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
junction capacitance (C
C-B
) between the collector and the base of the phototransistor is great (up to 20 pF) and as a result its
response speed (turn-off time t
off
) is low. The relationship between turn-off time t
off
and collector-base capacitance C
C-B
is
expressed by:
t
off
×
C
C-B
x h
FE
x R
L
................(1)
where
t
off
: Turn-off time (See Fig. 2-2.)
C
C-B
: Collector-base capacitance
h
FE
: D.C. current amplification factor
R
L
: Load resistance
Cc-
B
R
L
Figure 2-1. Collector-Base Capacitance
C
C-B
of Phototransistor
3
NEC [ NEC ]
