Circuit forms adjustable bipolar clamp

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. The circuit in Figure 1 is a bipolar clamper with a range of ±1 to ±10V, with the clamping level a function of the input V_CLAMP. IC_1A, IC_1B, and IC_3A are unity-gain buffers. IC_2A is a positive clamper, and IC_2B is a negative clamper. Figure 2 shows the transfer function, with V_CLAMP set at –5V. You can change V_CLAMP over the range of –1 to –10V and thereby change the clamping level. If V_IN is within –V_CLAMP to +V_CLAMP, then V_OUT=V_IN. If V_IN exceeds V_CLAMP, then V_OUT=V_CLAMP. To explain how the circuit works, assume four cases, with four values of V_IN. 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 7V, V_CLAMP is –5V, D₁ conducts, and D₂ is an open switch. The feedback loop around IC_2A regulates the anode of D₁ to 5V and the output of IC_2A to 4.4V. In cases B and C, both diodes are open switches. In Case D, D₂ conducts, and D₁ is an open switch.

Is this the best Design Idea in this issue? Vote at www.ednmag.com.