Sometimes it is desirable to limit the range of a signal (i.e., prevent it from exceeding certain voltage limits) somewhere in a circuit. The circuit shown in Figure 1 will accomplish this. The diode prevents the output from exceeding about +5.6V, with no effect on voltages less than that (including negative voltages); the only limitation is that the input must not go so negative that the reverse breakdown voltage of the diode is exceeded (e.g., −75V for a 1N4148). The series resistor limits the diode current during clamping action; however, a side effect is that it adds 1 kΩ of series resistance (in the Thevenin sense) to the signal, so its value is a compromise between maintaining a desirable low source (Thevenin) resistance and a desirable low clamping current. Diode clamps are standard equipment on all inputs in contemporary CMOS digital logic. Without them, the delicate input circuits are easily destroyed by static electricity discharges during handling.
A voltage divider can provide the reference voltage for a clamp (Figure 2). In this case you must ensure that the resistance looking into the voltage divider (Rvd) is small compared with R because what you have looks as shown in Figure 3 when the voltage divider is replaced with its Thevenin equivalent circuit.
When the diode conducts (input voltage exceeds clamp voltage), the output is really just the output of a voltage divider, with the Thevenin equivalent resistance of the voltage reference as the lower resistor (Figure 4). So, for the values shown, the output of the clamp for a triangle-wave input would look as shown in Figure 5. The problem is that the voltage divider doesn’t provide a stiff reference, in the language of electronics. A stiff voltage source is one that doesn’t bend easily, i.e., it has low internal (Thevenin) resistance.
In practice, the problem of finite impedance of the voltage-divider reference can be easily solved by use of a transistor or an op-amp. This is usually a better solution than using very small resistor values, because it doesn’t consume large currents, yet it provides a voltage reference with a Thevenin resistance of a few ohms or less. Furthermore, there are other ways to construct a clamp, using an op-amp as part of the clamp circuit.
Alternatively, a simple way to stiffen the clamp circuit of Figure 2, for time-varying signals only, is to add a so-called bypass capacitor across the lower (1 kΩ) resistor. To understand this fully we need to know about capacitors in the frequency domain. For now we’ll simply say that you can put a capacitor across the 1k resistor, and its stored charge acts to maintain that point at constant voltage.
For example, a 15 μF capacitor to ground would make the divider look as if it had a Thevenin resistance of less than 10Ω for frequencies above 1 kHz. As we’ll learn, the effectiveness of this trick decreases at low frequencies, and it does nothing at dc. One interesting clamp application is “dc restoration” of a signal that has been ac coupled (capacitively coupled). Figure 6 shows the idea.
This is particularly important for circuits whose inputs look like diodes (e.g., a transistor with grounded emitter); otherwise an ac-coupled signal will just fade away, as the coupling capacitor charges up to the signal’s peak voltage.
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