ltspice
Above is a screen-shot from an LTSpice simulation. It is a freq. response of an LC tank circuit with a 5V sinusoidal AC source.
When I plot it, the y-axis is in dB.
When I look at the voltage it is 13.979dB which corresponds to 5V from the formula 20*log(V1/1).(1 is the reference voltage?)
But the current is plotted in negative dB. Why is that so? What does -dB mean here for the current? is that something to do with reference current in 20*log(I1/Iref)?
You didn't measure the base current, you measured the current through R3.
Even though the symbol doesn't show it, LTSpice is keeping track of which pin is 1 and which is 2, and defining the sign of current with some fixed convention relative to those pin numbers. Either current is defined as going in to pin 1, or into pin 2, though I don't know which one.
To get the current through R3 to have the same sign as the current in to the BJT base, you just need to turn the resistor around 180 degrees.
Edit: User W5VO made some images showing how the cursor shows the reference direction when you hover over the resistor:
And how it changes after you rotate the resistor by 180 degrees:
So from this plot it looks like it's about 2.9K. Is this correct?
Yes
Obviously with the bias high enough the impedance of the transistor is 0 - it's just a diode. Right?
Probably not. The diode model probably includes parasitic resistance in the base and emitter contacts, so the input resistance will never go all the way to zero. Also, your R3 (multiplied by the transistor beta) will set a lower limit on how low a resistance you measure looking in to the base of the transistor in your circuit.
Best Answer
0 dBI is exactly 1 amp (not to be confused with dBi which is the gain of an antenna compared to the benchmark (i)sotropic antenna)
A current of 100 mA is -20 dBI etc..
So when you have decibels for current as the Y axis then it is implicit that they the Y axis is dB relative to 1 amp (or dBI).
One further thing to consider is that the graph may be plotting peak current and peak current is \$\sqrt2\$ times bigger (for a sinewave). Read the LTSpice help advice to know exactly.
I've just checked on google about usage of the term "dBI" as representing current and nothing comes up. You can find dBuA for dB relative to 1 uA but nothing seems to mention dBI - I'm interested if anyone can provide some evidence to substantiate the usage of the term "dBI".
Sure, dBV are used and this is quite common and one website I visited compared decibels for current and voltage by calling them dB(volts) and dB(amps) - maybe "dBI" just isn't used so as not to confuse folk with dBi?