3

There are two kinds of optocouplers (a light emitting diode (LED) as an input and a phototransistor as an output) according to the

type of output transistor: Single transistor type and Darlington-transistor type.

The single-transistor type optocouplers are used to perform high-speed switching (with high-speed response). The Darlington-

transistor type optocouplers are used to obtain a large output current by utilizing a small input current (independently of switching

speeds).

Designing the circuits properly will improve the PS2601 optocoupler (Single Transistor type) by having a base pin in terms of

switching speed, elimination of noise in input signals, and output leakage current (collector dark current, and application to high-

voltage circuits).

APPLICATIONS OF OPTOCOUPLER BASE PINS

INCREASING SWITCHING SPEED

The switching speed of an optocoupler with a base pin can be increased by inserting a resistor between the base and the emitter

of its phototransistor even when the optocoupler is applied to a large load resistance.

Generally, the phototransistor of an optocoupler such as the PS2601 has a large photo-sensitive area on it. Accordingly, the

expressed by:

where

: Turn-off time (See Fig. 2-2.)

: Load resistance

Figure 2-1. Collector-Base Capacitance