I was studying the CC, CB and CE Amplifier. And I have found a sentence somewhere that,
"Maximum power transfer occurs when load impedance is equal to source impedance."
As much as I understand impedance is something that opposes current. But, what are this load impedance, output impedance, input impedance? I mean how are the related to.
In the case of CC, CE, CB amplifiers, how are they related to each other and what are the meaning in this case?
Thanks in Advanced. Hope somebody will help.
Best Answer
To give a mathematical definition, if we were to excite the input of a two-port with a sinusoidal current source
$$i_{in}(t) = I_0 + I \sin(\omega t)$$
with \$I\$ small enough to not cause any nonlinear behavior, we'd find
$$v_{in}(t) = V_0 + V_i \sin(\omega t) + V_q \cos(\omega t)$$
Then we could define
$$R_{in} = \frac{V_i}{I}$$
and
$$X_{in} = \frac{V_q}{I}.$$
Then the input impedance would be \$Z_{in}\$ if we defined
$$Z_{in} = R_{in} + j X_{in}.$$
In words, this means that the real part of \$Z_{in}\$ tells us how the in-phase component of the input voltage depends on the input current. And the imaginary part of \$Z_{in}\$ tells us how the quadrature (90 degree phase shifted) part of the input voltage depends on the input current.
We would also find that
$$R_{in} = \frac{{\rm{d}}V_0}{{\rm{d}}I_0}.$$
The output impedance is defined the same way, but as a relationship between the output current and output voltage.
All the other things we know about input and output impedance and how they relate to circuit behavior can be connected back to these definitions.
This isn't quite correct.
First, when talking about possibly complex-valued impedances, maximum power transfer occurs when the source impedance is the complex conjugate of the load impedance.
Second, this maximum power transfer condition is for choosing the load impedance when the source impedance is fixed. If source impedance can be controlled, then either very high impedance (with a fixed-current source) or very low impedance (for a fixed-voltage source) should be chosen to deliver maximum power to a fixed impedance load.