1

Diode Clipper Circuits These circuits clip off portions of signal voltages above

or below certain limits, i.e. the circuits limit the range of the output signal.

A clipper can process any type of signal.

2

There can be single clippers : – Positive A clipper which removes a portion of

positive half cycle of the input signal is called positive clipper.

Negative A clipper circuit that removes the negative half cycle is called negative clipper.

Two categories : Series – Diode and load in series. Parallel – Diode and load in parallel.

3

Series negative clipper

4

The circuit The transfer

characteristics

Processing by series negative clipper

5

Triangular wave

Square wave

This clipper is like a half-wave rectifier.

Draw the output.

Draw the output.

What happens if the diode in a half-wave rectifier is reversed ?

6

Ans. It still remains a half-wave rectifier, but it

now works as a positive clipper.

Input Output ?

Diode Clipper Circuits When the diode is off the output of these circuits

resembles a voltage divider

7

i

SL

Lo v

RR

Rv

Diode Clipper Circuits If RS << RL

The level at which the signal is clipped can be adjusted by adding a d.c. bias voltage in series with the diode.

8

v0 vi

Diode Clipper Circuits

9

10

Parallel Clipper ( Shunt) The diode is put in parallel with the load.

This too is the familiar half-wave rectifier circuit.

Input wave. The circuit. Output wave.

11

Practical Aspects of Parallel Clipper. If we take into account the threshold voltage,

VT = 0.7 V, the clipping level is not zero, but 0.7 V.

12

Simple Parallel Positive Clipper

The Circuit.The Input. The Output.

Draw the output.

Biased Clippers

13

Bias means applying a dc voltage to change the dc level of a circuit.

If bias voltage is placed in series with diode then the circuit is called

biased clipper or limiter circuit.

This bias determines the point where the diode begins to conduct and

duration of conduction. With bias clipping can be done to any percent of the

input signal ranging from 1% to 99%.

Guidelines to Solve

Determine the transition level at which the diode turns ON.

With diode ON, find relation between vo and vi.

Draw the transfer characteristic of the clipper.

Plot the waveshape of vo for given input.

14

We find that

Diode is ON for vi > VB.

Therefore, vi(tr) = VB.

When diode is ON, vo = vi – VB.

When diode is OFF, vo = 0.

Plot the transfer characteristic of the clipper.

15

16

Characteristic

(Graph between input and output)

Draw the output wave corresponding to a

sinusoidal input with Vm > VB

Input/Output Characteristics

Note the clipping of the output voltage.

17

18

Now, draw the

output.

Example 1 Determine the output waveform for the clipper circuit,

if the input is a sinusoidal wave of peak value 15 V.

19

Solution

20

(tr) (tr)

0V and 0A; so that 0

Writing KVL, 3 0 3V

d d o d L

i i

v i v i R

v v

•The direction of the diode suggests that it will

be ON for positive values of vi.

•At transition level,

21

3o iv v After diode is ON,

22

Transfer

characteristic

Draw the output.

Example 2 Determine the output waveform for the clipper

circuit of Example 1, if its input is as follows

23

Solution

24

Problem is simpler. Only two levels :

vi = +15 V and vi = -5 V

25

Now, you can draw the output.

26

Note that

1. Total swing of vi is 15 – (-5) = 20 V.

2. Total swing of vo is 18 – 0 = 18 V.

3. Clipper circuit clipped off 2 V, and raised the dc

level by 3 V.

27

Example 3 Determine the output of the parallel biased clipper for

the given input.

28

Solution

29

Now, draw the output

wave, and get credit.

Now, draw

the transfer

characteristic

30

Example 4 Repeat Example 3, taking a silicon diode with VT = 0.7 V,

instead of an ideal diode.

Solution :

To determine transition level, we use the

condition, 0A at 0.7Vd di v

0VR d SV i R

31

Applying KVL,

(tr) 0i T BV V V

(tr) 4 0.7i B TV V V 3.3V

For inputs less than 3.3 V (including negative values),

the diode is ON, and vo = 3.3 V

For inputs greater than 3.3 V, the diode is OFF, and

vo = vi

as shown in figure.

32

Note that VT reduces Vi(tr) to 3.3 V from 4 V.

Limiting Circuit Using a Diode: Positive Cycle Clipping

As was studied in the past, the combination of resistor-diode creates limiting effect. 33

Limiting Circuit Using a Diode:Negative Cycle Clipping

34

Limiting Circuit Using a Diode:Positive and Negative Cycle Clipping

35

36

Design Problem You are given

PN-Junction diodes (assumed ideal),

Resistors,

Two batteries of 1.5 V each, and

A sinusoidal voltage source.

Design a circuit that gives an output as

37

Solution

• D1 clips off positive parts above the positive

bias level.

• D2 clips off below negative level.

• This circuit is called a combination clipper.

38

Applications of Combination Clippers

If the input voltage is very large compared to the bias level, the output signal is a SQUARE WAVE.

Thus, the circuit can be used for wave-shaping.

It can also be used in a completely different way, as a limiter used to protect a sensitive circuit (e.g., OP-AMP, Galvanometer).

The diodes conduct only when something abnormal happens.

Sensitive

Circuit