Small Signal CB Amplifier Model Electronics Help

An equivalent circuit for an electronic device is called a model. Using just the parameters we have discussed so far, we can construct a simple but reasonably accurate small-signal model for a transistor. Figure 5-23 shows a transistor in the common-base configuration and its approximate small-signal model. Remember that all voltages and currents are ac quantities, so all polarities periodically alternate. The polarities shown in the figure should be interpreted as reference directions for instantaneous values. For example, Figure 5-23(a) shows that an increase of current into the emitter terminal is accompanied by an increase of current out of the collector terminal

An equivalent circuit for an electronic device is called a model. Using just the parameters we have discussed so far, we can construct a simple but reasonably accurate small-signal model for a transistor. Figure 5-23 shows a transistor in the common-base configuration and its approximate small-signal model. Remember that all voltages and currents are ac quantities, so all polarities periodically alternate. The polarities shown in the figure should be interpreted as reference directions for instantaneous values. For example, Figure 5-23(a) shows that an increase of current into the emitter terminal is accompanied by an increase of current out of the collector terminal

It is clear that the transistor input resistance equals r, in our approximate Cfs model. This is a relatively small value, usually less than 100

The transistor output resistance, on the other hand, can be seen to equal the large value

Thus, the current gain of a CB amplifier is always less than

Figure 5-26(a) shows a CE amplifier that is driven by a source having internal resistance rs. The amplifier load is the resistor RL• Figure 5-26(b) shows the ac equivalent circuit that results when the assumptions R£ II r, “””r, and r, II Rc “””Rc are once again imposed, Figure 5-26(c) shows the amplifier circuit when the transistor is replaced by a single block having the parameters derived in equations 5-26 through 5-28, Note that the voltage source in Figure 5-26(c) is the (Thevenin) equivalent of the current source in Figure 5-25.

An increase in emitter-to-base (input) voltage reduces the forward bias on the. emitter-base junction and thus reduces the emitter current. But a decrease in emitter current causes a decrease’ in collector current, since Ie = pth. Decreasing /e causes the quantity I.R; in equation 5-31 to decrease and therefore causes VCII to increase. Recapitulating, an.increase in emitter (input) voltage causes an increase in collector (output) voltage, and we conclude that input and output are in phase. When a transistor is connected in.a circuit to produce gain; the transistor and all the associated external components it needs to operate properly (such as bias resistors) are referred to collectively asan amplifier stage. It is important to distinguish between the input and output resistances of the transistor alone and those’ of the stage of which it is a part. Hereafter, we will use the notation (stage) after the symbol rin ‘or ro when-we wish to emphasize that it refers to a stage characteristic. Figure 5-27 illust:ates these distinctions iq a CB amplifier stage

Small-Signal CE Amplifier Model

To develop a model for the transistor in its common-emitter configuration, we will first investigate the input resistance in that configuration. Figure 5-30 shows the CE input circuit with r, drawn inside the emitter terminal, to emphasize that it is an internal transistor parameter. The ac input resistance is rIll = ul)i/>. Recalling

Equation 5-35 shows that the input resistance in the CE configuration is approximately {3 times greater than that in the CB configuration. The quantity ({3 + l)r, is often designated as r, in texts and data sheets. It:.. can also be shown that the output resistance of a transistor in its CE configuration is approximately f3 times smaller than it is in the CB configuration: ro “”” rc1f3. Because the CE input resistance is f3 times greater and the output resistance is f3 times smaller than the corresponding values in the CB configuration, the common-emitter amplifier is inherently better suited for voltage amplification than its CB counterpart,Figure 5-31 shows an approximate model for the common-emitter transistor. Note that the controlled current source has value f3ib, reflecting the fact that ic = f3i/>. Once again, we neglect the feedback effect, whereby the value of if> depends somewhat on V

Figure 5-32 shows a common-emitter amplifier stage and its ac equivalent circuit. Note that the dc supply voltages are treated as ac short circuits, as before. In Figure 5-32(b), it can be seen that r.lf3 is in parallel with Re. In most practical circuits, r.lf3 ~ Re and so (r.lf3) ” He “” Re. For example, typical values are r, = 10 MH, f3 = 100, and R( = I kil, for which (rJf3) II Re = (100 kH) II (1 kO) = 990 n. For the approximate analysis methods we are developing now, we will assume that the parallel combination equals Re. Under that assumption, the output resistance of the stage is

Posted on November 18, 2015 in SMALL SIGNAL BJT AMPLIFIERS

Share the Story

Back to Top
Share This