# Transistor Common Base Configuration – a Hidden Champion

The transistor common base configuration is just as simple as the other two (common collector, common emitter) configurations, but far less known and used – which is a regrettable mistake, because it is absolutely awesome and has little-known characteristics which we’ll look into right away!

## Operating principle

At first look, the common base configuration seems “impossible” (ground at the base???) and even if it worked, the benefits wouldn’t be obvious. The base configuration circuit diagram above is the most commonly shown. It suggests that some voltage will be applied to the transistor emitter which should cause a voltage change at the collector output. But since the base is grounded the transistor will never conduct and hence the collector voltage will never change. So how does this circuit work?

What most of these simple diagrams don’t tell you is that they actually assume that the “input” voltage drops below 0V so that the potential difference between base and emitter becomes positive and the transistor conducts current which causes a voltage drop at “output” (collector). Since negative voltages aren’t practical, all we need to do is “lift” the base voltage higher than the input signal.

Assuming an input signal between 0V and 1V, the simplest working configuration consists of biasing the transistor base to a voltage higher than the input signal voltage.

In the diagram above the input signal (green) is a sine wave with 0,5V amplitude and creates an almost rectangular output signal (red) between 0V and 5V with pretty steep slopes.

What is there to be learned about the common base configuration from this experiment?

• It has steep amplification characteristics
• It amplifies voltage (a lot)
• It doesn’t change the phase (unlike ie. common emitter configuration)

For an amplifier, the resistor values are rather low and the power consumption is probably high. Let’s try this again, with a few changes. We’re going to bias the base with a low-impedance voltage source (0,8V) and a larger resistor (10KΩ) at the collector. There’s also a small resistor between emitter and input signal just to limit the maximum current through the transistor.

The output signal has again a rectangular shape with even steeper slopes; so increasing the collector resistor increases amplification. The 0,8V at the base and the very low resistor at the emitter (1Ω) are worth a closer look. This configuration potentially creates a large current which could destroy the transistor, so clearly this isn’t a real-world circuit, but it hints at another characteristic of the common base configuration: is has a low input impedance. So let’s summarise what we know so far about the common base configuration:

• It has steep amplification characteristics
• It amplifies voltage (a lot)
• It doesn’t change the phase
• It has a low input impedance
• It has a high output impedance

## Applications

Wikipedia mentions three fields of application: high frequency amplification, digital circuits and low impedance amplification such as in microphones.

The common base configuration has a low input capacitance and isn’t affected much by the Miller effect which caps high frequencies. The easiest way to understand the Miller effect is to imagine the NPN (or PNP) transistor as a semiconductor sandwich stacked with three layers.

In a way, this looks like two capacitors in series with the collector, base and emitter bridged by those capacitors. While low, the transistor’s own capacitance will attenuate high frequencies, reduce amplification and increase leaking. Common base configuration doesn’t suffer from this effect so much because the base-emitter current is already high while the base-emitter voltage is low: there isn’t much potential difference left to bridge for the intrinsic capacitance.

The next application is digital circuits, if the steep amplification slopes didn’t give this application already away. The utility in discrete circuits is kind-of obvious; the clear separation between “high” and “low” voltage levels, the low power requirements for driving the output signal and the low input capacitance (see Miller effect) make the common base configuration an interesting choice for fast switching digital circuits.

The last application is circuits where a low input impedance is required. Counter-intuitively, this is a great use-case for buffer circuits. A buffer circuit is meant to feed the output signal of one circuit to the input of another circuit without drawing much power from the first circuit. A classic example is the power amplification of an LC oscillator which produces a weak sine signal. We want to grab that sine and amplify its power, eg. because we want to send it into an antenna.