The easy way to clamp a signal to a given value is to use two zener diodes, connected back-to-back. This method has several disadvantages. The accuracy of the clamping depends on the tolerance of the zener diodes, and the clamping is not adjustable, except by changing diodes.
|Figure 1.||This circuit provides adjustable clamping over the range of ±1 to ±10 V.|
The circuit in Figure 1 is a bipolar clamper with a range of ±1 to ±10 V, with the clamping level a function of the input VCLAMP. IC1A, IC1B, and IC3A are unity-gain buffers. IC2A is a positive clamper, and IC2B is a negative clamper. Figure 2 shows the transfer function, with VCLAMP set at –5 V. You can change VCLAMP over the range of –1 to –10 V and thereby change the clamping level. If VIN is within –VCLAMP to + VCLAMP, then VOUT = VIN. If VIN exceeds VCLAMP, then VOUT = VCLAMP.
|Figure 2.||With VCLAMP set at –5 V, the output clamps firmly at ±5 V.|
To explain how the circuit works, assume four cases, with four values of VIN. Basically, the circuit works in two modes: the linear mode, in which diodes D1 and D2 are open switches, and the clamped mode, in which the diodes are closed switches. Table 1 gives results for the four cases. In Case A, the input is 7 V, VCLAMP is –5 V, D1 conducts, and D2 is an open switch. The feedback loop around IC2A regulates the anode of D1 to 5 V and the output of IC2A to 4.4 V. In cases B and C, both diodes are open switches. In Case D, D2 conducts, and D1 is an open switch.
|Table 1.||Results for clamped and linear modes|
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