![Positive Voltage Clamping V2](https://upload.wikimedia.org/wikipedia/commons/0/0f/Positive_Voltage_Clamping_V2.svg)
A clamper (or clamping circuit or clamp) is an
electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electrical ...
that fixes either the positive or the negative peak excursions of a signal to a defined voltage by adding a variable positive or negative DC voltage to it. The clamper does not restrict the peak-to-peak excursion of the signal (clipping); it moves the whole signal up or down so as to place its peaks at the reference level.
A diode clamp (a simple, common type) consists of a
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
, which conducts electric current in only one direction and prevents the signal exceeding the reference value; and a
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
, which provides a DC offset from the stored charge. The capacitor forms a
time constant In physics and engineering, the time constant, usually denoted by the Greek letter (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system.Concretely, a first-order LTI system is a sy ...
with a
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
load, which determines the range of frequencies over which the clamper will be effective.
General function
A clamper will bind the upper or lower extreme of a waveform to a fixed DC voltage level. These circuits are also known as DC voltage restorers. Clampers can be constructed in both positive and negative polarities. When unbiased, clamping circuits will fix the voltage lower limit (or upper limit, in the case of negative clampers) to 0 volts. These circuits clamp a peak of a waveform to a specific DC level compared with a capacitively coupled signal, which swings about its average DC level.
The clamping network is one that will "clamp" a signal to a different DC level. The network must have a capacitor, a diode, and optionally a resistive element and/or load, but it can also employ an independent DC supply to introduce an additional shift. The magnitude of R and C must be chosen such that the time constant RC is large enough to ensure that the voltage across the capacitor does not discharge significantly during the interval the diode is nonconducting.
Types
Clamp circuits are categorised by their operation: negative or positive, and biased or unbiased. A positive clamp circuit (negative peak clamper) outputs a purely positive waveform from an input signal; it offsets the input signal so that all of the waveform is greater than 0 V. A negative clamp is the opposite of this—this clamp outputs a purely negative waveform from an input signal. A bias voltage between the diode and ground offsets the output voltage by that amount.
For example, an input signal of peak value 5 V (V
INpeak = 5 V) is applied to a positive clamp with a bias of 3 V (V
BIAS = 3 V), the peak output voltage will be:
:V
OUTpeak = 2 × V
INpeak + V
BIAS
:V
OUTpeak = 2 × 5 V + 3 V
:V
OUTpeak = 13 V
(The peak to peak excursion remains at 10V.)
Positive unbiased
![Positive Voltage Clamping Circuit (simulated)](https://upload.wikimedia.org/wikipedia/commons/c/cc/Positive_Voltage_Clamping_Circuit_%28simulated%29.svg)
In the negative cycle of the input AC signal, the diode is forward biased and conducts, charging the capacitor to the peak negative value of V
IN. During the positive cycle, the diode is reverse biased and thus does not conduct. The output voltage is therefore equal to the voltage stored in the capacitor plus the input voltage,
so V
OUT = V
IN + V
INpeak. This is also called a
Villard circuit.
Negative unbiased
![Negative Voltage Clamping Circuit (simulated)](https://upload.wikimedia.org/wikipedia/commons/7/71/Negative_Voltage_Clamping_Circuit_%28simulated%29.svg)
A negative unbiased clamp is the opposite of the equivalent positive clamp. In the positive cycle of the input AC signal, the diode is forward biased and conducts, charging the capacitor to the peak positive value of V
IN. During the negative cycle, the diode is reverse biased and thus does not conduct. The output voltage is therefore equal to the voltage stored in the capacitor plus the input voltage again, so V
OUT = V
IN − V
INpeak.
Positive biased
![Positive Biased Voltage Clamping Circuit (simulated)](https://upload.wikimedia.org/wikipedia/commons/a/a9/Positive_Biased_Voltage_Clamping_Circuit_%28simulated%29.svg)
A positive biased voltage clamp is identical to an equivalent unbiased clamp but with the output voltage offset by the bias amount V
BIAS. Thus, V
OUT = V
IN + (V
INpeak + V
BIAS).
Negative biased
![Negative Biased Voltage Clamping Circuit (simulated)](https://upload.wikimedia.org/wikipedia/commons/9/91/Negative_Biased_Voltage_Clamping_Circuit_%28simulated%29.svg)
A negative biased voltage clamp is likewise identical to an equivalent unbiased clamp but with the output voltage offset in the negative direction by the bias amount V
BIAS. Thus, V
OUT = V
IN − (V
INpeak + V
BIAS).
