Biasing That Transistor: The Common Base Amplifier


We’ve previously remarked upon a generation lucky enough to be well-versed in microcontrollers and computersised electronics through being brought up on the Arduino or the Raspberry Pi but unlucky enough to have missed out on basic electronics such as how to bias a transistor, and to address that gap we’ve taken a look at the basics of transistor biasing.

All the circuits we worked with in the previous article had the transistor’s emitter taken to ground, took their input from the base, and their output from the collector. This configuration, called a Common Emitter amplifier is probably the most common, but it is far from the only way to use a transistor. Once you have set up the bias voltage as we described to the point at which the transistor is in its linear region, there are several other ways in which the device can be used as an amplifier. The subject of this article is one of these configurations, so described because it takes the transistor’s base to the ground instead of the emitter, as a Common Base amplifier.

The common base amplifier circuit
The common base amplifier circuit

Take a moment to consider the most basic common base amplifier. The transistor’s base is held at a constant bias potential, its emitter forms the amplifier’s input, and the collector its output. The DC biasing arrangement is exactly the same as the common emitter amplifier we have previously described, in that the base is held sufficiently higher than the emitter that the transistor is conducting in its linear region, and there is an emitter resistor to limit the current through the device.  The emitter circuit has a small load in it across which the input is placed, and the output comes from the collector as with he common emitter circuit.

When the voltage at the emitter is varied by changing the amplifier input, it in turn varies the voltage between base and emitter. As the voltage between base and emitter varies, so does the base current, which in turn causes a much larger variation in  the collector current due to the transistor’s gain. This causes a correspondingly larger voltage variation across the collector resistor which is available at the output as an amplified version of the smaller signal on the emitter.

There are two consequences of this mode of operation that give the common base amplifier significantly different properties to its common emitter siblings. It is a voltage amplifier rather than a current amplifier, it is non-inverting, and it has an extremely low impedance at its input.

When we examined the common emitter circuit we took a look at the fundamental of how a transistor works, that the current flowing through its collector is a multiple of that through its base, that multiple being referred to as its gain. Thus on changing the base current we generate a correspondingly larger change in the collector current from which we can derive an amplified voltage across a collector resistor, but we will always require our input to be a source of current rather than of voltage. By comparison with the common-base circuit the base current comes from the bias network rather than the input, so the input can be much more like a voltage source than a current source, and thus the common base circuit is a voltage amplifier.

The common emitter amplifier that we saw last time is an inverting amplifier, in that as an increasing current on the base causes more collector current to flow it pulls the collector voltage down. Thus a voltage rise on the input causes a corresponding voltage fall on the output, and a cyclical waveform such as a sine wave is inverted by its passage through it. The common base amplifier, on the other hand, reacts to an increased voltage on the collector, its input, by drawing less current through it, and the voltage left over on the output increases.

Finally, the input impedance of a common base amplifier is significantly lower than its common-emitter equivalent. The collector circuit of a transistor amplifier can either be a very low resistance or a reactive circuit with a zero resistance DC path. By comparison the impedance of the base circuit on the common emitter amplifier with its resistor network is much higher. In most cases the high impedance of a common emitter amplifier input is a huge advantage over the low impedance of the common base configuration because it does not load the circuit from which it derives its input, but in cases where an impedance match is required with a low impedance source, the common base amplifier’s low impedance becomes an advantage. Thus you will find common base amplifiers used in small-signal RF amplifiers where a match with a low-impedance feeder, and in amplifiers for low-impedance audio sources such as moving-coil microphones. The antenna preamplifier in an analog TV tuner was frequently a common-base design for this reason.

You may never build a common-base amplifier unless you work with RF, but it remains a useful configuration to know about and a handy weapon in your arsenal of circuits for when you need a voltage amplifier with a low impedance input. We will continue this series by looking at the final configuration for a transistor amplifier, the common collector amplifier or emitter follower, before examining the equivalent circuits using field effect transistor, or FET.



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