CGS39 quad logic gate

[CJ Miller]

I was just mulling over some CMOS logic/divider ideas and thinking about buffering, followers, conditioning, etc. I thought I should check on my CGS39 boards I haven't used yet to see how those are done.

Looking over the board and schematic I was surprised because it calls for TL074 op-amps instead of the LM324 single-supply types I was sure it needed. Turns out it was the older version which used 324s.

Can anybody explain to me what the benefit is of using the dual-supply amps for this? I might still use 324s like I intended but there must be some advantage to going the dual-rail route, otherwise Ken would probably not have changed the design. Any ideas?

[Mongo1]

Looking over the board and schematic I was surprised because it calls for TL074 op-amps instead of the LM324 single-supply types I was sure it needed. Turns out it was the older version which used 324s.
Can anybody explain to me what the benefit is of using the dual-supply amps for this?

Hi CJ,

I'm no expert, but the first thing that occurs to me is a range improvement. The 324s would probably not be happy if you stuck a -5V signal in the input, while the 074s would be fine. That means you could use something like the +/- 5V output from a VCO as an input with the new circuitry.

When you get down to it, I'm not even sure what happens when you put 0 volts into the 324s input. Op amps typically have some trouble operating at the supply rails, so there might well have been some stability or oscillation issues doing that.

I don't have the old schematic in front of me, but it's possible Ken addressed that issue by biasing the input to half of the V+ supply. The problem with doing that is that you can't run as large a signal into the input.

It looks like the new design with the 074s allows bipolar operation all the way up to the final stage. The output transistor clips off any negative levels. If we use 15V, and allow 1.2V drop across the transistor, the output at the junction of the 1.8K and 1K resistors would be 5V when the transistor is on, and 0V when it's off. Sweet!

It looks like a definite improvement in the design.

Hope that helps
Gary

[Mongo1]

As a follow up - I was just looking at the old 324 based design, and it looks like Ken had handled the negative input issue by using a series diode on the input.

In any case, the newer design is definitely simpler.

[cthulu]

The new schematic (cgs39 ver 2.0) seem to have an error in it. All IC's are powered between +15 and -15 Volt (30V in all). This wouldn't work as the 4k series chips are rated to more like 15V. On the PCB the Logic circuit is correctly tied to +15 and ground. The output from the comparators (op's) are then lifted up to 0 - 15V operation through the 2x100k resistor net at their outputs. I don't think the op amp is critical at all in this circuit.

[Mongo1]

Thanks Cthulu - that's interesting info.
So based on that point, the only real difference is that we can get rid of the diodes on the inputs.

Gary

[Mongo1]

I was just looking out on the web for issues related to the 324. I kept thinking I'd heard someone say some bad things about that chip in the past. I found this posted on a message board:

>Has anyone found this problem,
>
>When a negative voltage is connected to the input of this op amp the output
>will go positive, and the other outputs will go slightly positive.
>
>Do you know what causes this?
>
>Thanks
>
>

If you go below the negative supply, even a few tenths of a volt, a
324 (or similar-process parts, like LM339) will go crazy internally;
charge is being sprayed everywhere. There's a tiny fine-print footnote
on the original National datasheet, and some hints in the application
examples near the end.

Also: If one section of a 324 swings to the output rails, it can/will
also mess up the other amps in the same can. It's a nasty part,
overall.


So - maybe this was just a generally wise move. I know the 324 is very old design - I was using them back in 1977 or so. Maybe it's just a chip whose time has come and gone in general.

Gary

[CJ Miller]

Ok, thanks for the input.