Emitter Follower Applications

The emitter follower has many applications for impedance matching and buffering other active circuit modules.

Buffer for Voltage Regulator

The impedance matching and current amplifying characteristics of the emitter follower provide buffering for the zener and draw less current from it. It is a useful adjunct to a Zener regulator. The circuit on the right adds a capacitor to reduce ripple. 发射极跟随器的阻抗匹配和电流放大特性为其提供了缓冲作用，从而减轻了对齐波二极管的电流抽取。它是一个有用的辅助电路，用于Zener调节器。右边的电路增加了一个电容器以减少纹波。

Emitter-Follower Buffer

An emitter follower can serve as a buffer for a voltage source. The voltage divider at left is a poor voltage source because it is so strongly affected by the value of the load resistor. The same voltage divider with the transistor buffer at right will supply power to keep the voltage constant over its range of operation.

The simple voltage divider will give a 4V output for an open circuit, but quickly drops when larger loads (smaller load resistors) are connected. The buffer helps to keep the output voltage more nearly constant. You can change the value of the load resistor below to explore the effect of the buffer.

A 0.6 V diode drop was assumed for the transistor. The input impedance to the transistor buffer is assumed to be so high as to present essentially an open circuit.

NPN Darlington Configuration

This emitter follower has a pair of transistors in the Darlington configuration. In this arrangement, the emitter current of one transistor becomes the base current of the second. The Darlington configuration acts like one transistor with a beta which is the product of the betas of the two transistors. They are used where high output currents are needed. The input impedance of the Darlington configuration is quite high. 这种射极跟随器采用达林顿配置，其中第一个晶体管的射极电流成为第二个晶体管的基极电流。达林顿配置相当于一个具有β值为两个晶体管β值乘积的晶体管。它们用于需要高输出电流的场合。达林顿配置的输入阻抗相当高。 Switching of the second transistor may be slow, so a resistor is commonly tied between the emitters to increase the speed switching. Darlington pairs are available as single packages, usually with the resistor included. 第二个晶体管的开关可能较慢，因此通常会在发射极之间接一个电阻以提高开关速度。达林顿对通常以单个封装形式提供，通常包含该电阻。