Common Emitter Amplifier

When the base is made positive relative to the emitter, the junction is forward biased and the transistor is turned on. The collector current is given by the al and the current in the base circuit is (1 a) . The ratio of collector to base current is 

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The frequency components of a signal can be obtained directly from PSpice by enabling the Fourier option in the Time Domain (Transient) setup. We will use the common-emitter amplifier circuit shown on page 361. To modify the Time Domain (Transient) setup select PSpice and then Edit Simulation Profile. 

When we design a circuit with tolerance, we may sometimes want to find the worst case upper or lower 3 dB frequency with component tolerances. Unfortunately, calculating a 3 dB frequency requires that we find the mid-band gain and then find the frequency where the gain is 3 dB less than the mid-band. This type of calculation cannot be specified in the Monte Carlo/Worst Case dialog box. However, we can run a Monte Carlo analysis and then determine the 3 dB frequency using the Performance Analysis capabilities available in Probe. In this example, we will illustrate finding the maximum lower 3 dB frequency (FL), minimum upper 3 dB frequency (FH), and maximum and minimum bandwidth (FH - FL) for a common-emitter amplifier. Wire the circuit below ... 

The common-base circuit for an npn transistor (Fig. 9.19A) seems logical and simple, but its efficiency in amplification is not obvious and must be explained. The npn transistor in a common-emitter circuit (Fig. 9.19B) is a bit easier to understand as a current amplifier. There are four rules ... 

Fig.4. Data plot for a 20 nA current pulse. The pulse width was 16.5 fis. The circuit used for this data consisted of a single transistor in a common emitter configuration with a light emitting diode as the load element. This circuit was used instead of the operational amplifier circuit in Fig.l because it provides improved response at low current levels.

The BJT is a three electrode or triode electron device. When connected in a circuit it is usually operated as a two-port, or two-terminal, pair device as shown in Fig. 7.8. Therefore, one of the three electrodes of the BJT must be common to both the input and output ports. Thus, there are three basic BJT configurations, common emitter (CE), common base (CB), and common collector (CC), as shown in Fig. 7.8. The most often used configuration, especially for amplifiers, is the common-emitter (CE), although the other two configurations are used in some apphcations. 

The circuit will be as shown in Fig. 15.4 (on the next page). An advantage of this hookup is the fact that only one battery is needed, so the common emitter is used very often. Another advantage is that either voltage or current can be amplified. In this case, a meter can measure the input current. 

It should be noted that the common emitter (CF) arrangement can amplify both voltage and current, but the common collector (EF) only amplifies current. The reason for the latter is that the base voltage must be higher than the emitter voltage in order for base current to flow. Therefore the emitter voltage must be low, and we can not get increased voltage. This is why common collectors are not used very often, except for buffers and a few other applications where there is a need for low output impedance (equal to output resistance, in this example). 

BASIC COMMON EMfiTER TRANSISTOR AMPLIFIER The basic common-emitter transistor amplifier circuit represents the electronics work done in the Active Area instrumentation lab. 

There are varieties of ways to use junction transistors in circuits as shown in the following examples. The common emitter circuit shown in Figure 22.4 is widely used for amplifier as well as switching applications. 

Transistors are always operated so that amplification is obtained for a signal whose voltage is small on the input side (relative to the bias on the emitter) and small on the output side (relative to the collector voltage). Therefore small-signal theory applies. The designer obviously wants all signals to be amplified linearly, that is, by a common factor this makes the circuit behave more reasonably. 

Figure 9.6. Differential amplifier performance with common-mode interference signals, (a) Upper trace 10-kHz sine wave with superimposed common-mode noise as amplified by conventional amplifier lower trace same signal amplified by differential amplifier, (b) Upper trace 10-kHz square wave with superimposed 60-Hz hum and broad-band noise as amplified by single-ended amplifier middle trace same signal as amplified by differential amplifier lower trace residual noise removed by filtering, (c) Circuit schematic for transistor differential amplifier with constant-current emitter stage, (d) Vacuum-tube differential amplifier in long-tailed pair configuration. (See Ferris, 1963.)...

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