Play Meter

circuit, another MOSFET in a constant current ource configura-
+Voo
tion can replace the drain resistor as used in the two input NOR
gate shown in Fig. 14-4. If T 1 or T 3 is turned on, the source of
T 1 is returned to ground which allows a constant drain current
through T 1 and T 3 • Since this current will be the value of 1 0
(on) for each device, the need for a large drain resistor is no
longer necessary. With the aid of T 1 , it is possible to simulate
large values of resistances with a surface area no larger than one
MOSFET. Fig. 14-5 shows another version of the MOSFET RS
f1i~flop
with constant current drain loads. With this scheme, it is
possible to build several hundred of these devices into one
Fig. 14-5. Constant Current Source Version of the RS Flip-Flop.
integrated circuit.
Lesson Fifteen: Schmitt Triggers
Lesson Fifteen - Schmitt Triggers. This lesson includes the
design of a Schmitt Trigger Circuit using a JFET as the input
device.
SCHMITT TRIGGERS:
Another bistable circuit which is somewhat similar to the
f1i~flop is the Schmitt Trigger . This circuit is often used as a
voltage sensing device and FETs are very useful when a high
input impedance is required. Fig. 15-1 shows a JFET and an NP
bipolar transistor connected in a positive feedback loop to form a
Schmitt Trigger.
Any positive increase in V 8 will not affect the circuit since T 1 is
already on . However, as V 8 decreases enough to allow T 1 to start
turning off, T 1 begins to turn on and Vs begins to increase which
helps to tum T 1 off even more. This source voltage regeneration
causes a "snap" effect, such that as T 1 turns on, it aids T 1 in
v.
T,
----
+Voo
~ hystr~s i:s
T,
,.---~r--Q
Ro
<~If
hand __.,
u,
v.ron I
+Voo
Rc
v,
Ys ,
Y.,
Fig. 15-2. Output Voltage vs. Gate Voltage Showing Hystresis
Band of the Schmitt Trigger.
v,
turning off, which again aids T 1 to turn on. The condition for T 1
to start turning off is that
V 8
Fig. 15-1. JFET- NPN Schmitt Trigger.
:::;
0,
(15-1)
where
1
0
Voo
(on)= _ _ _
;;..::;. _ _ _
Ro + R2 + rd ,(on)
R1
Ro + R1 + rds (on)
· V 0
0 - -
V
S 1
( I 5-4)
On ce T 1 is on, the input voltage must increase to
When the input voltage V 1 is at a value such that V 8 ,
T 1 will be on and T 1 will be off as long as
lo (on)· rd• (on)< Vbe + R1 le
<
-
( 15-2)
V
>
R1
8 - Rc+RI +R2
. Voo = Vs2
(I 5-5)
which causes T 1 to tum on and T 1 to turn off. These two voltage
levels create a hy tresis band which is con trollable by R 1 . Fig.
15-2 illustrates this band in a graphical manner. One of the less
desirable features of this circuit is that when T 1 is on , the output
voltage will be somewhat above ground. This output voltage is
given by
and
(15-6)
Voo
(15-3)
which is also determined by R 1
.
PLAY METER. October 1. 1963