Electronic I Biomedical Engineering Second Year

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Experiment No. (4)

The Diode Clipper & Clamper Circuits

Object:

To demonstrate the operation of a diode clipper and clamper circuits.

Apparatus:

1. Dual Beam Oscilloscope.

2. Function Generator

3. Bread Board

4. Silicon Diodes

5. Power Supply

6. Capacitors

7. Resistors

Theory:

(A) The Diode Clipper Circuit:

Diode clippers are wave – shaping circuits in that they are used to prevent

signal voltages from going above or below certain levels. The clipping level may be

either equal to the diode’s barrier potential or made variable with a d.c source

voltage. Because of this limiting capability, the clipper is also called a limiter. There

are two general categories of clippers: series and parallel. The series configuration is

defined as one where the diode is in series with the load, while the parallel variety

has the diode in a branch parallel to the load.

Series Clipper Circuit:

Consider the circuit of fig. (4.1 a). An a.c generator applies a sine – wave

voltage Vin to a diode in series with a resistor R. In fig. (4.1 b) the input voltage Vin

and output voltage Vout are shown in proper time phase. During the positive

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alternation the cathode of the diode is positive relative to its anode; that is the diode

is reverse – biased. Hence no current flows in the circuit, and the output Vout across

R is zero. During the negative alternation the diode is forward – biased, acting as a

closed switch and permitting current to flow in R. The voltage across R is the

negative alternation with the polarity shown. This simple circuit is a positive series

limiter, “positive” because the positive alternation has been limited or eliminated

from the output. Fig. (4.2) shows that the series – connected diode may also be used

as a negative limiter by reversing the polarity of the diode in the circuit. The

waveforms of fig. (4.2 b) show that during the positive alternation the diode is

forward – biased, permitting current to flow in R. The voltage developed across R is

positive, following the input alternation. During the negative alternation the diode is

reverse – biased and no current flows. Hence, no output voltage is developed across

R. Thus the series – connected diode in fig. (4.2) act as negative limiter.

Parallel – Clipper Circuit:

The circuits in fig. (4.3 a & c) are examples of parallel connected diode

limiters. They are called parallel limiters because the output is in parallel with the

diode. During the positive alternation, diode D1 (see fig. (4.3 a)) is reversed biased

and exhibits high reverse resistance Rp.R and Rp constitute a voltage divider. If R is

much smaller than Rp, practically the entire positive alternation appears as the

output voltage Vout across the diode. During the negative alternation D1 is forward –

biased. The diode act as closed switch with very low forward resistance Rp. Because

R is sow so very much larger than Rp, there is negligible output across the diode.

That is, the diode conducting acts like a short circuit. Hence no voltage is developed

across the diode, as in fig. (4.3 b). Since the negative alternation is removed

(limited) from the output, this circuit is an example of negative limiter. By reversing

the polarity of the diode, fig. (4.3 c) the parallel diode limiter is used to remove the

positive alternation fig.(4.3 d).

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Biased – parallel limiters: Partial limiting:

The circuit of figs. (4.4) and (4.5) accomplish partial limiting of the negative

and positive alternations, respectively of an input sinewave. Diode D1 in fig. (4.4 a)

is reverse – biased by a battery VAA which maintains the anode VAA volts negative

relative to its cathode. During the positive alternation of the input voltage Vin the

cathode of D1 is held positive. The diode acts like an open switch and the positive

alternation appears in the output, Fig. (4.4 b). During the negative alternative the

cathode is driven negative, but the diode will not conduct until VAA is more negative

than the biase voltage VAA which maintains the anode VAA volts negative. Hence,

that part of the negative alternation which is less negative than VAA appears in the

output when the negative alternation of Vin reaches the level where it is more

negative than VAA, the cathode is driven more negative than the anode and the diode

conducts, limiting that portion of the negative alternation between –VAA and –Vm

peak. Similarly the biased parallel limiter of fig. (4.5) limits that portion of the

positive alternation of VAA, which is more positive than VAA (see fig.(4.5 a) & (4.5

b))

Diode Clamper Circuits:

The diode limiter or clipper modifies the input waveform by limiting or

“clipping” part of the waveform. Other diode circuits called clampers (or dc

restorers), do not change the shape of the input waveform; rather, they add a dc level

to it. There are positive, negative and biased clampers.

