The Imperfect Bipolar Transistor

We like to pretend that our circuit elements are perfect because, honestly, it makes life easier and it often doesn’t matter much in practice. For a normal design, the fact that a foot of wire has a tiny bit of resistance or that our capacitor value might be off by 10% doesn’t make much difference. One place that we really bury our heads in the sand, though, is when we use bipolar transistors as switches. A perfect switch would have 0 volts across it when it is actuated. A real switch won’t quite get there, but it will be doggone close. But a bipolar transistor in saturation won’t be really all the way on. [The Offset Volt] looks at how a bipolar transistor switches and why the voltage across it at saturation is a few tenths of a volt. You can see the video below.

To understand it, you’ll need a little bit of math and some understanding of the construction of transistors. The idea of using a transistor as a switch is that the transistor is saturated — that is, increasing base current doesn’t make much change in the collector current. While it isn’t perfect, it is good enough to switch a relay or do other common switching tasks.

Then again, you can do better. FETs can work much better and while there was a time when you still had to use a bipolar device for its current handling, low cost, or due to the switching frequency, FETs now can handle nearly any job a bipolar transistor can.

Still, good to understand the theory behind these devices and they still have their place, too. After all, there are quite a few options when you need a swtich.

15 thoughts on “The Imperfect Bipolar Transistor

  1. MOSFETs of course have their own rules. In the ideal world they just work, but in the real world you need to worry about having enough gate voltage to turn them on and if you are switching them on and off you need to deal with the gate capacitance.

    1. There are a few cases in wich you will be better using bipolar transistors rather than FETs. Most notable case I found is very low noise signal chain amplification in audio applications. Bipolar transistors have much lower 1/f noise.

      1. The worst part though are the manufacturing variations. Come on, you can’t give me 3-5V range for Vth.
        They work great as switches, but while analog design with them is possible, you have to measure and rebias
        the transistors every time one dies.

    2. The Vth of a MOSFET has such a wide range in the datasheet than the usual 0.6-0.7V of a BJT. When it comes to making mickey mouse discrete logic or working with low voltages, I wonder if there is a device with the drive requirement of a BJT and the on/off characteristics of a MOSFET.

        1. Bipolar transistor saturates and has a constant Vce voltage independent of current so the wasted power is Vce*Ic plus the base power Vbe*Ic/beta.
          Mosfets work differently, and have contact resistance Rdson, so the power is just I*I/Resin. Therefore, for large currents the bipolar transistor is more efficient, which is why IGBTs are used in high power situations.. they behave like bipolar without the base current term

          1. odd that you’d mention constant Vce because one of the few things i know about BJTs is that’s not true :)

            i wanted to find out if i’m high so i tested a 2N4401 NPN. assuming I didn’t foul this up..


            Ic Vce
            102mA 76mV
            18mA 22mV
            5mA 11mV
            0.3mA 7mV

            even today, i am pleasantly surprised how low Vce is on this cheap transistor!

            anyways i’m not saying you’re wrong. obviously, you’re right — when it really matters, the BJT’s Vce hits a plateau. i’m just pointing out what i love about BJTs: i’m pretty novice and yet i have some passing familiarity with the default transistor that i have a big pile of on my workbench. the few times i’ve used an FET i wound up getting small counts of relatively expensive items on the understanding that i’d want a different transistor for a different project. i’ve never just had a big bag of “generic nFET” to thoughtlessly throw in any project. they aren’t like capacitors where i bought a big assortment of hundreds of them just to round out the collection.

            of course when i am playing with power supplies these BJTs don’t suit and i still have to order some..

          2. Greg: have a try at reversing the BJT and then measuring the Vec.

            It still works, but because it’s not made symmetrical, the current gain in reverse is basically about 1-2 or thereabouts. However, the saturation voltage should be extremely low, which is sometimes used in applications where you want precise voltage levels.

          3. @Greg A
            You aren’t comparing the same thing. Chances are your projects with MOSFET have wider ranges of voltage/current than the ones with generic BJT.

            My BJT collection has a big hole above 1A and/or 80V while it is 200V and something silly like 100+A range (not at the same part) for my MOSFET.

            I was contemplating on a power project that needs parts outside of the ranges in my collection. I was surprised to find $1 IGBT on aliexpress. They seem fast enough (compared to BJT) vs what they were. I guess all those inverter microwave oven would need something to drive them.

  2. In my company, we are still in love with BJTs, as they allow you to build robust fail-safe circuits, that are not not nearly as affected by proton and heavy ion radiation in low earth orbit as FETs and CMOS.

  3. In some ways, a BJT is more perfect than a MOSFET. MOSFETs have quite variable VTH, and don’t follow the ideal square-law characteristic particularly closely [ ID = K.(VGS-VTH)^2 ]. BJTs, nearly no matter how they are constructed, follow their ideal law very closely over 6+ decades of current: IC = K.exp(VBE/kT). This predicability makes them great for precision circuits including bandgap-based references.

    MOSFETs are good at zero: zero gate current and 0 offset on VDS.

  4. It was even worse for the GEO and deep space satellites. There were no class S approved FET devices until fairly recently. BJT were a challenge, all kinds of leakage and degradation even at 100Krad. But we had those all figured out. They were still struggling to get a logic level FET approved when I left.

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