An amplifier circuit with a bipolar transistor. Component designators are discussed in the text.
In the previous topics, you saw some circuits that use bipolar and field effect transistors. A signal can be applied to some control point, causing a much greater signal to appear at the output. This is the principle by which all amplifiers work.
In above figure, an NPN bipolar transistor is connected as a common emitter amplifier. The input signal passes through C2 to the base. Resistors R2 and R3 provide the base bias. Resistor R1 and capacitor C1 allow for the emitter to have a dc voltage relative to ground, while keeping it grounded for signals. Resistor R1 also limits the current through the transistor. The ac output signal goes through capacitor C3. Resistor R4 keeps the ac output signal from being short-circuited through the power supply.
In this amplifier, the optimum capacitance values depend on the design frequency of the amplifier, and also on the impedances at the input and output. In general, as the frequency and/or circuit impedance increase, less capacitance is needed. At audio frequencies and low impedances, the capacitors might be as large as 100 μF. At radio frequencies and high impedances, values will be only a fraction of a microfarad, down to picofarads at the highest frequencies and impedances. The optimum resistor values also depend on the application. In the case of a weak-signal amplifier, typical values are 470 Ω for R1, 4.7 kΩ for R2, 10 kΩ for R3, and 4.7 kΩ for R4.