The Big Idea – Using Shunt Clippers to Protect Circuits

Biased Shunt clippers are a simple way to protect inputs from excessive positive or reverse voltage.

Drill Down:

From our previous discussion, we know that clipper circuits eliminate either the positive or the negative portion of a waveform – the unwanted output is limited to a maximum of Vf (forward voltage drop) above or below ground. The biased shunt clipper is normally used to protect a device or circuit that has both positive and negative input signals. The bias voltage is selected to prevent the input from exceeding a maximum safe level.

In Figure 15-1, diode D1 has its cathode connected to a bias of +2.4 V and D2 has its anode connected to -2.4V. As shown, D1 will be reverse biased while the output of the clipping circuit is below 2.4V.The positive output will be limited to a maximum of (2.4V+Vf). Similarly, D2 will be reverse biased until the output is more negative the -2.4v. The negative output will be limited to a maximum of – (2.4V+Vf)

Figure 15-1 Biased diode Shunt Clipper

The Zener clipper shown in Figure 15-2 does not require separate bias voltages. When the input signal becomes positive, D1 operates like an ordinary forward biased diode while D2 goes into breakdown. The output voltage at this time is (Vf1+Vz2). When the input is negative, D1 is in Zener breakdown and D2 is forward biased. The output voltage is now – (Vf2+Vz1).

Figure 15-2 Zener shunt Clipper

Example 1:

Assume we require a shunt clipper (Fig.15-1) to protect a circuit. The input voltage to the circuit cannot exceed Vo = +/- 3.1V. The input to the clipper will be +/- 8V (square wave) and the output current is to be 1mA. Specify the diodes and calculate R1.

Given Ifwd (forward current) = 10mA.

Vo = Bias voltage Vb + diode voltage drop Vf

Vb = Vo-Vf

Vb = 3.1V – 0.7V (assume a Si diode)

Vb = 2.4V

Voltage across R1 = (Ifwd + Io) X R1

(Ifwd + Io) X R1 = E – Vb – Vf

R1 = (VG_out – Vb – Vf) / (If + Io)

(8 – 2.4 – 0.7) / (10mA + 1mA) = 445 ohms

Use standard value 470 ohms

Figure 15-3 Diode Shunt Clipper Simulation

The diodes selected will be low current devices with a Vf (forward voltage) of approx 0.7V at If (forward current) = 10ma, and a peak reverse voltage greater than 10V

Example2:

Assume we would like to use a Zener shunt clipper (Fig.15-2) to protect a circuit. The input voltage cannot exceed Vo= +/- 7.5V. The input to the clipper is a +/- 24V square wave and the output current is to be 100 mA. Specify the diodes and calculate R1.

Given output is Vo = +/- 7.5V

Vo = Vf+Vz

Vz = Vo-Vf = 7.5V – 0.7V = 6.8V

Assume a 1N2804. From the spec sheet Vz = 6.8V.

Therefore:

VR1 = VG_out-Vo

= VG_out – (Vf+Vz)

= 24V – (0.7V + 6.8V) = 16.5V

To ensure that Iz > Izk

Iz is approx 1/4 Izt = 1/4 X 1.86A = 465mA

IR1 = Iz+Io = 465mA+100mA = 565mA

R1 = (VR1) / (IR1) = (14.3V) / (565mA)=253 ohms

Nearest standard value is 270 ohms

Figure 15-4 Zener Shunt Clipper Simulation

What Have We Learned?

1) Biased shunt clippers are a simple way to protect circuits from excessive positive and/or negative input voltages. Inputs are limited to an amount equal to the applied bias voltage plus the forward diode drop.

2) Zener clippers do not require the bias voltage.

What’s Next?

We will examine positive and negative diode clamping circuits

The Big Idea – Using Shunt Clippers to Protect Transistor Input Circuits

Shunt clippers are a simple way to protect transistor inputs from excessive reverse voltage.

Drill Down:

Most general purpose transistors will not survive a reverse voltage greater than 5 volts applied across their base-emitter junction. Shunt clippers (Figures 14-1 and 14-3) are a simple way to protect these input circuits

Fig.14-1: Negative Clipper Circuit Simulation

As shown in Fig 14-2 the negative portion of the input signal is clipped to protect the NPN transistor.

Fig.14-2: Negative Clipper Simulation Response

When the signal becomes positive, D1 is reverse biased and all of the output current from the clipper circuit (Iout) flows to the transistor input. The voltage at the transistor base will be:

Input voltage – Vdrop across R1 = VG_out – (Iout X R1)

When the input becomes negative the transistor’s base emitter junction is reverse biased. Notice that the forward biased diode is effectively in parallel with the transistor input. This limits the maximum base-emitter voltage to the diode forward drop Vf (0.7V).

If the transistor is a PNP (Fig.14-3) a positive clipper circuit is used.

