Basic bipolar amplifier circuit

Chapter Basic bipolar amplifier circuit

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Basic bipolar amplifier circuitIn the previous chapters, you saw some circuits that will work as amplifiers. The princi-ple is the same for all electronic amplification circuits. A signal is applied at some con-trol point, causing a much greater signal to appear at the output.In Fig. 24-2, 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 bias. Re-sistor R1 and capacitor C1 allow for the emitter to have a dc voltage relative to ground,while being grounded for signals. Resistor R1 also limits the current through the tran-sistor. The ac output signal goes through capacitor C3. Resistor R4 keeps the ac outputsignal from being short-circuited through the power supply.Basic bipolar amplifier circuit43724-2An amplifier using a bipolar transistor. Component designators and valuesare discussed in the text.In this amplifier, the capacitors must have values large enough to allow the ac sig-nal to pass with ease. But they shouldn’t be much larger than the minimum necessaryfor this purpose. If an 0.1-µF capacitor will suffice, there’s no point in using a 47-µF ca-pacitor. That would introduce unwanted losses into the circuit, and would also makethe circuit needlessly expensive to build.The ideal capacitance values depend on the design frequency of the amplifier, and alsoon the impedances at the input and output. In general, as the frequency and/or circuit im-pedance increase, less and less capacitance is needed. At audio frequencies, say 300 Hz to20 kHz, and at low impedance, the capacitors might be as large as 100 µF. At radio fre-quencies, such as 1 MHz to 50 MHz, and with high impedances, values will be only a