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.

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. 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.