A bipolar junction transistor (BJT) is a device that can amplify signals thanks to its two p-n junctions and three terminals: base, collector and emitter. This device is operated by current. At the collector of the transistor, a signal with a modest amplitude applied to the base is available in its amplified form. This is the BJT's amplification in action. To run the amplification process, an external DC power source will be needed, so keep that in mind.
The goal of any small signal amplifier is to maximize the input signal strength while minimizing distortion in the output signal. Put simply, the output signal should be a larger version of the input signal that is identical to it (amplified). When utilized as an amplifier, achieving low distortion requires accurately selecting the operational quiescent point. It is possible to determine its location along the load line using an appropriate biasing configuration; it is the amplifier's DC operating point. This article will walk the reader through the three different ways a BJT amplifier may be set up.
[Learn more about BJT technology on GlobalSpec]
Common-emitter amplifier
Voltage divider biasing is used in the common emitter amplifier circuit shown in Figure 1. Two resistors form a potential divider network across the supply in this biasing arrangement; the base bias voltage needed by the transistor is supplied by the middle point of the resistors. By maintaining a constant and steady voltage level for the base bias, this technique of biasing the transistor significantly mitigates the impact of changing beta (β), enabling optimal stability. A potential divider network consisting of two resistors (R1 and R2) and the supply voltage (Vcc) determines the idle base voltage (Vb).
Common emitter amplifier. Source: Wikimedia Commons CC BY-SA-3.0
Coupling capacitors C1 and C2 isolate the AC signals from the DC biasing voltage. As the capacitors will only allow AC signals to flow through and reject DC components, this guarantees that the bias condition established for the circuit to function properly is unaffected by any extra stages of the amplifier. In the circuit for the emitter leg is a common emitter (CE), a bypass capacitor as well. Because it functions as an open circuit component under DC biasing conditions, this capacitor ensures that the biasing currents and voltages remain stable when adding it.
Common-collector amplifier
An additional configuration of BJTs that takes an input signal at the base terminal and outputs it at the emitter terminal is the common collector (CC) amplifier. This means that the input and output circuits share the collector terminal. The term "CC" describes this setup since the collector terminal is "grounded" or "earthed" via the power source.
Common collector amplifier. Source: Wikimedia Commons CC BY-SA-3.0
By switching the load resistor's connection from the conventional collector terminal (RC) to the more unusual emitter terminal (RE), the CC arrangement is functionally inverted compared to the CE setup. When a large current gain is required between an input source with a high impedance and an output load with a low impedance, the CC or grounded collector design is typically employed.
With the help of a basic voltage divider network consisting of resistors R1 and R2, the NPN transistor is biased into conduction. The VB can be easily determined using the simple formula for voltage dividers because this voltage divider lightly loads the transistor. A voltage drop across the emitter resistor RE will be produced by any collector current when the transistor's collector terminal is directly connected to VCC and there is no collector resistance (RC = 0).
The output voltage, VOUT, is represented by the voltage drop, VE, in the common collector amplifier circuit. ldeal conditions for an unclipped output signal include a DC voltage drop across RE that is equal to half of the supply voltage, VCC, so that the quiescent output voltage of the transistors is in the center of their characteristic curves. Thereby, IB and the current gain of the transistor are crucial factors in determining the choice of RE.
In an emitter follower circuit, the base input signal's voltage changes cause the emitter output to "follow" or track those changes, with the exception that it stays approximately 0.7 volts (VBE) below the base voltage. This configuration of transistors is frequently used in amplifiers because the output signal is taken from across the emitter load.
Common-base amplifier
Another BJT amplifier configuration is the common base amplifier, so called because the base terminal of the transistor is connected to both the input and output signals. Due to its unique input/output characteristics, the common base design is still utilized as an amplifier, even though it is less frequent than the more popular CE or CC configurations.
The emitter terminal receives the input signal, while the collector terminal sends out the output signal, in order for the common base configuration to function as an amplifier. Similar to other configurations, it needs to be appropriately biased to function as a common base amplifier; hence, the currents flowing through the emitter and collector double as input and output currents, respectively. The base-emitter connection is biased in a forward direction.
[See also: Bipolar junction transistor: Operating principles, types and applications]
Conclusion
When a high voltage gain is required, the CE amplifier configuration is the best option. If a high current gain is required, the CC arrangement is appropriate. Due to its lower gains, CB design is less prevalent in amplifier applications; yet it is utilized in specific radio frequency applications and impedance matching. The selection of configuration is determined by the application's particular specifications.
