Signal source for BJTs - Page 1
Forum section: electronics / beginners

author: bitman
author_rank: Unknown
timestamp: May 18, 2020, 03:48:48 am
content: When-ever I read about BJTs and how to use them as a switch, every circuit seems to have an incoming signal to the base and a constant voltage/current on emitter/collector. I've created several circuits like the one enclosed where I simply use the BJT (pnp) as an electronic switch and the signal entering the emitter is typically a clock or other varying voltages - frequencies in 100k->1mhz. I'm aware of the parasitic capacity in BJTs but to my knowledge that's present on the base->emitter/collector too - which means it will limit the frequency supported regardless of how I connect things.Question: What issues are there connecting a signal like this diagram shows? It seems to work - at least on a bread-board and small simulations. I just wonder why examples always show the signal/frequency coming in on the base?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 18, 2020, 04:40:44 am
content: Base input, common emitter has the highest voltageandpower gain. Its inverting gain is also useful in specific instances. That's probably good reason to focus on it.Emitter input, base to a fixed voltage ("common"), is not as often seen in switching circuits. The emitter must sink the full collector current, so you haven't really solved anything, and the collector output is noninverting so you haven't even begun to implement a logic function. (Presumably you could gang some together to make AND or OR gates, but these are not universal!)Similarly, the power gain is lower. The signal source must deliver Pin ~ ΔVbe*Ic; this will be much less than Pout ~ ΔVce*Ic for a typical load, but hardly zero. Whereas ΔVbe*Ib can be approx. hFE times smaller, on top of the advantage in voltage gain.It is relatively common in RF circuits, where the base acts as a shield between input and output -- Cce is much less than Ccb or Cbe are, and this allows more gain-bandwidth without running into stability problems. At high enough frequencies, the marginal drop in power gain may not be a concern anyway -- operation might be at such frequencies where the gain is already fairly low (due to capacitances and other limitations); you might as well go for the configuration which gives better performance in other respects.Note that I'm giving reasons drawn from analog amplification. This is for good reason -- a transistor "switch" is only a "switch" in as much as we call it that. It still necessarily goes through its linear operating range, even if only briefly; and in doing so, it takes some amount of voltage and current or charge at its input (the ratio of which gives an impedance, roughly speaking, and the product of which gives power or energy), and that causes its output to traverse its range (which has similar units, and the same meaningful quotient and product). We might not be talking about small-signal gain and impedance, but rather the average taken across the end points; we can be much more sloppy with them, since we're going to be passing around min/max margins, but the units themselves don't magically change, we can still use them just the same.The distinction between analog and digital, is hierarchical: digital is a subset of analog, in which we can make certain useful assumptions, and use them to quickly solve different kinds of problems (i.e., combinatorial and sequential logic). This is only possible as far as our base assumptions remain true; and when we forget them, nature has a way of gently (or not so much) reminding us of this, from time to time.Point being -- this circuit more easily violates those assumptions, so it also makes a poorer example of a "switching" circuit, and hence why you might not see it so often in textbooks say. So, basically coming around to my second paragraph, but with more background; and again, it's perhaps a less universal starting point anyway, so most digital logic isn't very concerned about it.So, that said -- you will perhaps be pleased to know that such circuitsdoshow up from time to time, in professional digital circuitry. Example:https://www.nxp.com/docs/en/application-note/AN10441.pdfHere (Fig. 1), MOSFETs are used instead of BJTs, because the gate terminal draws no current; no resistor divider is needed for the base. It could be implemented just as well with a circuit like yours, it just takes extra parts.Note that the body diodes (shown redundantly in the symbols, to make extra sure you're reminded of their presence!) allow current to flow from the left side to the right, when its voltage becomes higher. The supplies are shown 3.3V on the left and 5V on the right, for precisely this reason. If the supplies were reversed, D and S would have to be swapped. Such a circuit does not work as a general level translator (any voltage on left, any voltage on right), but when you have a fixed configuration, that's perfectly acceptable!Tim

author: bitman
author_rank: Unknown
timestamp: May 18, 2020, 05:42:14 am
content: Quote from: T3sl4co1l on May 18, 2020, 04:40:44 amThanks for a long a thorough explanation. If I was looking for a gain - a traditional transistor function I fully understand that what I presented here makes no sense. In my case all I want to ensure is that a given signal is controlled if power is provided or not. You're right that there is a bit of a voltage loss from e->c which is pretty much why I've been breadboarding to ensure "it would work". This is for TTL so I have a rather wide range where operations will still work.So I want to control if the signal is passed thru - not amplify it. And I needed it to be negated - ie. when there is power, the signal is blocked - hence a simple PNP. Is there a better/simpler way to do this? I was looking for a function that would work like a mechanical relay just using simple non-mechanical components. At least with a relay i wouldn't have a voltage drop or a parasitic capacitor - but it would be rather large.Of course I could probably have used a simple NOT->AND gate - that would definitely result in the required voltage o the out-pin. So I think you hit it on the head - I was thinking very analog here. Perhaps I need to reconsider.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 18, 2020, 07:23:51 am
content: Well, a diode or even a resistor might meet your description -- but that all depends on the voltage ranges corresponding to each state! I take it TTL is the desired output level and load, but the inputs?Tim

author: magic
author_rank: Super Contributor
timestamp: May 18, 2020, 08:40:40 am
content: I suppose it will work.The 100kΩ resistor limits possible base current of the PNP, so the load will not be able to draw more than a few mA from CLK - this may be a good thing or a bad thing, depending on what you want.When base goes high and CLK is high, the PNP may take a fraction of µs to turn off due to charge storage.Base voltage needs to be roughly equal or higher than CLK voltage for reliable turn-off.Base voltage shouldn't be more than 5~6V to prevent breakdown of the base-emitter junction when CLK is low.Some of those limits could be increased by using a P channel MOSFET instead.

author: bitman
author_rank: Unknown
timestamp: May 18, 2020, 04:27:48 pm
content: Quote from: T3sl4co1l on May 18, 2020, 07:23:51 amThe load goes to a simple 74LS chip - part of SPI data. I've seen good results with just 4.3 peak voltage (for high) so far. As to the use of a diode, if I'm lucky a simple diode I've added will prevent the 74LS chips to be powered on, so instead of using this simple "switch" a simple diode ensures I don't need to even be concerned - but typically I'm not that lucky. Even when the chips are powered off they'll sink current or do other "weird" things interfering with the transmission happening on the same line. Hence this "backup" method.I don't think a diode can do it - maybe a Triac but I have none of those fellas lying around. I need to block conditionally - which is why I think you were spot on that I was thinking analog instead of digital when I came up with this design. I now realize a simple NAND chip would have been able to do this for me, and not having caused me the headaches I've had trying to understand enough of BJTs to make it work. I didn't even consider MOSFETs for this - I presume because I don't have any in my spare parts drawers.

author: bitman
author_rank: Unknown
timestamp: May 18, 2020, 04:40:57 pm
content: Quote from: magic on May 18, 2020, 08:40:40 amI'm able to power a 10mA LED so I have enough for my purpose. Having a low current was one of the things I struggled with initially. I'm more concerned that I don't get the full 5v this way, since I can always change the resistors if the current isn't large enough.Your second point was why I thought I should post this. At the frequency generated the parasitic capacitor is very dominant on the scope. I may end up simply slowing down the clock if it becomes a problem - for now I found a better PNP than the 2N3906 jelly bean I began with and the numbers look a lot better now. My question is that I still see similar slopes using the base in a traditional manner, so regardless it seems to be a problem with "high" frequencies (< 1MHz not exactly high).