Electronics > Beginners
HEF4013 flip flop question
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Chris Wilson:
I have been bread boarding a circuit and have seen that for a given input signal level of about 2.2V p to p at around 950kHz I cannot use a Vdd for the HEF4013 above a certain level or the output stops. I see this is correct in the spec sheet, but I am curious as to why Vdd is linked to the drive level required. Do all flip flops have this trait? Thanks.
Audioguru:
It is on the datasheet.

A HEF4013 is old Cmos logic that works with a supply that is 3V to 18V. A Cmos logic output driving the input of another Cmos logic produces an output swing the same as the entire power supply voltage so they make an input work when the input voltage swing is 0.3 times the supply voltage or less for a logic low and 0.7 times the supply voltage or more for a logic high, so that noise does not affect it much.

 
Zero999:
Yes, nearly all CMOS ICs have this characteristic: the logic thresholds scale with the supply voltage.

Look at the schematic of a NOT gate. It output won't properly go from high to low, until the lower N-MOSFET is on and the upper P-MOSFET is off. For a MOSFET to turn on, the gate voltage must be a couple of volts above (N-channel) or below (P-channel) the source. At higher supply voltages, the sources will also be at higher voltages, so the gate will need to swing closer to the supply rails for them to turn on properly. Applying voltages between the logic levels, isn't good, because both transistors will turn on, causing excessive power consumption.

https://en.wikipedia.org/wiki/CMOS
Chris Wilson:
Thanks Zero999

If I did not want use a separate 6V supply for the HEF4013 in order for it to divide a low level input signal of around 950kHz could I bias the input so a low level input will still wok yet have it run off the rest of the circuit's 12V supply?
Zero999:
Unfortunately, it's not that simple. Some sort of level shifter would be required to boost the voltage.

Common base configuration is probably best suited to this because it's non-inverting and is fast. Here's an example. It will probably need some tweaking to suit your application.


Another possiblty is a level shifter IC such as the CD4504 but it's probably not worth it for one channel.
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