Common Base amplifiers

    Depending on where you apply the input at, and where you sense the output from, there are 3 basic single stage BJT amplifiers: Common Emitter, Common Collector (a.k.a. Emitter Follower) and Common Base amplifiers. Each single stage amplifier have different properties. Designing practical amplifiers generally involve using multiple stages together, in order to satisfy the desired specifications simultaneously. The objective of this lab is to familiarize you with the design of common emitter and common collector amplifiers and then to design a multi-stage amplifier based on a set of given specifications. You can ignore the Early effect (?? = ∞) in your hand calculations. 1. Common-emitter amplifier is the most common configuration for BJT amplifiers since a single stage provides both voltage and current gain. The common emitter stage, using a single power supply, is shown in Fig 1. Figure 1. Common-emitter amplifier. The AC behavior of an amplifier can be quantified by three properties: Input impedance, gain and output impedance. You can model any amplifier as a twoport network, by using those 3 properties. The two-port representation of a voltage amplifier is shown in Fig 2. The AC properties are greatly affected by the DC operating point. It is necessary to stabilize the DC bias point against fluctuations in Process, ??? and Temperature variations (PVT). With a resistor connected to the emitter node, an increase in the collector current is compensated due to the negative feedback. Remember that, in + _ ???         ? 2 order to have ?-independent DC bias point, current through and 2 must be chosen to be much larger (at least 10 times larger) than the base current. The voltage at the emitter node (??) must be chosen to be much larger than the variations in ??? for a stable collector current, however, as ?? gets large, the available voltage swing at the output node decreases. Figure 2. Two-port network model for a voltage amplifier Another consideration in the design of amplifiers is the output voltage swing. This is the largest value of the unclipped signal amplitude at the output node. Output voltage swing depends on the DC bias point. Typically, the DC voltage at the output node is set to ???/ to allow for maximum voltage swing. Remember that, a good operating point , is somewhere in the middle of the load line. Design procedure to design a CE amplifier: • Choose the voltage at the emitter node, ?? ≥ [?] and calculate ??. • Choose ?? ≈ [??]. • Choose ? so that ?? ≈ ???/ = .5[?]. • Choose the current through and 2 to be at least 10 times larger than ??, in order to reduce the sensitivity due to ? variations. • Calculate the values of and 2. Note that, when you build your circuit on breadboard, you will need to use 5% or 10% standard resistors, so choose your resistors that you use in simulation accordingly. You can find a list of standard resistors online. • Calculate the total emitter resistance ( ? + ?3). • Determine ? to achieve the desired input resistance and gain. Note that, increasing the emitter resistance, increases ?? but decreases ??. In the circuit shown in Fig. 1, a small amount of the emitter resistance ( ? ) is not shorted to have large ??. (Remember that the emitter resistance will be multiplied by ? when seen at the base terminal.) Therefore, the value of ? must be chosen to be much smaller than the value of ?3. Design the CE amplifier shown in Fig.1, to meet the following design specifications: ➢ ??? = 5[?], ? = 0[?Ω], ➢ ?? > 0[?Ω], |??| > 0 ➢ Output voltage swing (???) > .6[?] peak-to-peak, ➢ Operating frequency = 5[?Hz].