ELE 124 - Electronics (1)
Lecture 9
Diode Limiters (clippers) and Diode Clampers
Dr. Said Emam
Diodes Applications
• OR Gate & AND Gate
• Half-Wave Rectifiers
• Full-Wave Rectifiers
• The Basic DC Power Supply
• Diode Limiters (clippers)
• Diode Clampers
Diode Limiters (Clippers)
A clipper is a circuit that removes either positive or negative parts of a waveform.
The Positive Clipper: removes the positive
parts of the input signal.
When 𝑣𝑖𝑛 ≥ 0.7V
When 𝑣𝑖𝑛 ≤ 0.7 V
D is ON and 𝒗𝒐𝒖𝒕 = 𝟎. 𝟕V
D is OFF and 𝒗𝒐𝒖𝒕 = 𝒗𝒊𝒏
In case of a load resistance :
During the OFF time
The Negative Clipper: removes the negative
parts of the input signal.
When 𝑣𝑖𝑛 ≤ −0.7V D is ON and 𝑣𝑜𝑢𝑡 = −0.7V
When 𝑣𝑖𝑛 ≥ −0.7V D is OFF and 𝑣𝑜𝑢𝑡 = 𝑣𝑖𝑛
EXAMPLE 1 What would you expect to see displayed on an oscilloscope
connected across RL in the limiter shown in Figure?
When 𝑣𝑖𝑛 ≤ −0.7V D is ON and 𝑣𝑜𝑢𝑡 = −0.7 V
When 𝑣𝑖𝑛 ≥ −0.7 D is OFF and
𝑅𝐿
𝑣𝑜𝑢𝑡 = 𝑣𝑖𝑛
𝑅𝐿 + 𝑅1
100
𝑣𝑃(𝑜𝑢𝑡) = 𝑣𝑃(𝑖𝑛)
100+10
= 9.1 V
The scope will display an output waveform as
shown in Figure
Biased Limiters
By adding a dc voltage source in series with the diode, we can change the clipping level.
(a) Biased positive clipper.
(b) Biased negative clipper.
Transfer characteristics (𝒗𝒐 𝒗𝒔. 𝒗𝒊𝒏 )
EXAMPLE 2 Figure shows a circuit combining a positive limiter with a negative
limiter. Determine the output voltage waveform.
• When 𝑣𝑖𝑛 reaches +5.7 𝑉, diode D1 conducts
and limits the waveform to +5.7 𝑉.
• D2 does not conduct until the voltage reaches
− 5.7𝑉 .
• Therefore, positive voltages above +5.7 V and
negative voltages below −5.7𝑉 are clipped
off.
• The resulting output voltage waveform is
shown in Figure.
Determining the transition level for the circuit
Determining the transition level for the circuit
Determining v o for the diode
in the “on” state.
Using the transition voltage to
define the “on” and “off” regions.
Example 4 Find the output voltage for the applied square wave signal.
A Limiter Application
1- Many circuits have certain restrictions on the input level to avoid damaging
the circuit.
For example, In a circuit like this, normal
operation means that the input signal is always
smaller than 0.7 V in either polarity.
2- signal shaping
When the input voltage is very
large compared to the bias levels,
the output signal is a square
wave.
Diode Clampers
• A clamper is a circuit that shifts a waveform to a different dc level without
changing the appearance of the applied signal.
• i.e. A clamper adds a dc level to an ac voltage.
Assuming ideal diode
• When 𝒗𝑰 < 𝟎 D is ON and C charges to 𝒗𝑪 = 𝟔𝑽 & 𝒗𝑶 = 𝟎𝑽
• When 𝒗𝑰 goes +ve D goes OFF and 𝒗𝑶 = 𝒗𝑰 + 𝒗𝑪 = 𝟏𝟎𝑽
• The output waveform will be identical to that of the input, except that it is shifted
upward by 𝒗𝑪 volts.
Positive clamper :
• When 𝒗𝑰 goes negative, the diode is F.B.,
allowing C to charge to 𝑽𝒑(𝒊𝒏) − 𝟎. 𝟕 𝑽
• Just after the negative peak, the diode is R.B.
• If RC >> T ➔ 𝒗𝑪 can be considered constant
i.e. The capacitor voltage acts as a battery in
series with the input voltage, and therefore :
𝒗𝑶 = 𝒗𝑰 + 𝑉𝐶
Negative clamper :
𝒗𝑶 = 𝒗𝑰 − 𝑉𝐶
EXAMPLE 5 What is the output voltage that you would expect to observe across RL in the
clamping circuit of Figure shown? Assume that RC is large enough to prevent significant capacitor
discharge.
Solution
• During the 1st quarter, the diode is F.B., allowing C to
charge to 𝑽𝒑(𝒊𝒏) − 𝟎. 𝟕 𝑽
𝑉𝐶 = 24 − 0.7 = 23.3V
• Just after the positive peak, the diode is R.B. and hence
𝒗𝑶 = 𝒗𝑰 − 𝑉𝐶 = 𝒗𝑰 − 23.3
The output waveform will be identical to that of the
input, except that it is shifted downward by 𝟐𝟑. 𝟑𝐕, as
shown in figure.
𝑽avg ≅ −23.3V
EXAMPLE 6 Determine 𝑣𝑜 for the network of Fig. shown for the input
indicated.
EXAMPLES
Homework
•
• Draw the transfer characteristics of each circuit.
1)
2)
3)
4) Describe the output waveform for the circuit of Fig. shown for the input
indicated.