Hi all!
TL;DR: when a BJT sits naked (no DC resistances on either the emitter or collector, but well controlled base current) between Vcc and GND, is it safe to use ONLY the base current to prevent it from releasing magic smoke? Details below.
I'm building a transmitter for a wired interface (Ethernet). I chose to drive the transmission line itself with a transformer which is in turn driven by a common-collector BJT (a voltage follower/buffer), but with no DC current limiting resistors.
The inductance of the transformer has been selected to provide a good impedance at the low end of the bandwidth, but that left me with such an impedance at the high end of the range that I can not really afford to put a series limiting resistor to control the current (and thus power) through the BJT when the transmitter is idling.
Stated differently, when the transmitter is not sending any frames, the output of the voltage follower is pure DC. It passes only through a transformer primary whose impedance at DC is near-zero.
I can not put a resistor in series with the transformer because the amplitude of the output oscillations would be too low. I can also not put a resistor in series with the collector for the same reason.
The option of using a DC resistor with a capacitor AC bypass is, again, DOA because during the operation the capacitor will get charged up to maybe Vrms of the output signal and will clip the output signal when the transistors input signal falls below that value.
I want to avoid the common-emitter conformation if anyhow possible because of problems with the signal distortion. I'm also not yet willing to experiment with switching the Vcc wire on and off to provide power-on- demand, so to speak. Same goes with the baseline base voltage.
So I'm basically stuck with a transistor which, when idling, sits naked between Vcc and GND. To enable proper voltage following, its base needs to be biased to a pretty high voltage - I'm keeping it at 1-1.2V - but this opens up a possibility of a short circuit.
The only way I figured out to prevent it from exploding is to use a well- selected series resistor with the base to moderate the baseline current and basically abuse the BJTs core operating principle (moderation of resistance via current) for fun and profit.
How reliable do you judge this to be, given random operating conditions the users are sure to put the interface in? Does inter-transistor difference (especially hFE differences between individual transistors) become important? Will I need to tightly match the base series resistance for each individual transistor?