![]() Yes, it matters to have either impedance reduction(CC) or voltage gain. Starting at around 30 Hz, the gain goes up so that eventually the emitter resistor looks like (1 kΩ // 470 Ω) = 320 Ω, for a gain of about 30. The collector resistor RC should be chosen to limit collector current to the suitable level. At low frequencies, the gain of the second transistor stage is about 10. ![]() ![]() When the input is zero, the transistor is cut. Output impedance or source impedance will be higher for the common base arrangement. The rolloff frequency of 470 Ω and 10 ♟ is 34 Hz, so this isn't quite a "HiFi" audio amp. Figure 1 shows a simple npn common-emitter digital amplifier, inverter, or switch, in which the input signal is at either zero volts or a substantial positive value, and is applied to the transistor’s base via series resistor R b, and the output signal is taken from the transistor’s collector. This makes biasing easier and more predictable, but still gives higher gain at the audio frequencies where we care about gain. The 470 Ω resistor with 10 ♟ cap in series effectively make the emitter resistor lower at audio frequencies. In this experiment we will build a two-stage amplifier using two bipolar transistors. Since the capacitor blocks DC, only the 1 kΩ resistor matters in setting the DC operating point of the transistor, also called biasing the transistor. The difference between these two types is that for bipolar devices an input current controls the large current flow through the device, while for field-effect transistors an input voltage provides the current control. Your circuit actually has even higher gain, due to additional resistor and capacitor in series on the emitter. Since roughly the same current flows thru the collector as emitter, the voltage variations as the collector will be about 10x higher (voltage gain of 10) at the collector than the emitter. Note that in the second stage of your circuit, the collector resistor is 10x the emitter resistor. In the end, most "amplifiers" need overall voltage gain, so output from the collector is going to be needed somewhere in there. In that case a emitter follower (output from emitter) can be useful. Sometimes it's useful to have the same signal but at a lower impedance. The finished schematic, along with voltage sources ready for simulation is shown below.At a high level, the main difference is that output from the collector can give you voltage gain, while output from the emitter gives you current gain, but not voltage gain. ![]() Sometimes this is called the load resistor 2, however this can be confusing, as typically the “load” is placed after the output AC coupling capacitor. The small-signal design equations listed in Table 3are for the input resistance (ri), output resistance (ro), voltage gain (av vo/vi), and current gain (ai. \(R_C\) is the collector resistor which helps set the voltage gain of the amplifier.\(C_E\) is the emitter bypass capacitor and is used to bypass \(R_E\) so that the AC signal essentially sees the emitter connected directly to ground. \(R_E\) adds emitter degeneration 1 and makes the amplifier gain more stable with variations in \(\beta\).With R b and R c in place, according to AoE formula, Z source 2Mohm/ (100 1) 10kohm 1kohm 30,801. \(C1\) is used to AC couple the input signal to the DC bias point – it’s value is chosen so that it appears as a short for the AC signal frequencies of interest but blocks DC. Output Impedance: if removing R e and R c, Z out 2Mohm/ (100 1).\(R1\) and \(R2\) are used to provide a DC bias point for the base of the transistor, using the standard resistor divider technique (to be exact, you also have to take into account that the transistor draws some current from the output of the resistor divider, but generally you can ignore that).Schematic for a common emitter amplifier with DC bias and AC coupling. ![]()
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