Clipping and clamping diode circuits

 

Introduction

 

 

CLAMPERS

Certain applications in electronics require that the upper or lower extremity of a wave be fixed at a specific value. In such applications, a CLAMPING (or CLAMPER) circuit is used. A clamping circuit clamps or restrains either the upper or lower extremity of a waveform to a fixed dc potential. This circuit is also known as a DIRECT-CURRENT RESTORER or a BASE-LINE STABILIZER. Such circuits are used in test equipment, radar systems, electronic countermeasure systems, and sonar systems. Depending upon the equipment, you could find negative or positive clampers with or without bias. Figure 4-16, views (A), illustrates some examples of waveforms created by clampers. However, before we discuss clampers, we will review some relevant points about series RC circuits.

 

 

 

 

 

 

 

 

Clippers

Power amplifiers, when driven out of their linear  range of operation, sound particularly bad, and can produce damage to themselves or the transducers to which they are connected. The design of traditional protection circuits is complicated by the various performance, cost, and sonic tradeoffs involved. There is certainly no one right answer to the limiter puzzle. The

circuits presented here, however, are designed to maintain a high level of sonic integrity, while remaining cost-effective.

These circuits combine active limiting with a diode- based clipper to provide excellent driver protection while avoiding the sonic degradation of simpler designs. An innovative nonlinear capacitor circuit further improves the sonic performance of the limiter.

 

Elements of Clipping & Clamping Circuits

 

 

a) Limiters(clippers):

– Diodes can be used to clip off portions of signal voltages (above or below

certain levels).

 

 

– Diode will become forward biased as soon as VA becomes larger than

VBIAS+0.7.

– When diode is forward biased, VA cannot become larger than VBIAS + 0.7 V!

– Thus, the voltage across the load, RL, will also be equal to VBIAS + 0.7.

– When diode is reverse biased, it appears as an open, so the output voltage is

the voltage of RL alone.

 - Desired voltage levels can be attained with a voltage divider.

 

 

 

– We replace the voltage source with a resistive voltage divider.

VBIAS = R3/(R2 + R3) VSUPPLY

 

b) Diode Clampers

– A clamper adds a dc level to an ac voltage.

– Also called dc restorers.

– When input voltage goes initially negative, diode is forward biased.

– Capacitor charges to near peak of inpt (Vp(in) – 0.7).

– Right after the negative peak, diode is reverse biased (because cathode is held

near Vp(in) – 0.7 by charge on capacitor).

– Capacitor can only discharge through the RL.

– Since RL has high resistance, the capacitor discharges very little each period.

– Note that time constant should be large (at least 10 times the period of the

input voltage).

– Since capacitor retains charge, it acts like a battery in series with the input

voltage.

 

 

 

 

Kinds of Clipping & Clamping Circuits

 

Schottky Diodes

 

General Description

A concern  in many  electronic  systems  is  the presence  of  impulses  of  noise  or  .voltage spikes.  on  data  or  signal  lines.    These undesired transient signals can originate from within  or  outside  of  a  given  system,  and  if severe  enough,  can  cause  permanent damage  to  system  components  such  as microprocessors.    Long  transmission  lines connecting  different  parts  of  a  system  are particularly  susceptible  to  noise  pickup  and are often the major culprits in noise and spike problems.

Clipping  and  Clamping  is  the  deliberate limiting of a voltage at a circuit node or supply bus.    Generally,  in  the  context  of  using

Schottky  diodes  for  such  purposes,  clipping is  the  limiting  of  a  circuit  node  voltage  by using  the  forward  threshold  voltage  of  a diode.    Clamping  is the limiting of  node voltage  in  two  directions  via  use  of  an  anti- parallel  pair  of  clipping  diodes.    In  other words,  clipping  limits  node  voltage  below  a certain  threshold;  clamping limits node voltage  within  a  certain  .window.  set  by  an anti-parallel  diode  pair.  The  absolute  limiting voltage  level(s)  can  be  adjusted  by  biasing the diode(s) with an offset voltage via use of pull-up  or  pull-down  circuits.    Please  refer  to

Figure 1.   Note that if the input signal voltage rises above  the supply  voltage  due  to  .noise spikes., the upper diode will turn on, dumping current  into  the  VS  node,  limiting  the  voltage at Node .A. to  1 diode drop above the supply voltage.    Similarly,  if  a  negative-going transient on the data stream dips more than 1 diode drop below ground, the lower diode will begin to conduct, limiting Node .A. voltage to one  diode  drop  below  ground.    In  summary, the .window. of voltages allowed at Node A is limited to a maximum of  VS+VD,  and  a minimum of  .VD  (one  diode  drop  below

ground).    This  assumes  that  the  diodes behave as perfect switches.  In reality, non- ideal  diode  properties,  including  the  current- handling capability of the diode used,  play a major  role  in  how well such clipping and clamping  circuits  function  in  actual  practice. Note Figure 1.  Diagram of a Clamping circuit using two Schottky Diodes.

Desireable  Characteristics  of  Clipping  /

Clamping  Diodes,  or  .Why  Use  Schottky

Diodes?.

