LM393 Comparator input bias

Can anyone explain the biasing of the LM393 input (pin 5) in the circuit below.

formatting link

The above reference forms part of a discussion available at

formatting link


Gerhard van den Berg CSIR

Reply to
Loading thread data ...


formatting link

Assuming internal workings as indicated in the schematic in the current data sheet available on the National Semiconductor web site: If the (+) input is unbiased, that side of the input diff pair won't conduct. That doesn't keep the other side from conducting, though. Don't expect fast response--there's nothing to suck base charge out of Q7 when the (-) input side turns off.

In general, it's bad practice to rely on characteristics that aren't guaranteed--or even implied by the actual specifications. (The simplified schematic in the data sheet should not be taken as a specification.) Seems to me you can get the same general performance while using the comparator in a more conventional biasing circuit, using the same number of parts (or likely even fewer parts), and the performance will be more stable over things like die and temperature changes.

Cheers, Tom

Reply to
Tom Bruhns

The comparator has pnp inputs requiring bias current from the negative rail. As a result, the non-inverting input of the second stage must eventually float high, preventing the output indication.

Quite lame, with undetermined response to initial power-on or short-term power removal.

It could only have been designed by accident, so you should probably consider the entire schematic to be suspect.


Reply to

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.