Positive Clamper:

The effect of a positive clamper on an ac waveform with OV as its axis is

shown in Fig. (4.6). The clamper has added +20 V dc to the 40 Vpp waveform. The

results that the input waveform, which varied from +20 to –20 V, appears in the

output of the circuit as a signal varying between 0 and +40 V, with +20 V dc as its

axis. The output waveform acts as though a +20 V battery where connected in series

with the input.

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Fig. (4.7) is the circuit diagram of a dc clamper. The circuit operates as

follows: On the negative alternation of the 10 Vp-p input sinewave the cathode of

diode D is driven negative relative to the anode. Therefore D conducts, charging C

through the resistance of the forward – biased diode. Capacitor C will charge to the

peak of the negative alternation, 5 V, with the polarity shown in fig. (4.7). On the

positive alternation D is cut off since its cathode is positive relative to the anode.

Capacitor C tries to discharge through R when D is cut off. However, if the time

constant RC is large compared with the period of the sinewave, the capacitor will

loose very little of its charge and will hold the 5 V across it. The effect of the circuit

is illustrated in fig. (4.8). Capacitor C has been replaced by a +5 V battery which is

in series with the input signal. If the sinusoidaly varying voltage is added, point by

point, to the +5 V of the battery, the results is a waveform which varies between 0

and +10 V peak.

Negative Clamper:

It adds a negative dc level to an ac signal. This is accomplished by reversing

the polarity of the diode, as in the circuit of fig. (4.9). In this circuit, C charges on

the positive alternation of the input signal. If the input signal varies between +15

and –15, the net effect is to charge C to –15 V. The output waveform now varies

from 0 to –30 V.

Biased Clamper:

A biased negative clamper is shown in fig. (4.10). A battery, 3 V in this case,

biases the cathode at +3 V. Since D cannot conduct until its anode is positive

relative to the cathode, the diode must wait until the positive alternation of the input

has risen more than +3 V on the anode. The diode therefore conducts between the

+3 V and +10 V level of the input signal. As a result, capacitor C charges to –7 V.

After this initial charge, the action of the circuit is similar to that of fig. (4.9). The

output waveform is therefore clamped below +3 V, and varies between +3 and –7 V.

Other clamper arrangements, for example positive – biased clamper are possible.

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Procedure:

(A) Clipper Circuits:

1. Positive Clipper.

(i) Connect the circuit shown in fig. (4.11 a).

(ii) Set your oscilloscope to the following approximate setting;

Channels 1 & 2 : 1 V / division, dc coupling

Time base : 1 ms / division

(iii) Adjust the signal generators output level at 6 Vp-p at a frequency of 200 Hz.

(iv) Draw the input and the output waveforms.

2. Negative Clipper.

(i) Reverse the polarity of the diode in the circuit shown in fig. (4.11 a) as

shown in fig. (4.11 b).

(ii) Draw the input & the output waveforms.

3. Positive – biased Clipper.

(i) Connect the circuit shown in fig. (4.11 c).

(ii) Draw the input & the output waveform.

4. Negative – biased Clipper.

(i) Modify the circuit shown in fig. (4.11 c) to that shown in fig. (4.11 d).

(ii) Draw the input & the output waveforms.

(B) Clamper Circuits:

1. Positive Clamper.

(i) Connect the circuit shown in fig. (4.12 a).

(ii) Set your oscilloscope to the following approximate settings:

Channels 1 and 2 : 2 V / division, dc coupling.

Time base : 0.5 ms / division

(iii) Adjust the signal generators output level at 5 Vp-p at a frequency 1 kHz.

(iv) Draw the input & the output – waveforms.

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2. Positive – biased Clamper.

