In praise of the common-base: Bench quickie

Article By : Michael Dunn

The common-base configuration is so little used, it doesn’t even register on many engineers’ radar. If that describes you, refresh yourself on the "basics."

The common-base configuration is so little used, it doesn’t even register on many engineers’ radar. So, as a self-professed analog guy, I’m embarrassed to say that, while it’s on my radar, I don’t recall ever designing a C-B circuit. As part of the EDN CDEL project, a C-B stage seemed the perfect choice to provide voltage gain for the MOSFET gate driver, but I required more thought than expected to design a circuit that might actually work, and I wasn’t happy till I’d actually confirmed operation on a breadboard, and worked through some very basic math.

I find I can’t really learn and internalize a circuit until I have a need for it, and have done at least some small hands-on activity with it. Unlike basic C-E & C-C arrangements that most of us can design in our sleep, it took me a few tries to draw a workable C-B stage. Roughly speaking, I needed to convert a 0-3 V swing to a 0-12 V one. One side of my brain knew that all the output current came from the input, yet the other side kept looking at the biased E-B junction and couldn’t get past the fact that it wouldn’t just turn on the transistor fully. Eventually though, the voltages and currents fell into place. True enough though, the C-B can still saturate like a C-E when driven hard enough.

Once I realized the base had to be biased for my application, not grounded, the circuit fell into place. That was after realizing I didn’t want a PNP … unless R_o were to be connected to a negative supply!

Although I prefer to understand things visually and intuitively, not just mathematically, I worked through some simple math to confirm my thinking:

Assume I_C = I_E (it doesn’t, but close enough with a good transistor)

I_E = (V_I – V_E) / R_i  (set V_E to V_ref, or V_ref – 0.7V. Just don’t forget about your approximations.)

V_O = 12V – I_C·R_o

Since current through R_i & R_o is the same, gain = R_o/R_i

The output supply must generally be stable, as there’s no power supply rejection. A small bonus of this “coming down from 12V” behaviour for my MOSFET driver is that I can set V_O to around 3V when V_I is 0V, thus not wasting precious driving-DAC resolution in the MOSFET’s sub-threshold range.

I like how C-B is somewhat op-amp-like. Gain is R_o/R_i, and the emitter input behaves similarly to a summing node.

Here’s an exceedingly boring video demonstrating input vs. output, albeit on a classic 549 scope:

Of course, C-B can be used in ways beyond this basic voltage amp. It can make a digital level converter, and is regularly seen in RF and cascode circuits.

Have you designed or encountered interesting common-base circuits? If so, how, where, and why?

Related articles:

• Trits vs. Bits: 5-Trit ternary DAC has (almost) 8-bit resolution and high PSRR
• Off-line flyback uses HV transistor in common-base configuration
• LED-driver level-shifter includes fault detection
• Cascode MOSFET increases boost regulator’s input- and output-voltage ranges
• Choosing Transistors

—Michael Dunn is Editor in Chief at EDN with several decades of electronic design experience in various areas.

Follow Michael Dunn on Twitter and EDN on Instagram.

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