Op-amp circuit
![Precision op-amp clamp](https://upload.wikimedia.org/wikipedia/commons/e/ef/Precision_op-amp_clamp.svg)
The figure shows an
op-amp
An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
-based clamp circuit with a non-zero reference clamping voltage. The advantage here is that the clamping level is at precisely the reference voltage. There is no need to take into account the forward voltage drop of the diode (which is necessary in the preceding simple circuits as this adds to the reference voltage). The effect of the diode voltage drop on the circuit output will be divided down by the gain of the amplifier, resulting in an insignificant error. The circuit also has a great improvement in linearity at small input signals in comparison to the simple diode circuit and is largely unaffected by changes in the load.
Clamping for input protection
Clamping can be used to adapt an input signal to a device that cannot make use of or may be damaged by the signal range of the original input.
Principles of operation
During the first negative phase of the AC input voltage, the capacitor in a positive clamper circuit charges rapidly. As ''V''
in becomes positive, the capacitor serves as a voltage doubler; since it has stored the equivalent of ''V''
in during the negative cycle, it provides nearly that voltage during the positive cycle. This essentially doubles the voltage seen by the load. As ''V''
in becomes negative, the capacitor acts as a battery of the same voltage of ''V''
in. The voltage source and the capacitor counteract each other, resulting in a net voltage of zero as seen by the load.
Loading
For passive type clampers with a capacitor, followed by a diode in parallel with the load, the load can significantly affect performance. The magnitude of ''R'' and ''C'' are chosen so that the time constant,
, is large enough to ensure that the voltage across the capacitor does not discharge significantly during the diode's non-conducting interval. A load resistance that is too low (heavy load) will partially discharge the capacitor and cause the waveform peaks to drift off the intended clamp voltage. This effect is greatest at low frequencies. At a higher frequency, there is less time between cycles for the capacitor to discharge.
The capacitor cannot be made arbitrarily large to overcome load discharge. During the conducting interval, the capacitor must be recharged. The time taken to do this is governed by a different time constant, this time set by the capacitance and the internal impedance of the driving circuit. Since the peak voltage is reached in one quarter cycle and then starts to fall again, the capacitor must be recharged in a quarter cycle. This requirement calls for a low value of capacitance.
The two conflicting requirements for capacitance value may be irreconcilable in applications with a high driving impedance and low load impedance. In such cases, an active circuit must be used such as the op-amp circuit described above.
Biased versus non-biased
By using a voltage source and resistor, the clamper can be biased to bind the output voltage to a different value. The voltage supplied to the potentiometer will be equal to the offset from zero (assuming an ideal diode) in the case of either a positive or negative clamper (the clamper type will determine the direction of the offset). If a negative voltage is supplied to either positive or negative, the waveform will cross the x-axis and be bound to a value of this magnitude on the opposite side.
Zener diode
A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached.
Zener diodes are manufactured with a great varie ...
s can also be used in place of a voltage source and potentiometer, hence setting the offset at the Zener voltage.
Examples
Clamping circuits were common in
analog television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, instantaneous phase and frequency, ...
receivers. These sets have a DC restorer circuit, which returns the voltage of the video signal during the "back porch" of the line blanking (retrace) period to 0 V. Low-frequency interference, especially power line hum, induced onto the signal spoils the rendering of the image and, in extreme, cases causes the set to lose
synchronization
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
. This interference can be effectively removed via this method.
See also
*
Clipper (electronics)
In electronics, a clipper is a circuit designed to prevent a signal from exceeding a predetermined reference voltage level. A clipper does not distort the remaining part of the applied waveform. Clipping circuits are used to select, for purpose ...
, a circuit that imposes a fixed limit and does not offset the signal
*
Envelope detector
An envelope detector (sometimes called a peak detector) is an electronic circuit that takes a (relatively) high-frequency amplitude modulated signal as input and provides an output, which is the demodulated ''envelope'' of the original signal.
...
, a circuit that outputs the maximum (or minimum); a clamper with the diode and capacitor exchanged
*
Schottky diode
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage ...
*
Snubber
A snubber is a device used to suppress ("snub") a phenomenon such as voltage transients in electrical systems, pressure transients in fluid systems (caused by for example water hammer) or excess force or rapid movement in mechanical systems.
Ele ...
, a circuit that reduces dV/dt or limits peak voltage, in order to reduce arcing or breakdown
References
Further reading
*
*
* {{cite book , author=J. A. Coekin , title=High-Speed Pulse Techniques , year=1975 , publisher=Pergamon , isbn=978-1-4831-0548-2 , pages=60–64
Electronic circuits