Fig.14-3: Positive Clipper Circuit Simulation

D1 is now forward biased when the input signal is positive (Fig.14-4). Again, the transistor is protected because the diode limits the voltage at the base-emitter junction to its Vf. When the input becomes negative, D1 is reverse biased and Iout flows through the transistor base

Fig.14-4: Positive Clipper Simulation Response

Consider the following example:

Assume we require the negative shunt clipper circuit in Fig.14-1 to have an output voltage (Vout) of 10V and an output current of approx 2mA. If the input voltage is +/- 12V, calculate the value of R1 and the diode forward current.

When the input is +12V:

Vout = 10V = VG_out – (Iout X R1)

(Iout X R1) = VG_out – 10V = 12V – 10V = 2V

R1 = 2V / Iout = 2V / 2mA = 1Kohm

When the input is -12V:

D1 is forward biased, assume Vf = 0.7

Vf = VG_out – (Iout X R1)

Iout = (VG_out – Vf )/ R1

Iout = (12V – 0.7V / 1kohm) = 11.3mA.

What Have We Learned?

1) Shunt clippers are a simple way to protect transistor inputs. They shield the base-emitter junction by limiting excessive reverse voltage to an amount equal to the clipping diode’s forward voltage drop (Vf).

2) The diode orientation determines whether the circuit is negative or positive clipping.

What’s Next?

We will examine a biased shunt clipper circuit

The Big Idea – Series Noise Clipping Circuits.

Frequently, unwanted noise in signals can trigger sensitive circuits. A series noise clipper can be used to eliminate this noise.

Drill Down:

If the noise is smaller than a forward diode drop and the signal is larger, then the diode circuit shown in Figure 12.1 can be used. Since the noise voltage peaks are not large enough to forward bias either D1 or D2, the output during the time between signals is zero (Fig.12-2). Desired signals will forward bias the diodes and the output peak voltage will be VG_out – Vf  (Fig.12-3). This creates a dead zone of +/- Vf around ground potential at the output - for signals to be passed to the output they must exceed +/- Vf  (Fig. 12-4).

Figure 12-1: Series Noise Clipper Circuit

Figure 12-2: Series Noise Clipper Circuit at 0.2V

Figure 12-3: Series Noise Clipper Circuit at 0.5V

Figure 12-4: Series Noise Clipper Circuit at 6V

When the noise is too large for ordinary diodes Zener diodes are used (Fig. 12-5).

Figure 12-5: Series Noise Clipper Using Zener Diodes

Figure 12-6: Zener diode noise clipper at 2V.

When the signal is negative, Z1 is forward -biased and Z2 is in breakdown (Fig. 12-8).

Figure 12-7: Zener diode noise clipper at 2.6V

The dead band is now +/- (Vf + Vz) as shown in Fig.12-6 and Fig.12-7. Comparing Fig. 12-6 to Fig. 12-8 clearly illustrates only signals greater than the dead band will be passed to the output. 

Figure 12-8: Zener diode noise clipper at 8V signal

The signals must be large enough to drive one diode into breakdown and forward bias the other. Since the voltage drop across the two diodes is subtracted from the input signal the output peak is now +/- (VG_out – Vf – Vz).

Let’s look at an example using a Zener circuit of the type shown in Fig 12-5. We will assume the input signal is +/- 8V with noise amplitude of +/- 2V.

1) Find an appropriate Zener diode and calculate the value of R1.

2) Calculate the amplitude of the output signals.

From the noise amplitude we know that Vz > 2V.

A review of some data sheets shows the 1N746 (Vz = 3.3V) to be a suitable device.

The output signal (Vo = VRES) is therefore:

= +/- (VG_out – Vf – Vz)

= +/- (8V – 0.7V – 3.3V)

= +/- 4V

When the input signal is positive, Z2 behaves as an ordinary forward-biased diode, while Z1 goes into breakdown (Fig.12-8).

In the absence of a load current, R1 must pass enough current to keep the diode conducting when the signal is present. From the 1N746 data sheet Izt is given as 20ma.

To ensure that IR1 is greater than Izk, make IR1 approx 1/4 Izt (5mA) and VR1 = Vo (+/-4V). Therefore R1 = VR1/IR1 = 4 / 5mA = 800 ohms. The nearest standard 5% value would be 820 ohms.

The power dissipation in R1 (PR1) will be equal to Vo^2 / R1.

Therefore PR1 will be equal to (4V)^2 / 820 ohms = 19.5mW.

What Have We Learned?

1) If the noise voltage is smaller than the forward diode drop and the signal is larger a series clipper circuit comprised of two opposite polarity silicon diodes in parallel can be used.

2) If the noise voltage is too large for general purpose silicon diodes a clipper circuit comprised of series Zener diodes can be used.

