# Impedance relation between 2 Bjts

#### samy555

May 11, 2010
63
If there are two BJT small signal amplifiers (common Emitter) cascade connected using a suitable decoupling cap
The first preamp has an o/p imeadance Zo, and the second has an i/p impedance Zin
Now we have Zo and Zin and a cap betwwen them
My question is: Is the connection series or parallel? That, is the o/p voltage divided or the o/p current?

#### BobK

Jan 5, 2010
7,682
Zout and Zin form a voltage divider. The actual input voltage seen by the second amplifer is is calcluated from that, i.e. V = Vin * Zin / (Zin + Zout)

Bob

#### samy555

May 11, 2010
63
but, why in many electronic books, when there is a load resistance RL connected to the collector of a transistor has a collector resistance RC,,, why take RL and RC in parallel?
note: there is a coupling cap between RC & RL.
thanks alot

#### BobK

Jan 5, 2010
7,682
The coupling capacitor is chosen to pass the AC signal of interest, so effectively it has no resistance for the signal.

Bob

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