BJT Amplifiers

The BJT Amplifier | The AC Analysis of the BJT Amplifier


The BJT Amplifier

The amplifier is designed to take an input signal and deliver that signal to another amplifier or to a load. The load can be a resistor or a transducer, such as a speaker. The amplifier will either increase the input signal amplitude or increase the current delivered to the load.

A variety of amplifiers exists. Our initial study is on the BJT amplifier. The BJT amplifier is classified into Class A, Class B, Class AB, Class C, and Class D. Additionally, the BJT amplifier is categorized into the circuit configurations--common emitter, common collector, and common base.

Our study will analyze the common emitter (CE) bipolar junction transistor amplifier, shown below.

Bipolar Junction Transistor Amplifier

To evaluate the circuit, the circuit analysis will utilize superposition. Recall that superposition applies only to linear circuits and involves calculating the voltages and currents with one supply grounded, and then analyze the circuit with the other supply grounded. The AC voltage source will be grounded first. The focus will be on calculation of the DC operating condition for the BJT common emitter amplifier.

For DC voltages, all capacitors will be open circuits. The remaining circuit will be separated into an analysis of the base-emitter loop, the transistor gain characteristics, and the collector-emitter loop. Values will be calculated for the base voltage, base current, collector voltage, collector current, emitter voltage, emitter current, and the collector-emitter voltage.

Next week, we will be looking at the results through an interactive on the DC operating conditions. We will then continue our superposition analysis with the AC circuit analysis.


The AC Analysis of the BJT Amplifier

Prior to performing the analysis, it must be reiterated: for the BJT amplifier to operate properly, the DC power supply must be connected to the circuit while the AC input signal is applied to the circuit.

As we analyze the circuit on paper, we will be replacing the DC power supply with a connection to ground in order to evaluate the effects of the AC signal on the circuit. The reactance of the capacitor depends upon the signal input frequency. To simplify the analysis, the capacitors will be assumed to be shorted out for the sinusoidal waveform inputs. Thus, the sine wave passes through the circuit to the output load. Through the AC analysis, the magnitude of the amplifier voltage gain is determined to be dependent upon the collector resistor, load resistor, unbypassed emitter resistor, and the BJT emitter dynamic resistance.

Amplifier Circuit

The interactive provides you with the DC and AC signals throughout the amplifier circuit.

While viewing this demonstration you can right-click and choose "Zoom In" to get a more detailed view of the graphs.

Additionally, we notice that the output waveform is 180 degrees phase shifted from the input signal. The voltage gain will be described as a negative value. Note that throughout the amplifier circuit, the AC signal is riding on the DC voltage levels.