First, backtracking the DC analysis of transistors, there results a Q point which makes the output signal stable, taking the linear portion of Shockleys equation. Similarly, in AC small-signal analysis, the circuit functions along the same portion of the Shockley equation. Through the small-signals, approximation are more accurate, where a model can be developed by assuming the changes in voltages and currents because of the AC signal are small as compared to the resulting Q point in the DC analysis.  Therefore, in the analysis of such circuit, there must be a separation of nalyses between DC and AC.
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Second, the model used for this activity is the re model, which includes a connection between the input and the output to produce the feedback effects of the input and output signals . In this model, there exists the system called the two- port network system, which defines certain parameters: input impedance (Zi), output impedance (Zo), input current and output current (10). As a result of these parameters, voltage gain (Av) and/or current gain (A’) is/are produced. Simply put, impedance is that resulting resistance of all resistors seen by a certain voltage erminal in the circuit during an AC analysis .
Conversely, this is also equivalent to the total resistance in a DC analysis. In other words, the impedance is the resulting Thevenin resistance using Thevenin’s theorem. For the AC analysis of the circuit, certain steps are followed. First, all capacitors are shorted. Second, independent sources are turned off; and third, AC equivalent circuit is drawn with its two-port network. Ill. DISCUSSION AND RESULTS The common-emitter amplifier circuit explored in the lab activity is coined such because both the input and output voltages share the emitter part of the transistor.
Wiring up the circuit shown in fg 1, the Vin and Vout are obtained: Vout(peak-peak) Figure 2. Vin(peak-peak) vs Vout(peak-peak) We then find the voltage gain (Av) of the circuit with the formula: Hence, we get: Av = -5 The voltage gain, or the ratio between the output and the input, is important to be determined because the circuit investigated is
Page 2 Common-emitter amplifier Essay
a type of amplifier; hence, it is known to have the “ability to increase the magnitude of the input signal. ”  As noticed above, the voltage gain is a unitless quantity – this is because it only hows the relationship of the input and output signals received in the circuit.
Also, noticeably, the output voltage has a phase shift from the input, giving the voltage gain a negative sign, despite the amplification. This will be further explained later on. Next, we acquire the input (lin) and output (lout) currents: lout 75. 273 PA 619. 576 nA These values make sense because, first of all, lin is equivalent to the base current, and lout to the collector current. Since the input and output voltages are seeked from the base and the collector in a common-emitter circuit, these currents ave a bearing to the voltages. As we know, the current varies directly with the current.See More on Common