# Talk:Basic Circuit Building Blocks

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Talk about basic circuits and BB

Got some ideas?

Hey, can you put the eagle cad schematic sources up? That way other folks can copy and paste for new circuits, or adapt the ones there (need a better meter symbol, for example).

This is russ_hensel who did a lot of this yet unfinished page. I will try to pull something together on the eagle files in the next few days and post it somewhere, but do not get too excited, I had to create some of the parts and I did just enough for the schematic, the internal details and footprints are typically junk.

It occurs to me that the individual circuits should be broken out into seperate pages. This would make the whole thing a lot easier to work with. Wackyvorlon 23:20, 13 April 2008 (PDT)

## high side switch

I find the high-side switch section (Basic Circuits and Circuit Building Blocks#Transistor High Side Switch) confusing. The text seems to describe a PNP high side switch (or perhaps a Pfet?), while the diagram shows a NPN high side switch.

I've seen PNPs on the high side in some circuits. I've also seen NPNs on the high side in other circuits.

Would mentioning both kinds on this page lead to unnecessary confusion? If so, which kind is the best for the purposes of this page? --DavidCary 20:06, 2 August 2008 (PDT)

Russ says: I wrote the original and did the schematic, I do not think the text indicates which type of transistor is used? But that aside, I now realize that you are right that either type of transistor can be used. With the one in the schematic you have positive logic in that a + input turns on the switch but requires a drive voltage higher than the power supply. If the reverse polarity transistor is used then a low voltage on the input turns on the switch, this does not require that a voltage higher than the main powersupply be used. With fets I think that the npn and pnp are not as symetrical in price and preformance as for bypolar transistors so the positive logic is the prefered approach with some high side switches incorporating their own charge pumps. Some of this should perhaps be noted on the page, but I would rather do it with external references as I would like to keep the circuits on the page nice and simple ( we could also have a follow up page on the wiki ) In any case I am not sure enough about all this stuff to document it without more research that I do not have time to do now ( lots of summer projects including getting ready for burning man, will any of our wiki members be there? ). The upshot is that the item could be improved, but I am not up for it now.

## High side driver -2nd that motion.

As mentioned by another contributor, the "Transistor high side switch" and "Transistor Push Pull circuit" are less than ideal. You state yourself that "these may need to be driven at a higher voltage than the supply", where this often not possible e.g. when under battery power. The PNP tranistor should be the high side driver and the NPN the low side. This way the base voltage requirements are well within the range of the supply. If this inverts the logic level then handle it in software.

Additionally a current limiting resistance in the base is needed, this value is chosen to provide enough current to saturate when the transistor is passing its maximum current. Without the resistance your poor PIC port pin is effectively shorted to ground via diode.

russ_hensel says: I think the earlier comment applies only to the "Transistor high side switch". As for the push pull amp I think it is ok as it stands for a simple circuit. I have seen it in many references. It is basically a dual emitter follower so its input impedance is the input inpedance of the load resistor times the beta which is often high enough to make base resistors unnecessary. It is hardly an ideal circuit as it would normally have cross over distortion because of the 0 bias on both transistors. However as a booster for an op amp circuit where it is inside the feedback loop it can do pretty well. I think ( but not very hard or long ) that reversing the transistor polarities would lead to a feed thru short in the output stage with both transistors conducting with heavy currents for 0 input voltage. A special driver might get over that problem. The output transistors are often darlingtons so we cannot drive to the power supply limits. A Sziklai pair might remove this problem, or higher voltage in the drive circuit could also be used. I did not think of this circuit as output for a pic, it is a bipolar device for analog circuits, although it can be used in some digital applications. To repeat the circuits here are meant to be basic enough to work but with out the bells .... for a carefully designed circuit. However the links for each circuit may link to advanced versions of same, either on the wiki ( where you can write up a version ) or off. Let me know if you think the circuit is actually in error or just simplified ( but still operational ).