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The common-base circuit

Schematic of the common-base circuit with NPN- and PNP-BJT

The pictures show the schematics of the common-base circuit with NPN transistor and with PNP transistor.

Fields of application for the common-base circuit

The common-base circuit is a basic transistor amplifier with voltage gain. It has a low input impedance and a medium output impedance, which is significantly determined by the collector resistor R2. The current gain is slightly lower than one.

Because of its low input impedance, the common-base circuit is eligible as a current output follower circuit, i.e. a current voltage converter (I-U-converter). In such an application, the output voltage is almost the input current multiplied by the collector resistor R2.

How to design a transistor amplifier in common-base configuration

Transistor amplifiers in common-base configuration can be designed with TransistorAmp 2 software. Start TransistorAmp 2 and click the Common-Base Circuit button:

design a new amp in common-base configuration

Now, the dialog Common-base circuit appears. Here you must input all parameters of your amplifier circuit:

specify a new amp in common-base configuration

To select the transistor type for your circuit, you must click the Select transistor type from list button and choose the transistor type within the next dialog:

select a transistor type for the amplifier in common-base configuration

TransistorAmp 2 contains almost every usual type of bipolar transistor (thousands of types). So, you will certainly find there the transistor type you need. After having chosen a transistor type, you must click on OK and you will get back to the previous dialog (where you specify your amplifier). Now, that dialog contains the transistor type which you have selected and you can click on OK. Then, the dialog closes and you will see immediately the design result of your new transistor amplifier:

design result of your new transistor amplifier in common-base configuration

Here, you can see your input parameters, the circuit and the values of all components. Furthermore, you can see the most important parameters of the operation point, i.e. collector current, collector emitter voltage and power consumption.

If you want to change any parameter of the circuit, click the Create New button and then again the Common-Base Circuit button, and you will see that the input dialog has remembered all of your inputs, so it is easy for you to make changes.

If you want to save your result, click the Save Result button, and input a file name and save the file.

Here, you can see the result file from our example: Amplifier in common-base configuration

If you want to simulate your circuit, click the Simulate button.

The first time you do this, you must input some settings:

The simulation (AC sweep) in LTspice shows the output signal (input amplitude: 1 mV):

LTspice simulation of a common-base circuit