What is the importance of the input and output impedance of a BJT
The gain of an amplifier, a single device, depends on the GM, the dIout/dVin, also called transconductance. GM is a vacuum tube holdover, meaning conductance-mutual or how the input and output interact.

Knowing the GM, multiply that by the impedance at the output node, and you have the gain of that stage, of that single device.

But you must include ALL the resistances, all the impedances, hanging on the output node. That requires you know the Rload, the R_collector, the slope_of_the_collector_IV_curve, and the capacitive parasitics. All these in parallel affect the gain.

Should you implement a constant-current load on the collector, then the slope of the collector IV curve, described with the name of Early Voltage, or Vearly or just Va, will set the low frequency gain.

Here is a complete bipolar gain-stage, with a resistive Rcollector (not the constant-current load):







simulate this circuit &ndash; Schematic created using CircuitLab

What is the bandwidth of this circuit? The lower end is set by the capacitors. The upper end, the Tau, is the 10pF in parallel with Rcollector || R5 || r_early || Rload. If you build this, add 5 or 10pF more, for wiring capacitances. And a scope probe will add 10 ---20pF more. A quick number: 10pF + 4Kohm is 400 nanoSeconds Tau, or 2.5MegaRad/2*pi ~~ 0.4MHz F3dB.

Now, regarding input impedance, given voltage sources have some resistance or impedance, call it Zsource, the Zin of the bipolar will form a Voltage Divider with the Zsource. Darlington bipolars will greatly incread the Zin, and the voltage division becomes

Zsource / (Zsource + Zin)

Also notice the C_collectorbase, of value 10pF, which about right for a 2N3904. The collector end sees the 10pF; the base end sees 10pF * (1 * stage_gain), thus the base Zin (at high frequencies) suffers because of the signal charge being stolen by the C_collectorbase.

Notice the self-biasing, using 220,000 ohms in series, the total being BETA * Rcollector. This rather nicely sets the Vcollector at 4.5 volts +-, allowing a large "linear" swing. But you have to know the BETA rather accurately, if you want VDD/2 as the Qpoint. On the other hand, this method of biasing does give you a fine opamp (inverting input only) with over 100,000 Hz bandwidth. Note the 220,000 Rs do load both the Vin and the Vout.