Why Sink current is higher than Source current?

[Shahriar]

Hi

Can anybody tell that why Sink Current in most of Integrated circuits are higher than Source current. i.e. a TTL Gate can sink around 20mA while it can not source more than 10mA. (The Numbers I said are not very sharp.)

ThanX
Shahriar

 

[audioguru2]

Hi Shahriar,
The input of an old TTL gate is the emitter of an NPN transistor and has a max low current of 1.6mA. If a TTL output must drive 10 of them then its output current must sink 16mA. The input high current of a TTL gate is only 40uA.

Newer CD4xxx Cmos gates have no input current, they need to be driven with current only to charge their small input capacitance quickly. Their output sink and source currents are perfectly symmetrical and therefore are exactly the same. 

 

[Staigen1]

Hi both of you

How was it with the old RTL digital ICs, didn't they source more than sink?
Or was it an another family, that have PNPs output, only similar to RTL?


//Staigen

 

[audioguru2]

Hi Staigen,
Very old RTL logic chips used Vacuum toobes, didn't they?? ;D ;D

 

[Staigen1]

Hi Audioguru

Hehe, not that old, theres a digital family called RTL, hehehe

 

[Kevin Weddle]

I think it's really arbitrary. The goal is to design a circuit that is sufficient for a large signal. It's like cracking open any old circuit and asking, " Why is that current there, higher than that current there?" The large voltage swing lends itself to that.

 

[Tony_Stoynov]

Hi,
this is depend from the technology, in some chips have a curent sources, on some chips the limit is from high side structures. Not in every chips have a different between two curents.

RTL is a rezistor transistor logic, in this type of logic is a normal to have a much higer current t grond, becouse the high level is provide from pull up rezistors. In the new TTL or CMOS, output is PNP-NPN or P-N chanel transistors, but actually this is like more easy that is in reality to obtin the high frequency have a many diferents way to improove the circuit, but some of the limitation is this differents, but how i say before, this is depend from the chip.