Intro to Pulse Circuits – Biased Shunt Clippers

Welcome back to my workbench,

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)Biased Diode Clipper Model

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).

Zener Clipper Model

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

Biased Shunt Clipper Simulation

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

Zener Clipper Simulation

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

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Intro to Pulse Circuits – Protecting Discrete Transistors with a Shunt Clipper.

Welcome back to my workbench,

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

 Negative Clipper Circuit Simulation

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.

 Negative Clipper Sim Response

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.

 Positive Clipper Circuit Simulation

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

Positive Clipper Sim Response

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

 

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