![]() If they don't share a common ground then one or none of those stages is allowed to be called a CE amplifier and the gains don't multiply. Only then can you multiple the gains of each stage to get the overall gain. If you have two CE amplifiers the emitters must share the same "Common" ground (the power rail obviously being treated as ground for ac signals as well.). You are not free to choose a different ground node if you want to call the connection a CE amplifier. Where there are external input and output connections these have a pre-defined ground node. The concept of gain only means something with that constraint. When you use the term CE amplifier the inputs *and* outputs of the circuit must use the "E" as the common ground reference that's why it's called *Common* Emitter. So they all have an input between B and E. Sure, all transistors, independent of the circuit configuration, are transconductance devices. Here's an AC coupled version, (R2 on this schematic has no equivalent on the Boss Tone) So the presence of RB reduces the gain by a factor of 1/2.8 from the straight DC connected buffer view. If we now choose RB = RE as per the circuit (the 18k's) ![]() If you crunch the numbers you end up with Rpi approximately 1.8 * RE. The collector current is about 84% of I_RE because the bias current through RB (=VBE/RB) steals some of the RE current. The collector current is set by the bias point voltage across RE, ie. We can simplify a bit by realizing Rpi depends on the Q2's collector bias current. However, the resistor RB between the Base and Emitter of Q2 takes some of the gain away so you end-up with the impedance looking into the RB +Q2 as, ![]() That view would make the effective collector resistance of Q1 (hfe+1) * RE. ![]()
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