Clippers and Clampers

Clippers and Clampers
Clippers
Clipping circuits (also known as limiters, amplitude selectors, or slicers), are used to
remove the part of a signal that is above or below some defined reference level.
We’ve already seen an example of a clipper in the half-wave rectifier – which circuit
basically cut off everything at the reference level of zero and let only the positive-
going (or negative-going) portion of the input waveform through.
To clip to a reference level other than zero, a dc source is put in series with the diode.
Depending on the direction of the diode and the polarity of the battery, the circuit
will either clip the input waveform above or below the reference level (the battery
voltage for an ideal diode; i.e., for Von=0). This process is illustrated in the four parts
of fiure:1.
# Without the battery, the output of the circuit below would be the negative portion
of the input wave (assuming the bottom node is grounded). When vi > 0, the diode
is on (short-circuited), vi is dropped across R and vo=0. When vi <0, the diode is off
(open-circuited), the voltage across R is zero and vo=vi.
Figure: 1
# With the battery in the orientation shown in Figure:2, the diode doesn’t turn on
until vi > VB (If this looks strange, revisit the definition of forward bias). This shifts
the reference level up and clips the input at +VB and passes everything for vi < VB.
1
Figure: 2
Figure: 3 has the battery with the same orientation as in figure:2, but the diode has
been flipped. Without the battery, the positive portion of the input waveform
would be passed (i.e., a reference level of zero). With the battery, the diode
conducts for vi < VB. This means that the reference level is shifted to +VB and only vi
> VB appears at the output.
Figure: 3
Again refereeing figure:2, the diode is in the original position but the polarities
on the battery have been switched in Figure:4. The discussion follows the same
logic as earlier, but now the reference level has been shifted to –VB. The final
result is that vo = vi for vi < -VB.
2
Figure: 4
Finally, circuit of figure : 5 behaves the same as in figure: 3, but the polarity on the
battery has been switched, shifting the reference level to -VB. The signal that
appears as the output is vi as long as vi > - VB.
Figure: 5
A parallel-biased clipper is a circuit that clips the positive and negative- going
portions of the input signal simultaneously. This is designed by using two parallel
diodes oriented in opposite directions – note that it is very important that the
diodes are oppositely oriented. Just as in our previous discussion, the path
containing diode D1 will provide the upper limit with reference level VB1+VON (with
the VB1 polarity shown) and the path containing D2 will provide the lower limit with
reference level VB2+VON (with the VB2 polarity shown). An example of this type of
clipper, with the resulting output waveform is shown below Figure:6
3
Figure: 5
Clampers
Clamping circuits, also known as dc restorers or clamped capacitors, shift an input

signal by an amount defined by an independent voltage source. While clippers limit
the part of the input signal that reaches the output according to some reference
level(s), the entire input reaches the output in a clamping circuit – it is just shifted
so that the maximum (or minimum) value of the input is “clamped” to the
independent source.
Let’s look at Figure 3.48 (input and circuit shown to the right) and see if we can turn
this into something that makes sense! Basically, we have a sinusoidal input of
magnitude Vm with zero offset (i.e., symmetric signal) fed to the clamper circuit.
Taking the input by sections to build the output…
4
➢ If vin > VB, the diode is on, R is shorted, and the output is VB.
➢ When vin < VB, the diode is off, current flows through the resistor, and the
capacitor charges to a voltage vc=VB – vin. The maximum voltage on the capacitor
will be related to the maximum swing of the input by:
VC Vm VB . After fully charging, the capacitor acts like series source in
the circuit (with the RC time constant conditions discussed below). After steady
state is reached, the output voltage is found by loop analysis as
vo= vin + VC.
In the circuit shown above,

VC = - Vm + VB
(KVL at maximum negative input, vin=-Vm) or vo = vin – Vm+ VB. Specifically, for the
extreme values of the input signal:
if vin=+Vm, vo=VB and if vin=-Vm, vo=- Vm – Vm + VB = -Vm – (Vm – VB) = -Vm + VC = VC -
Vm
Keep in mind that the above analysis was for the diode orientation and VB polarity
shown. If the diode had been flipped, the minimum rather than the maximum of the
input would have been clamped to VB.
A clamping circuit has to have an independent source, a diode, a resistor and a

capacitor. To keep a constant voltage on the capacitor over the period of the input,
the RC time constant must be large. A design rule of thumb is to make the RC time
constant at least five times the half-period of the input signal, which results in
approximately an 18% error over a half-period due to capacitor discharge. If this
error is too large, the RC time constant may be increased but, as with everything in
design, there comes a point where factors such as size and power dissipation may
make any further improvements impractical.
5
Parts (b) and (c) of this figure, shown below, demonstrate the output of the clamper with
a long time constant and the distortion introduced by the capacitor discharging for a short
time constant.

Clippers and Clampers

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Clippers and Clampers

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Clippers and Clampers

Clamping circuits, also known as dc restorers or clamped capacitors, shift an input

vo= vin + VC.

In the circuit shown above,

A clamping circuit has to have an independent source, a diode, a resistor and a

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