In some circuits, it is necessary to symmetrically limit or clip the peak output voltage of an amplifier stage. Limiting may be required to prevent over driving a following amplifier stage. This type of circuit is also used in volume compressor and amplitude leveler designs.
The limiting function may be accomplished by several methods. Figure 3-6 shows a simple back-to-back diode limiter.
If standard small signal diodes such as 1N914 are used for 01 and 02, ±0.6 V would be available at the limiter output. If germanium diodes are used for 01 and 02. ± 0.4 V will be the output voltage. Limiting may also be accomplished utilizing resistor/zener diode networks. The low-voltage shunt limiter shown in Figure 3-7 is useful when the signal level must be limited at a very low level, such as several hundred millivolts. Such levels are, of course, below the range of conventional diodes, so alternate methods are necessary to accomplish limiting at these levels.
In this circuit, the operational amplifier is used to shift the apparent threshold of a conventional IN914 silicon diode. The output voltage limit can be adjusted to any fraction of the diode voltage. This circuit divides the signal level, even when below the threshold, because of the feedback through resistor R2. As an example, assume a 10-V peak-to-peak input sine-wave signal at a frequency of 1 kHz. With the values shown the output will be a 150-mV peak-to-peak square wave as shown in the scope photos in Figure 3-8.