If your symptom is 'stuck low' you may have miss matched transistors...
If your symptom is 'stuck low' you may have miss matched transistors...
When you say "mismatched," do you mean two wildly different HFE values?
Did you try the ~250mV threshold resistor values ?
If your symptom is 'stuck low' you may have miss matched transistors...
When you say "mismatched," do you mean two wildly different HFE values?Yes, the threshold spice gives assumes roughly matched transistors. With a triangle wave injection your circuit is now working, it then becomes a matter of setting the threshold
If Vbe or hFE are too different, will move the operating point slightly.
Did you try the ~250mV threshold resistor values ?
With the waveform entirely below 0 V, and the High level right at 0 V, I do get an output of about 2V zero to peak. But when I raise the HI part of the waveform to just a little bit above 0V, using the DC offset adjustment of my function generator, the output all but vanishes.
With the waveform entirely below 0 V, and the High level right at 0 V, I do get an output of about 2V zero to peak. But when I raise the HI part of the waveform to just a little bit above 0V, using the DC offset adjustment of my function generator, the output all but vanishes.The output should swing to close to 5V, so something is incorrect. Maybe resistor values are incorrect ?
With the waveform entirely below 0 V, and the High level right at 0 V, I do get an output of about 2V zero to peak. But when I raise the HI part of the waveform to just a little bit above 0V, using the DC offset adjustment of my function generator, the output all but vanishes.The output should swing to close to 5V, so something is incorrect. Maybe resistor values are incorrect ?
With the input simply grounded, (150K 12k 4k7) Spice gives
Vbb = 680.33099mV
Vbase = 344.21611mV so that means Q1 is comfortably OFF.
When the input lifts to 258mV, the collector has dropped from 5V to ~2.5V
I suspect that the OP has connected R3 (150K) to the collector of Q2 instead of the base.
However, if I continue to increase the input voltage to 326.3 mV, the output will drop to 2.553 V. In other words…
Input=0v, Output=5.0V
Input=252mV, Output=4.851V
Input=326.3mV, Output=2.553V
When I simulate that same circuit, I get this…
Input=0v, Output=5.0v
Input=252mV, Output=3.25v
Input=326.3mV, Output=90.94mV
At this point, I'm not sure we are really making progress.
The only thing I can suggest is that I purchase a dual BC847B to ensure the hFE of both NPNs is the same and then I can breadboard that. I found 8 part numbers in stock at DigiKey from reputable vendors. Please let me know which part number I should buy, and I will go proceed with that. Of course, I will also be testing the comparators when they arrive as well.
However, if I continue to increase the input voltage to 326.3 mV, the output will drop to 2.553 V. In other words…
Input=0v, Output=5.0V
Input=252mV, Output=4.851V
Input=326.3mV, Output=2.553V
When I simulate that same circuit, I get this…
Input=0v, Output=5.0v
Input=252mV, Output=3.25v
Input=326.3mV, Output=90.94mV
At this point, I'm not sure we are really making progress.That's good, you have found a 50% threshold point. It's about 66mV higher than my sim, which is not massive.
Increase that Biased DC Vin slightly more, and it should give you the 0.5V point - useful spec points are 50% and the 10%/90% points.The only thing I can suggest is that I purchase a dual BC847B to ensure the hFE of both NPNs is the same and then I can breadboard that. I found 8 part numbers in stock at DigiKey from reputable vendors. Please let me know which part number I should buy, and I will go proceed with that. Of course, I will also be testing the comparators when they arrive as well.You could get BC846B / BC847B for lowest cost and the BCM846B / BCM847B are the Nexperia / Diodes matched pair specs.
I'm not sure you really need the tight matching those give (2mV on Vbe) but you could try some of each.
Single package give you same-vendor/batch/date code and same temperature, which is a good improvement over separate packages.
Addit: I skewed the gain in spice, by selecting a badly-wrong-suffix part and it moves the threshold roughly +/- 25mV
Skew Q2=BC847A, Q1=BC847C = 233.33334mV @ 50%
Match Q2=BC847B, Q1=BC847B = 258.26559mV @ 50%
Skew Q2=BC847C, Q1=BC847A = 284.01084mV @ 50%
As I said in my earlier post, the problem is that when I use the DC offset adjustment of my function generator to make the waveform to be above 0 V, the output of your circuit basically goes to zero. You can see this in two attachments below. With the waveform entirely below 0 V, and the High level right at 0 V, I do get an output of about 2V zero to peak. But when I raise the HI part of the waveform to just a little bit above 0V, using the DC offset adjustment of my function generator, the output all but vanishes.
However, if I continue to increase the input voltage to 326.3 mV, the output will drop to 2.553 V. In other words…
Input=0v, Output=5.0V
Input=252mV, Output=4.851V
Input=326.3mV, Output=2.553V
As I said in my earlier post, the problem is that when I use the DC offset adjustment of my function generator to make the waveform to be above 0 V, the output of your circuit basically goes to zero. You can see this in two attachments below. With the waveform entirely below 0 V, and the High level right at 0 V, I do get an output of about 2V zero to peak. But when I raise the HI part of the waveform to just a little bit above 0V, using the DC offset adjustment of my function generator, the output all but vanishes.However, if I continue to increase the input voltage to 326.3 mV, the output will drop to 2.553 V. In other words…
Input=0v, Output=5.0V
Input=252mV, Output=4.851V
Input=326.3mV, Output=2.553V
Those two tests have contradictory results.
So if you also connect the scope, while doing the static DC test with the power supply and voltmeter, do the readings on the scope and voltmeter agree when measuring both the input and output?
If you used your signal generator as the input source at a really slow frequency (0.2Hz), what does the DC voltmeter show vs the scope?