What is a clamp circuit?

We will later use the output of the comparator circuit as an input to the MicroStamp11. There is, however, a big problem with this approach. The problem is that the voltage levels generated by the comparator circuit are too large. Recall that for the op-amp to work well, the supply voltage must be around 9 volts. This means that the output voltage of your comparator will be either 0 or 9 volts. If we were to apply the 9 volt output to an input pin of the MicroStamp11 we would immediately destroy the MicroStamp11. The MicroStamp11 is only designed to accept voltages that are either zero or 5 volts. Any applied voltages outside of this range will destroy the delicate circuitry within the device. So if we are to use the output of the comparator, we will need some way of reducing the 0-9 volt range produced by the comparator to a 0-5 volt range. This can be done using a clamp circuit.

A clamp circuit is a special type of circuit that is used to limit or clamp the output voltage to a specified range. The clamping action is accomplished through the use of diodes. Remember that a diode is like an electronic valve. When it is forward biased, it acts like a short circuit and when it is reverse biased it acts like an open circuit. Figure 10 depicts the schematic for a clamp circuit. When $V_{\rm in}$ is greater than 5 volts, then the bottom diode is reverse biased and the top diode is forward biased. As a result, the circuit can be replaced by the equivalent circuit shown in the top righthand drawing. In this case, we see that the input source is connected directly to the 5 volt supply and it is disconnected from the resistor $R_2$. This means, therefore that $V_{\rm out}$ will equal zero. The resistor $R_1$ is put in here to limit the current drawn out of voltage source $V_{\rm in}$. If $V_{\rm in}$ is less than 5 volts, then the bottom diode is forward biased and the top diode is reverse biased. The equivalent resistive network is shown in the bottom righthand schematic diagram. We now see that the source is connected to ground through resistors $R_1$ and $R_2$. Once again we choose $R_2$ to be larger than $R_1$ so that most of $V_{\rm in}$ drops over the second resistor.

Figure 10: Clamp Circuit

The choice of $R_1$ and $R_2$ in the above circuit is dictated by two concerns. First we see that $R_1$ is essentially a current limiting resistor that prevent the source from being unduly stressed if $V_{\rm in } > 5$. A logical value for this would be around 1 k-ohm. $R_2$ is then chosen to be much larger than $R_1$ in order to drop most of the input voltage across this second resistor. Choosing $R_2$ an order of magnitude greater than $R_1$ is a realistic choice.