Electronics engineering

1. Department of Electrical and Electronic
Engineering
Khulna University of Engineering & Technology
Khulna-9203
Course code : EE 2113
Digital Electronics
Presented By
Amit Kumer Podder
Assistant Professor
Switching Characteristics of Diode
And Transistor
1

2. Switching Circuit
10/5/2020 Amit Kumer Podder 2
A circuit which can turn ON or OFF current in an electrical circuit is known as
a switching circuit.
A switching circuit essentially consists of two parts viz.
(i)a switch and
(ii)associated circuitry.
What is Switching Circuit?
A switch is a device that can turn ON or OFF current in an electrical circuit. It is
the most important part of a switching circuit.
What is Switch?
The switches can be broadly classified into the following three types
(i) Mechanical switch
(ii) Electro-mechanical switch or Relay
(iii) Electronic switch

3. Switch
10/5/2020 Amit Kumer Podder 3
Mechanical Switch
Limitation
A switch which is operated mechanically to turn ON or OFF current in
an electrical circuit is known as a mechanical switch.
The familiar example of a mechanical switch is the
tumbler switch used in homes to turn ON or OFF
power supply to various appliances such as fans,
heaters, bulbs etc.
i. In the closed position, the switch carries the whole of the load current. For a large
load current, the switch contacts have to be made heavy to enable them to carry
the necessary current without overheating. This increases the size of the switch.
ii. If the load current carried by the circuit is large, there will be sparking at the
contacts of the switch during breaking operation. This results in the wear and tear
of the contacts.
iii. Due to high inertia of a mechanical switch, the speed of operation is very small

4. Switch
10/5/2020 Amit Kumer Podder 4
Electro-mechanical Switch or Relay
It is a mechanical switch which is operated electrically to turn ON or OFF current in an
electrical circuit.
When the switch S is closed, the lever is pulled
downward and the armature A closes the relay
contacts 1 and 2.
This turns ON current in the circuit.
When switch S is opened, the solenoid is de-energised
and the spring pulls the lever and hence the armature
A upwards.
Consequently, the relay contacts 1 and 2 are opened
and current flow in the circuit is interrupted.

5. 10/5/2020 Amit Kumer Podder 5
Electro-mechanical Switch or Relay
(i) The relay or electro-mechanical switch requires a small power for its operation.
(ii) The switch in the relay coil carries a small current as compared to the load current. This
permits the use of a smaller switch in the relay coil circuit.
(iii)The operator can turn ON or OFF power to a load even from a distance. This is a very
important advantage when high voltages are to be handled.
(iv) There is no danger of sparking as the turning ON or OFF is carried by the relay coil switch
which carries a small current.
Advantages
Limitations
 The speed of operation is very small; less than 5 operations per second.
 A relay has moving parts and hence there is considerable wear and tear.

6. 10/5/2020 Amit Kumer Podder 6
Electronic Switch
It is a device which can turn ON or OFF current in an electrical
circuit with the help of electronic devices e.g., transistors or tubes.
Electronic switches have become very popular because of
their high speed of operation and absence of sparking.

7. 10/5/2020 Amit Kumer Podder 7
Electronic Switch
(i) It has no moving parts and hence there is little wear and tear. Therefore, it
gives noiseless operation.
(ii) It has smaller size and weight.
(iii)It gives trouble free service because of solid state.
(iv) It is cheaper than other switches and requires little maintenance.
(v) It has a very fast speed of operation say up to 𝟏𝟎𝟗operations per second.
On the other hand, the mechanical switches have a small speed of operation
e. g. less than 5 operations in a second.
Advantages

8. 10/5/2020 Amit Kumer Podder 8
Study Area
We will learn mainly how
Diode and Transistor
acts as an switch

9. Diode as a Switching Device
10/5/2020 Amit Kumer Podder 9
 Whenever a specified voltage is exceeded, the diode resistance gets
increased, making the diode reverse biased and it acts as an open switch.
 Whenever the voltage applied is below the reference voltage, the diode
resistance gets decreased, making the diode forward biased, and it acts as a
closed switch.
The following circuit explains the diode acting as a switch.

10. Diode as a Switching Device
10/5/2020 Amit Kumer Podder 10
P-type
Material
N-type
Material
Anode cathode
𝑝𝑝𝑜
𝑛𝑝𝑜
𝑝𝑛𝑜
𝑛𝑛𝑜
P-type N-type
𝑥 = 0
The majority carriers in P-type (holes) =
The majority carriers in N-type (electrons) =
The minority carriers in P-type (electrons) = 𝑛𝑝𝑜
The minority carriers in N-type (holes) = 𝑝𝑛𝑜
𝑝𝑝𝑜
𝑛𝑛𝑜

11. Diode as a Switching Device
10/5/2020 Amit Kumer Podder 11
𝑝𝑝𝑜
𝑛𝑝𝑜
𝑝𝑛𝑜
𝑛𝑛𝑜
P-type N-type
𝑥 = 0
P-type
Material
N-type
Material
During Forward biased Condition − The
minority carriers are more near junction and
less far away from the junction.

12. Diode as a Switching Device
10/5/2020 Amit Kumer Podder 12
𝑝𝑝𝑜
𝑛𝑝𝑜
𝑝𝑛𝑜
𝑛𝑛𝑜
P-type N-type
𝑥 = 0
P-type
Material
N-type
Material
During reverse bias condition − Majority
carriers doesn’t conduct the current through
the junction and hence don’t participate in
current condition.
The minority carriers will cross the junction
and conduct the current, which is called
as Reverse Saturation Current.

13. Diode as a Switching Device
10/5/2020 Amit Kumer Podder 13

14. Diode as a Switching Device
10/5/2020 Amit Kumer Podder 14
 Storage time − The time period for which the diode remains in the
conduction state even in the reverse biased state, is called as Storage
time.
 Transition time − The time elapsed in returning back to the state of
non-conduction, i.e. steady state reverse bias, is called Transition
time.
 Reverse recovery time − The time required for the diode to change
from forward bias to reverse bias is called as Reverse recovery time.
 Forward recovery time − The time required for the diode to change
from reverse bias to forward bias is called as Forward recovery time.

15. Transistor as a Switching Device
OFF Condition
When the base input voltage is enough negative, the transistor
is cut off and no current flows in collector load.
As a result, there is no voltage drop across RC and the output
voltage is *ideally. i.e.,
𝐼𝑐= 0 and 𝑉𝑐𝐸 = 𝑉
𝑐𝑐
10/5/2020 Amit Kumer Podder 15

16. Transistor as a Switching Device
ON Condition
When the input base voltage is positive enough that transistor
saturates, then 𝐼𝑐(𝑠𝑎𝑡) will flow through 𝑅𝑐 .
Under such conditions, the entire 𝑉
𝑐𝑐 will drop across collector
load 𝑅𝑐 and output voltage is ideally zero i.e.,
𝐼𝑐 = 𝐼𝑐(𝑠𝑎𝑡) =
𝑉𝑐𝑐
𝑅𝑐
𝑉𝐶𝐸 = 0
10/5/2020 Amit Kumer Podder 16

17. Thank
You
10/5/2020 Amit Kumer Podder 17