- The Common Emitter Amplifier circuit is the most typical amplifier arrangement for an NPN transistor.
- The common emitter amplifier serves as a voltage amplifier and is a three-stage single-stage bipolar junction transistor. This amplifier’s base terminal serves as its input, the collector terminal serves as its output, and the emitter terminal serves as a common terminal for both terminals. The common emitter amplifier’s fundamental symbol is displayed below.
- An AC input signal that alternates between a positive value and a corresponding negative value is amplified by transistor amplifiers. The transistor must therefore be able to function between these two maximum or peak values, hence a method of “presetting” a common emitter amplifier circuit configuration is necessary. A technique called biasing can be used to do this.
- Biasing plays a crucial role in the design of amplifiers because it determines the transistor amplifier’s ideal operating point when it is ready to accept signals, minimizing output signal distortion.
- Additionally, we may view every potential operating point of the transistor, from fully “ON” to fully “OFF,” and to which the quiescent operating point or Q-point of the amplifier can be identified, by using a static or DC load line drawn onto the output characteristic curves of an amplifier.
Transistor Common – Emitter Circuit Design
- This standard emitter amplifier design for a logic buffer is about as straightforward as a design can get. The transistor is shown in the circuit design with both an input and a collector resistor. The base current is constrained by the input resistor, while the output voltage is developed by the collector resistor.
- Current flows through R1 and into the base when the input detects a logic high. The transistor turns on as a result of this. As a result, the voltage across resistor R1 develops to its full potential while the voltage on the collector drops to almost zero.
- It is fairly simple to create a common emitter amplifier that serves as a buffer for a logic IC. The next step-by-step guide could be followed to design the stage, albeit it is not the only option.
Select a Transistor
The following variables will affect the transistor designated.
- Dissipation of power is anticipated.
- Switching applications require a high switching speed; avoid using another type of transistor with large bandwidth.
- Current gain is necessary.
- Needed current capability
- volts at the collector-emitter.
- All of these can be foreseen with enough precision before the design’s beginning. The figures should all be verified after the design is finished to make sure the transistor is suitable for the selected values.
Calculate the Collector Resistor
- Once the type of transistor has been selected, the values of the remaining electronic components must be determined. By figuring out how much current must travel through the resistor, we can determine the collector resistor, R2. This will depend on factors like the required current for the circuit. Another possibility is that an LED indication must be connected in series with the collector resistor. Determine the current that will provide the required amount of light. By utilizing Ohms’s law and knowing the voltage across the resistor and the current flowing through it, the value of the resistor can be calculated.
Determine Base Resistor Value
- The collector current is divided by the value of or have, which is essentially the same, to get the base current. To turn the transistor ON for the lowest values, even at low temperatures when values will be lower, make sure there is enough current driving. It is important to avoid pushing too much current into the base because this will make switching more difficult. After all, the too much-stored charge needs to be discharged.
Re-evaluate Initial Assumptions
- After the design is finished, it is vital to reassess some of the initial choices and projections in case the outcome has altered.
Output Characteristics of Common-Emitter Transistor
- Transistor emitter and collector junction resistance aren’t always constant, which is unfortunate. Therefore, the relationship between the various currents and voltages in a transistor amplifier cannot always be expressed using Ohm’s Law. This information is typically provided by the manufacturer in graphical form for the reasons mentioned above, among others. A graph of a transistor’s common emitter configuration’s collector (output) characteristics can be seen in the picture below. The graph is made up of numerous curves. With the base current held constant, each curve depicts the fluctuation in collector current as the collector-to-emitter voltage varies.
- Even when no input signal is supplied, certain values for ie, vbe, ic, and VCE exist. The symbols IB, VBE, IC, and VCE stand for the no-signal values of ib, VBE, ic, and VCE, which are also known as the quiescent, or average, values. The quiescent point is the location on the output characteristic curve determined by IC and VCE (Q). An important aspect of analyzing an amplifier circuit is determining where the quiescent point on the characteristic curve is located.
- The steps are as follows: The supply voltage VCC and load resistance RL values that will be employed in the amplifier circuit’s “load line” are first determined. Assume that ib is so little that ic is equal to zero. If this were the case, there would be no voltage drop across RL, and VCE would be equal to VCC. On the output characteristic curve, point 1 is shown (figure above). When VCE = 0 and ic = VCC/RL, it is assumed that the base current is so great that the transistor turns into a perfect conductor.
- Place this point at position 2 on the typical curve and connect points 1 and 2 with a straight line as shown in the illustration. Because only the values of the load and VCC govern this line, it is referred to as the load line.
- The resulting values of VCE and ic must be along the load line for the chosen values of VCC and RL. The no-signal values for VCE and IC, as determined by the load line and the output characteristic for the chosen average base current IB = 100 A, are shown in the figure above at point Q. Moving along the load line will allow you to get the instantaneous values of VCE and ic from the graph for any given value of ib. The instantaneous base current, ab, relies on the addition of base bias current and input signal.
- Keep in mind that the load line should be well inside the collector-dissipation line’s maximum. A transistor’s maximum collector dissipation, or Pmax, is a property that is typically listed in transistor manuals or catalog descriptions. The locations that fulfill the equation ICVCE = Pmax are all connected to form the maximum collector-dissipation line. By assuming that the base-to-emitter junction consists just of the transistor input resistance hie, the quiescent base current can be computed. IB is equal to VBB/(RS + hie) if vs = 0 (quiescent state). RS will frequently be substantially higher than hie, causing IB to exceed VBB/RS. Therefore, the type of signal source will determine the bias voltage to be used. Later, we’ll talk about other biasing strategies.