Basic Electronics for Scientists and Engineers

180 Oscillators
The second of our voltage loop equations gives
I b = V cc − V be1
R B2
. (7.13)
So far our analysis is general. If the transistor is saturated, however, then I c =
Ic
sat ≈ V cc /R L1 (cf. Fig. 7.10). Also, to drive the transistor into saturation, the base
current should be more, for a given I c , than it would be in the linear active region.
Thus, I b > Ic
sat/β
1,whereβ 1 is the current gain factor for transistor 1. Using
Eq. (7.13) for I b and the above approximation for Ic
sat , we obtain
or, rearranging,
V cc − V be1
R B2
> V cc
β 1 R L1
(7.14)
( )
Vcc − V be1
β 1 > R B2
. (7.15)
V cc R L1
A similar analysis for the circuit driving transistor 2 yields
( )
Vcc − V be2
β 2 > R B1
. (7.16)
V cc R L2
Equations (7.11), (7.15), and(7.16), then, place important operating restrictions
on our circuit component values in addition to those (Eqs. (7.8) and (7.9)) thatset
the timing.
7.2.3 The 555 timer
The 555 timer is another “black box” device that can be used for a number of
purposes. It comes in an 8-pin package similar to the 741 op-amp. A functional
schematic of this device including pin assignments is shown in Fig. 7.11. Thisis
sufficient for us to understand how the device functions. We have three resistors that
form a flexible voltage divider, two comparators (basically, op-amps), a transistor,
and a new device called a flip-flop. The flip-flop has two outputs, labeled Q and ¯Q,
which have opposite states: when Q is high, ¯Q is low, and vice versa. The output
Q is available at pin 3, while ¯Q is fed to the base of the transistor, thus turning
the transistor on or off. The flip-flop also has two inputs, called set and reset. A
high input into set causes Q to go high (i.e., to be set), while a low input into reset
causes Q to go low (i.e., to be reset). The voltage into set comes from the output of
comparator 1 which compares the voltage on pin 2 with an internal voltage derived