Certain types of discrete transistors, the NPN 2N2222 and the PNP 2N2907, can be operated as inverted mode switches where "inverted" means that the collector is made to serve as an emitter and the emitter is made to serve as a collector. Having originally read about this in a long since misplaced General Electric application note back in 1964, this works because these two transistors types are of epitaxial planar construction and as such, can deliver extremely low saturation voltages in their inverted mode on-states where "low" means on the order of microvolts.

This property was put to good use in the design of a successive approximation analog to digital (A/D) converter that was part of the digital multimeter (DMM) called Building Block Ten (BB10) of the U.S. Navy's CVA Versatile Avionics Shop Test (VAST) system. (If you can say all of that in one breath, you have better lungs than I have!) An array of 2N2222 and 2N2907 inverted mode switches fed and switched the resistor ladder network of that A/D converter.

There are limitations to be observed when using the transistors in inverted mode service however. The emitter-serving-as-a-collector to base voltage must not exceed the base-to-emitter reverse breakdown voltage of that pn-junction. Also, the emitter to collector current needs to be kept small meaning being kept to less than 1mA.

Prior to encountering that DMM, I once designed a very simple switched attenuator using inverted mode 2N2222 switches as follows.

inverted mode switches (cr)

Each control signal is zero for its NPN being in cut-off and +5V for its NPN being saturated. The 10K input resistor limits the maximum emitter to collector current to 0.5mA.

Just to mention, I used this circuit as part of the "AM Leveller" module of the VAST BB21 signal generator whose RF output was from 10kHz to 40MHz and whose AM per cent modulations were 20%, 40%, 60% and 80%. There were four modulation levels and hence, four switches.

This article first appeared on EDN.