BJT and CMOS as switch

2. BJT
Bipolar junction transistor
Type of transistor that relies on the
contact of two types of semiconductors
for its operation.
Their operation involve both electrons
and holes.
BJT can be used as amplifiers , switches
,temperature sensors or in oscillators.
BJT can be found either as individual
discrete component or in large numbers
as parts of integrated circuits.

3. SCHEMATIC SYMBOLS FOR
BJT

4. BJT AS AN ELECTRONIC
SWITCH
As a switch , we use the cutoff and saturation regions of
BJT operation.
 Cutoff Region: Cutoff is easy to design. If v1<=0.5or so,
the EBJ will conduct negligible current. Also, the CBJ
will be reversed biased with a large Vcc.
Consequently,
Vo= Vcc
These are the cutoff conditions and the BJT is in the “off”
state.
 Saturation Region :Saturation is a bit more difficult to
design .We need V1 sufficiently large so that the
collector current becomes large enough for the CBJ to
become forward biased. For the “on” state of the switch,
we increase V1until the BJT saturates. This occurs
when the EBJ and the CBJ are both forward biased.

5. BJT AS AN ELECTRONIC
SWITCH
 When the input equals –Vin, the base-emitter
junction is reverse biased or off so no current
flows in the collector.
 When the BJT is in the cutoff, the circuit (ideally)
has the following values:
Vce=Vcc and Ic=0 A
This state is similar to an open switch.
 When the input equals +Vin, the transistor is
driven into saturation and the following condition
occur:
Vce~0V and Icsat=Vcc/Rc
This state is similar to a closed switch
connecting the bottom of Rc to ground.

6. The Characteristics for a BJT
switch assumes that:
1) -Vin is low enough to drive the transistor
into cutoff.
2) +Vin must produce enough base current
through Rb to drive the transistor into
saturation.
3) The transistor is an ideal component.
These conditions can be assured by
designing the circuit so that:
1) -Vin=Vbe
2) +Vin=Vbe+IbRb(Vcc is a good
maximum)

7. Condition 1: guarantees that the circuit
is driven into the cutoff region by the
input.
Condition 2 and 3: assures that the
transistor will be driven into the
saturation region.

8. CMOS AS A
SWITCH

9. INTRODUCTION
 Complementary metal–oxide–
semiconductor (CMOS) is a
technology for constructing
integrating circuits.
 It is used
in microcontrollers, static RAM,
microprocessors and other digital
logic circuits.

10.  CMOS technology is also used for
several analog circuits such as
image sensors, data converters
and highly
integrated transceivers for many
types of communication.
 Frank wanlass patented CMOS in
1963 .

11.  The words "complementary-
symmetry" refer to the fact that
the typical digital design style with
CMOS uses complementary and
symmetrical pairs of p-type and n-
type metal oxide semiconductor
field effect
transistors (MOSFETs) for logic
functions.[2]

12. CMOS AS A SWITCH

13. INVERSION
 CMOS circuits are constructed in such
a way that all PMOS transistors must
have either an input from the voltage
source or from another PMOS
transistor.
 Similarly, all NMOS transistors must
have either an input from ground or
from another NMOS transistor