![]() Input Impedance: Medium (about 3kΩ to 5kΩ). Therefore an increase in base/emitter voltage causes a decrease in collector/emitter voltage. This will in turn cause an increase in collector current, but as collector current increases, the voltage drop across the load resistor increases and as the voltage on the top end of the load resistor (the supply voltage) will not change, the voltage on the bottom end must decrease. This is because an increase in base/emitter voltage will cause an increase in base current. Notice that because of this method of connection the output waveform will be in anti-phase to the input waveform. This is because the larger the load resistor, the larger the change in voltage that will be caused by a given change in collector current. The value of the load resistor will affect the VOLTAGE GAIN of the amplifier. To give VOLTAGE amplification, a load resistor (or an impedance such as a tuned circuit) must be connected in the collector circuit, so that a change in collector current causes a change in the voltage developed across the load resistor. Therefore the circuit is that of a CURRENT amplifier. In this method of connection small changes in base/emitter current cause large changes in collector/emitter current. The most common function of a transistor is to be used in COMMON EMITTER mode. the reservoir capacitor in the power supply) so there can be no AC voltage difference between +V and 0V rails.įig. This is because, although there is obviously a voltage (the supply voltage) between these two points, the DC supply is always de-coupled by a large capacitor (e.g. 3.6.2 to 3.6.4, the +V supply line and the 0V line can be considered as the same point, as far as any AC signal is concerned. In a transistor amplifier circuit, such as those shown in Figs. Note that the diagrams are shown here reduced to their most basic form and are not intended to be practical circuits. ![]() These differences can be exploited by the circuit designer to give an amplifier with characteristics that are most suited a particular purpose. 3.6.2 to 3.6.4 would show quite different characteristic curves for each mode. The choice of which terminal is used as the common connection has a marked effect on the performance of the amplifier.Ī transistor connected in the three modes illustrated in Figs. Because an amplifier must have two input and two output terminals, a transistor used as an amplifier must have one of its three terminals common to both input and output as shown in Fig 3.6.1.
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