This probably is what I would do.Should I use a proper opamp amplifier?
Which is why you ultimately need to actually go into the lab and test some real hardware....the difference between post #15 and #16 is probably in selection of the SPICE integration method. I'm using Gear instead of default modified trap. Hard to say which setting better fits the real hardware behaviour.
If you remove Cc, your circuit behaves as an opamp with gain=-1. Cc introduces a positive feedback around 2nd-3rd stage making it an oscillator. As you are using it at low frequency, my suggestion is to go back to the original values, remove Cc, and remove the second and third HC04. A single stage can easly provide the gain of 10. If you don't have 1M/10M resistors, try to reduce their values, but keep the x10 ratio between the two. I would not go below 1M for the second resistor for two reasons: to avoid loading the output stage, but most important, as the gates have an input capacitance Cgate that can make the circuit oscillate if R x Cgate is too small. Having R larger, makes the loop gain small avoiding oscillations.Hello,
I've tried to use in a circuit this improved (high gain, 3 gates) 4069 amplifier from the old AN-88 (National Semiconductor),
instead of a better amplifier, because I had unused gates available.
This is used at very low frequency and DC, so low bandwidth is not a problem.
I built the circuit with 20 k resistors (values in red), VDD=5 V, and it oscillates at about 2.4 MHz.
View attachment 172706
Then I tried to compensate the amplifier by adding Cc:
Is it possible to compensate this amplifier, keeping the resistor values to 20 k and how to do it?
- for Cc = 22 nF, it oscillates at about 38 kHz
- for Cc = 100 nF, it oscillates at about 9.8 kHz.
Should I use a proper opamp amplifier?
Thanks
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