Editing Negative-feedback amplifier (section)

===Replacement of the feedback network with a two-port===
The first step is replacement of the feedback network by a [[two-port network|two-port]]. Just what components go into the two-port?

On the input side of the two-port we have ''R''<sub>f</sub>. If the voltage at the right side of ''R''<sub>f</sub> changes, it changes the current in ''R''<sub>f</sub> that is subtracted from the current entering the base of the input transistor. That is, the input side of the two-port is a dependent current source controlled by the voltage at the top of resistor ''R''<sub>2</sub>.

One might say the second stage of the amplifier is just a [[voltage follower]], transmitting the voltage at the collector of the input transistor to the top of ''R''<sub>2</sub>. That is, the monitored output signal is really the voltage at the collector of the input transistor. That view is legitimate, but then the voltage follower stage becomes part of the feedback network. That makes analysis of feedback more complicated.
[[Image:G-equivalent circuit.PNG|thumbnail|250px|Figure 4: The g-parameter feedback network]]
An alternative view is that the voltage at the top of ''R''<sub>2</sub> is set by the emitter current of the output transistor. That view leads to an entirely passive feedback network made up of ''R''<sub>2</sub> and ''R''<sub>f</sub>. The variable controlling the feedback is the emitter current, so the feedback is a current-controlled current source (CCCS). We search through the four available [[two-port network]]s and find the only one with a CCCS is the g-parameter two-port, shown in Figure 4. The next task is to select the g-parameters so that the two-port of Figure 4 is electrically equivalent to the L-section made up of ''R''<sub>2</sub> and ''R''<sub>f</sub>. That selection is an algebraic procedure made most simply by looking at two individual cases: the case with ''V''<sub>1</sub> = 0, which makes the VCVS on the right side of the two-port a short-circuit; and the case with ''I''<sub>2</sub> = 0. which makes the CCCS on the left side an open circuit. The algebra in these two cases is simple, much easier than solving for all variables at once. The choice of g-parameters that make the two-port and the L-section behave the same way are shown in the table below.
{| class="wikitable" style="background:white;text-align:center "
!g<sub>11</sub>
!g<sub>12</sub>
!g<sub>21</sub> 
!g<sub>22</sub> 
|-
|-valign="center"
| '''<math>\frac {1} {R_\mathrm{f}+R_2}</math>'''
| '''<math> - \frac {R_2}{R_2+R_\mathrm{f}}</math>'''
| '''<math> \frac {R_2} {R_2+R_\mathrm{f}} </math>'''
| '''<math>R_2||R_\mathrm{f} \ </math>'''
|}
[[Image:Small-signal current amplifier with feedback.PNG|thumbnail|400px|Figure 5: Small-signal circuit with two-port for feedback network; upper shaded box: main amplifier; lower shaded box: feedback two-port replacing the ''L''-section made up of ''R''<sub>f</sub> and ''R''<sub>2</sub>.]]