3) A dead band equal to +/- Vf will exist around the ground potential. Signals must exceed +/- Vf to be passed to the output.

What’s Next?

We will examine shunt noise clipping circuits using: 1) general purpose silicon diodes and 2) Zener diodes.

The Big Idea – Diode Clipping Circuits

Circuits that select part of an input waveform for transmission are referred to as voltage (or current) clippers, limiters, slicers or amplitude selectors..

Drill Down:

The positive series clipper shown in Figure 11-1 forward biases the diode when the input signal is negative.

Figure 11-1: Positive Series Clipper

Figure 11-2 clearly illustrates the resulting output voltage (VRES) is the peak input voltage (VG_out) less the diode drop (VD1). When the input becomes positive, the diode is reverse biased and only the reverse saturation (Is = approx 5uA max with a 1N4448) current flows through R1. As shown, the output is now the input waveform with the positive portion clipped off.

Figure 11-2 Positive Clipper Waveforms

The negative series clipper (Figure 11-3) operates in a manner similar to the positive clipper  except now the negative portion is blocked (Figure 11-4)

Figure 11-3: Negative Series Clipper

Figure 11-4 Negative Clipper Waveforms

What Have We Learned?

1) Circuits that select part of an input waveform for transmission are referred to as voltage (or current) clippers, limiters, slicers or amplitude selectors.

2) The resulting output voltage is equal to the peak input voltage less the diode drop. Depending on the direction of the series diode either the positive or the negative part of the input waveform has been eliminated

What’s Next?

We will examine a series noise clipping circuit using: 1) general purpose silicon diodes and 2) zener diodes.

Diodes designed with sufficient power dissipation capability to operate in the avalanche or breakdown region of their VI (voltage versus current) response curve are often applied as voltage references or regulators. These devices are called Zener, avalanche or breakdown diodes.

Drill Down :

Zener diodes (Fig. 10-1) are p-n junction devices designed to operate in the reverse breakdown region of their VI characteristic curve. By maintaining their reverse current within certain limits, the voltage drop across the diode will remain constant. In this mode of operation the zener diode will act as a voltage reference.

Figure 10-1: Zener Diode – Cathode is stripe

As shown in Fig. 10-2 Vz is the zener voltage measured at test current Izt. If we examine a typical breakdown diode specification we will find additional ratings: knee current Izk, is the minimum current that should pass through the device to maintain a constant Vz. Izm is the maximum zener current that will not to exceed the maximum permissible power dissipation rating. Violate this and you will watch your diode turn into smoke.

Figure 10-2: VI Characteristic Curve

For operation as a voltage reference a zener diode must be reverse biased. The cathode is connected to positive, anode to negative. When the reverse voltage is smaller than Vz only the normal diode reverse saturation current Is flows. When the zener diode is forward biased it behaves like an ordinary diode, a large forward current flows, and the forward diode voltage is typically 0.7 V. Avalanche diodes are available with maintaining voltages from several volts to several hundreds volts and with power dissipation ratings up to 50 Watts.

The temperature sensitivity of a zener diode (temperature coefficient) is given as a percentage change in reference voltage per centigrade degree change in diode temperature. This number is usually in the range of +/- 0.1 % per deg C. The direction of the change is related to the mechanism of breakdown (avalanche multiplication versus zener breakdown). Generally, if the reference voltage is above 6V the coefficient is positive, if below, negative.

Fig 10-3  Temp Coefficient Curve for 1N829 

Some manufacturers have produced temperature compensated reference diodes by combining a positive temp coefficient zener diode with a forward biased, negative temp coefficient, diode in a single package (i.e. Microsemi 1N829, a 6.2V reference diode with a temp coefficient of +/- 0.0005 % per deg C over a range of  -55 to +100 deg C). Rather than use a single larger diode it is often better to place multiple zener diodes in series when designing a high voltage reference. This combination allows higher voltage, higher power dissipation, lower temperature coefficient, and lower dynamic resistance (the reciprocal slope of the volt-amp curve in the operating region). Of course, this is a more costly solution than a single diode.

What Have We Learned?

1) Zener (a.k.a. avalanche, or breakdown) diodes are p-n junction devices designed to operate in the reverse breakdown region of their VI characteristic curve. By maintaining their reverse current within certain limits, the voltage drop across the diode will remain constant. In this mode of operation the zener diode will act as a voltage reference.

2) A zener diode must be reverse biased to operate as a voltage reference. The cathode is connected to positive, anode to negative.

3) Avalanche diodes are available with maintaining voltages from several volts to several hundreds volts and with power dissipation ratings up to 50 Watts.

4) It is often better to place multiple zener diodes in series when designing a high voltage reference. This combination allows higher voltage, higher power dissipation, lower temperature coefficient, and lower dynamic resistance.

What’s Next?

We will examine pulse clipping applications using diodes.