In  looking  at  the  circuit  shown  in  Figure  1,

one  can  envision  several  desired  properties

for the diodes used:

1)  Low forward voltage

2)  Fast switching

3)  High current-handling capability

 

 

Use of the Clipping & Clamping  Circuits

 

 

Basic Feedback Limiter with Diode Clipper

The circuit shown in Figure 1 demonstrates the basic feedback limiter with adjustable clipper. The input signal is fed to the limiter circuitry at the node labeled “Input”. The limiter’s output is sent to the power amplifier from the point labeled “To Power Amplifier”. In addition, the output from the power amplifier is fed back to the limiter circuit by way of the node marked “From Power Amplifier Output”. Under normal operation, the input signal is below the limiter’s threshold and so the VCA is at unity gain, its lowest distortion region. For peak output levels of short duration which exceed the predetermined clip level, the clipper circuit “hard limits” the output to this level, performing very much like the (adjustable) diode clipper that it is. If the output level remains above threshold for long, the signal’s rms value will exceed the limiter’s average power threshold, causing the limiter to quickly reduce the level of signal being fed to the amplifier. In

this way, inaudible (but potentially damaging) peaks of short duration will be clipped, while longer duration peaks will be handled by the limiter, and little audible impairment should occur.

 

 

 

 

 

The Clipper

Figure 2 shows the clipper circuit used in this design. A trans-impedance amplifier, OA3, converts the output current from the VCA to a voltage which drives the actual clipper circuitry. When OA3’s output voltage exceeds the threshold set by VR1, the transistor pair Q1 and Q2 combine to bypass R2 and clip the output to a fixed level. Using our design example, the peak allowable power is specified as 220 VRMS, and since we are ultimately clipping the signal to a square wave, this is equivalent to 220 Vpeak. Given the power amplifier’s gain of 40, the limiter must clip at 5.5 Vpeak. The two 1N4148 diodes prevent base-emitter breakdown in Q1 and Q2. The addition of these diodes means that the clipping voltage will be two diode drops (approximately 1.2 V) greater than the voltage at the bases of Q1 and Q2. VR1 adjusts the voltage at the base of Q1 between 0 and -7.5 V, and at the base of Q2 between 0 and +7.5 V. Since we want the limiter to clip at 5.5 V, VR1 should be adjusted to provide -4.3 V and 4.3 V at the bases of Q1and Q2, respectively.

 

 

 

The Limiter

To form the limiter block, the VCA in Figure 1 is configured as a high-compression-ratio feedback compressor. Under normal operation, the amplifier output is below the compressor’s threshold voltage, the VCA’s EC- control port is kept at zero volts, resulting in no compression or limiting action. Above the threshold level, the threshold amplifier conducts and closes the feedback loop from the RMS level-detector to the VCA, resulting in the desired limiter function.

 

 

 

Advanced applications  of  clipping & clamping circuits

 

 

 

DETAILED DESCRIPTION

The GB4550(A) is intended for video applications requiring coarse DC restoration coupled with flat frequency response. As shown in Figure 1, the signal path features a wide band operational amplifier designed to be unity gain stable. While this amplifer is not intended to drive 75 transmission lines,

it is ideal for applications where high capactive loads, up to several hundred picofarads, must be driven, such as input buffering and DC restoration of video signals. Optimal frequency response for the GB4550(A) occurs with load capacitances in the range of 80 pF to 100 pF as shown in Figure 4. For smaller loads, an external capacitor can be added to extend the bandwidth and improve the flatness of the device response. The clamping function is achieved through the use of a simple comparator. The inverting input of the comparator is connected to the GB4550(A) output, while the non-inverting input is connected to the clamp voltage reference. For output signal voltages more positive than the clamp reference the comparator output is essentially open-circuit, while signal voltages more negative than the clamp reference result in the charging of CX. The action of the comparator is to provide a positive current which is fed back to the op-amp non-inverting input under conditions where the op-amp output is more negative than the clamp reference voltage. This negative feedback raises the DC level of the input signal to the point where all signal fluctuations occur at voltages above the clamp reference level. This is the desired clamp action. The input to the op-amp must be AC coupled using an appropriate size of capacitor, which then acts as a DC "reservoir" for the corrective level shift.

Under equilibrium conditions the average current supplied by the comparator output is just sufficient to balance the current discharging the input capacitor. This discharge current is simply the input bias current of the op-amp, typically less than 20 µA . However, an external resistor can be added to increase the pull down current. Under dynamic conditions, where the system is adjusting for a change in the signal level, the charging current may be in the milliamp range. Because the corrective current is small under equilibrium conditions, the error voltage at the comparator input is small also, so clamping accuracy to within ±7 mV is achievable. The clamp circuit makes use of a "peak hold" capacitor, CX, at the output of the comparator . This gives rise to a more constant voltage at the comparator output which is translated to a more constant corrective current by an internal 100 k resistor connected between the comparator output and the signal input.

To avoid excessive phase shift and consequent instability of the clamp feedback loop, the peak hold capacitor needs to be considerably smaller (e.g. 1000 times) than the input coupling capacitor. If a faster clamp is desirable (e.g. for 60 Hz hum elimination) the peak hold capacitor can be removed and a

smaller input coupling capacitor employed. In this application some distortion of the signal "tip" is unavoidable.

 

 

 

 

 

 

 

Conclusion

 

In my opinion , clipping and clamping circuits plays important rule in electronics world , from protecting the delicate electronics circuits to producing desired voltage signals , and much more applications beside electronics