Using a Picoammeter to Investigate Leakage Currents in Clamp Diodes

Clamp diodes are widely used in analog electronic circuits, but, like all real-world components, these diodes have some non-ideal characteristics. This article investigates some of their characteristics.

Reverse Biased Leakage Current

Reverse biased leakage current is a characteristic of concern for a clamp diode. Ideally the leakage would be zero but this is the real world, so what can one live with? In a typical application there might be two clamp diodes, one to +15 V the other to -15 V and a common connection to the signal to be clamped. The leakage current drops as the voltage drops, so if that signal is nearer to +10 V, then the lower clamp diode would have more leakage than the upper one. So in such an application, a diode with both low leakage and a fairly flat leakage vs. voltage would be desirable.

   

Figure 1 is plotted data of leakage measurements versus reverse voltage. A quiet DC supply was used along

with an AMETRIX Instruments’ Model 101 Picoammeter to measure the current (and yes, that is single digit femtoamps in the diagram).

 Comments about reverse leakage:

1. The SST4391 is rated for 35 V gate-drain and gate-source

2. The SST4391 costs $0.057 each

3. The 2n3904 has a Vcb maximum of 60 V

4. The MMBT3904LT1G is $0.0231 each

5. The FLLD261 is $0.0275 each

6. The BAS16 costs $0.0151 each

7. The BAT54 costs $0.0204 each

8. The BAS70 costs $0.0323 each

9. The MMBD4148CC costs $0.0194 each

So even though there is a 3:1 price ratio between the most and least expensive, they are all <6 a="" cost="" decision="" is="" not="" of="" part="" process.="" serious="" so="" span="" the="">

Forward Voltage Drop

The forward voltage drop is another diode characteristic to be considered for a voltage clamp. These measurements were made as follows (the data is presented graphically in Figure 2):

1. Forward bias the diode with a quiet DC supply that has fine resolution; in this case an EDC calibrator

2. With function generator set to pulse, 1 Hz, 1% duty cycle, drive a mercury wetted reed relay. Connect

the relay contacts across DUT such that the DUT is shorted for 990 msec and unshorted for 10 msec; this is to minimize self heating effects.

3. Measure forward voltage with Tektronix MSO4104 scope with 10x probes to minimize the low forward current errors. Set the scope’s bandwidth limit to 20 MHz to minimize noise. Use the scope’s vertical channel’s offset mode and 10 mV range, adjusting the offset knob until trace is in center of screen, then document the offset voltage.

Conclusion

Some simple measurements with a precision picoammeter can provide much insight into the components that you think might be ideal in a given application; and they might reveal some things that are unexpected.