(i) Modify the positive Clamper circuit shown in Fig. (4.12 a) to a positive –

biased Clamper circuit shown in fig. (4.12 b).

(ii) Repeat steps (ii) – (iv) in part “1”.

3. Negative Clamper.

(i) Connect the negative Clamper circuit shown in fig. (4.12c)

(ii) Repeat steps (ii) – (iv) in part “1”.

4. Negative – biased clamper.

(i) Modify the negative Clamper circuit shown in fig. (4.12 c) to a negative –

biased Clamper shown in fig. (4.12 d).

(ii) Repeat steps (ii) – (iv) in part “1”.

Discussion:

1. How can you perform the same function of fig. (4.11 c) using a Zener

diode? Explain and Draw the circuit diagram.

2. Using ideal diodes, design a Clamper circuit to perform the function

indicated in fig. (4.13).

3. What happen to the output of the circuit shown in fig. (4.12 b) if the

polarity of the power supply is reversed?

4.

Vin Vout +30 V

+20 V

t Vin Vout t –10 V –20 V

Figure (4.13) A clamper circuit

?

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D1 A

B Vin

Vout + R –

+ Vin

–

+ Vin

–

+ Vin

–

Vout

–

+ Vout

+ Vout

Vout

–

+ Vin

–

Vin

Vin

Vout

Vout

Vout

+ R –

A

B

D2

D1

D2 Vin

R

R

RF

(a) (b) Figure (4.1) Series semiconductor diode to limit the positive

alternation of an ac signal.

(a) (b) Figure (4.2) Series – connected diode used to limit negative

alternation of an ac signal.

(a) (b)

(d)

Figure (4.3) Parallel – connected diodes used to limit negative &

positive alternation.

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D1

– C +

Vin

Vout – VAA

+

+ Vin

–

+20 0 –20

R

(a) (b)

Figure (4.4) Negative – biased clipper circuit.

+ Vout

–

Vout Vin VAA

D1

R

Vin

Vout

Figure (4.5) Positive – biased clipper circuit.

positive clamper Input

+40 +20 Output 0

Figure (4.6) Positive clamper add a+ dc axis to the waveform.

+5 0 –5

+10 +5 0

Input R D Output

Figure (4.7) Circuit diagram of a positive clamper.

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+5 +10 0 Input Output +5 –5 0

5 V – +

Figure (4.8) A + 5 V battery replaces the charged capacitor C, inserting a + 5 V axis into the output waveform.

+15 0 0 Input R D Output –15 –15 –30

C + –

+10 0 Input R Output –10

D

C + –

+ 3 V –

+3 V –7 V –17 V

Figure (4.9) Negative clamper addes a – dc axis to the waveform.

Figure (4.10) Biased negative clamper.

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Ch. 1 Vin

Ch. 2 Vout 15 K

6 Vp–p

200 Hz 6 Vp–p

200 Hz

Ch. 1 Vin

Ch. 2 Vout 15 K

Ch. 1 Vin

Ch. 2 Vout

15 K Ch. 1 Vin

Ch. 2 Vout 15 K

+ 6 Vp–p

200 Hz – + 1.5 V

–

+ 6 Vp–p

200 Hz – – 1.5 V

+

(a) Positive clipper circuit (b) Negative clipper circuit

(c) Positive – biased clipper (b) Negative – biased clipper circuit circuit

Figure (4.11) Schematic diagram of clipper circuits

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Ch. 1 Vin

Ch. 2 Vout

Ch. 1 Vin

Ch. 2 Vout

Ch. 1 Vin

Ch. 2 Vout

5 Vp–p

1 KHz

(a) (b) Positive clamper circuit Positive - biased clamper circuit

Figure (4.12) Schematic diagram of clamper circuits

f10

f10

f10

f10

5 Vp–p

1 KHz

5 Vp–p

1 KHz 5 Vp–p

1 KHz

Ch. 1 Vin

Ch. 2 Vout

2 V

3 V

(c) (d) Negative clamper circuit Negative - biased clamper circuit

100 100

100 100