Жалпы коллекторлық күшейткіш: Вольт түрлендіргіш ретінде қолданылуы

Type of transistor amplifier

In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit the base terminal of the transistor serves as the input, the emitter is the output, and the collector is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name. The analogous field effect transistor circuit is the common drain amplifier and the analogous tube circuit is the cathode follower.

The common collector amplifier's low output impedance allows a source with a large output impedance to drive a small load impedance without changing its voltage. Thus this circuit finds applications as a voltage buffer. In other words, the circuit has current gain (which depends largely on the hFE of the transistor) instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load. One aspect of buffer action is transformation of impedances. For example, the Thévenin resistance of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower (a small resistance). That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a small load resistance and a driving stage presents a large load to the driving stage—an advantage in coupling a voltage signal to a small load. This configuration is commonly used in the output stages of class B and class AB amplifiers. The base circuit is modified to operate the transistor in class B or AB mode. In class A mode, sometimes an active current source is used instead of RE (Fig. 4) to improve linearity and/or efficiency.