.
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