CMOS FET in RTL configuration...

[dtvonly]

Hi. I typically see a BJT used in an RTL configuration. What about N- or P-channel MOSFET? if not then why. I would think that due to their faster switching time, wouldn't they be better? Thank you.

 

[kubeek]

I would say that RTL is so old that no one bothered to redraw it's equivalent with FETs.
They could be marginally faster, but I don't think they would come close to CMOS process. Also think about the state of development of BJTs vs FETs at that time, so maybe an RTL with FETs would be slower than with BJTs then.

 

[dtvonly]

CMOS for RTL application...

Hi. Kubeek. So CMOS is "fine" for RTL application?

 

[kubeek]

Well if you can design a transistor circuit properly you should have no problem doing it with either FETs or BJTs.

 

[Roff]

RTL NOR gates work because, in an RTL circuit, BJTs are basically current controlled. MOSFETs are voltage controlled.
If you sum 3 identical resistors into the base of an NPN, current through any one of them can turn on the NPN.
If you sum 3 identical resistors into the gate of an NMOS, a high on only one input will not turn on the NMOS unless you use a high supply voltage, because of the typical threshold voltage of MOSFETs and the voltage divider created by the summing resistors.

See the attachment. Note that with one input switching, the NMOS circuit does not work with vcc=5V, while the NPN circuit works with either vcc.
Also note that I used devices with typical specs, not worst case. The NMOS circuit might not even work with vcc=10V if worst case Vgs(th) were used.

 

[Mike odom]

of course it would work, but it'd look like this:

 

[Roff]

of course it would work, but it'd look like this:

View attachment 69882

That's not RTL. The input resistors are redundant. It's NMOS logic.
I realize that an RTL NOR gate using multiple BJTs looks similar, but in it, the resistors are necessary.

 

[Mike odom]

ok then, this? Or are you going to claim this is DTL? Either way, it accomplishes the same thing... logic functions from discrete components...

 

[crutschow]

ok then, this? Or are you going to claim this is DTL? Either way, it accomplishes the same thing... logic functions from discrete components...

View attachment 69885

Well it looks like a duck, so...........

The OP was asking about RTL, not DTL, which your circuit plainly is.

 

[Roff]

ok then, this? Or are you going to claim this is DTL? Either way, it accomplishes the same thing... logic functions from discrete components...

View attachment 69885

It looks a little like a DTL duck, but it can't quack. The gate has no discharge path to ground (no return current paths for the diodes), and the series resistors are once again redundant.
See attachment for a circuit that will work.

 

[crutschow]

It looks a little like a DTL duck, but it can't quack. The gate has no discharge path to ground, and the series resistors are once again redundant.

Darn, I forgot to turn on my computer speakers.