Electronics: Three layouts of connecting transistors

The common-emitter ampliﬁer

One of the simpler transistor ampliﬁer circuits to study previously illustrated the transis-tor’s switching ability. (Figure 4.20)

It is called the common-emitter conﬁguration because (ignoring the power supply battery) both the signal source and the load share the emitter lead as a common connection point shown in Figure 4.21. This is not the only way in which a transistor may be used as an ampliﬁer, as we will see in later sections of this chapter.

Before, a small solar cell current saturated a transistor, illuminating a lamp. Knowing now that transistors are able to “throttle” their collector currents according to the amount of base

current supplied by an input signal source, we should see that the brightness of the lamp in this circuit is controllable by the solar cell’s light exposure. When there is just a little light shone on the solar cell, the lamp will glow dimly. The lamp’s brightness will steadily increase as more light falls on the solar cell. Suppose that we were interested in using the solar cell as a light intensity instrument. We want to measure the intensity of incident light with the solar cell by using its output current to drive a meter movement. It is possible to directly connect a meter movement to a solar cell (Figure 4.22) for this purpose. In fact, the simplest light-exposure meters for photography work are designed like this.

Although this approach might work for moderate light intensity measurements, it would not work as well for low light intensity measurements. Because the solar cell has to supply the meter movement’s power needs, the system is necessarily limited in its sensitivity. Supposing that our need here is to measure very low-level light intensities, we are pressed to ﬁnd another solution.

Perhaps the most direct solution to this measurement problem is to use a transistor (Figure 4.23) to amplify the solar cell’s current so that more meter deﬂection may be obtained for less incident light.

Current through the meter movement in this circuit will be β times the solar cell current. With a transistor β of 100, this represents a substantial increase in measurement sensitivity. It is prudent to point out that the additional power to move the meter needle comes from the battery on the far right of the circuit, not the solar cell itself. All the solar cell’s current does is control battery current to the meter to provide a greater meter reading than the solar cell could provide unaided.

Because the transistor is a current-regulating device, and because meter movement indications are based on the current through the movement coil, meter indication in this circuit should depend only on the current from the solar cell, not on the amount of voltage provided by the battery. This means the accuracy of the circuit will be independent of battery condition, a signiﬁcant feature! All that is required of the battery is a certain minimum voltage and current output ability to drive the meter full-scale.

Another way in which the common-emitter conﬁguration may be used is to produce an output voltage derived fromthe input signal, rather than a speciﬁc output current. Let’s replace the meter movement with a plain resistor and measure voltage between collector and emitter

in Figure 4.24

With the solar cell darkened (no current), the transistor will be in cutoff mode and behaves an open switch between collector and emitter. This will produce maximum voltage drop between collector and emitter for maximum Voutput, equal to the full voltage of the battery. At full power (maximum light exposure), the solar cell will drive the transistor into saturation mode, making it behave like a closed switch between collector and emitter. The result will be minimum voltage drop between collector and emitter, or almost zero output voltage. In actuality, a saturated transistor can never achieve zero voltage drop between collector and emitter because of the two PN junctions through which collector current must travel. However, this “collector-emitter saturation voltage” will be fairlylow, around several tenths of a volt, depending on the speciﬁc transistor used.

For light exposure levels somewhere between zero and maximum solar cell output, the transistor will be in its active mode, and the output voltage will be somewhere between zero and full battery voltage. An important quality to note here about the common-emitter conﬁguration is that the output voltage is inversely proportional to the input signal strength. That is, the output voltage decreases as the input signal increases. For this reason, the common-emitter ampliﬁer conﬁguration is referred to as an inverting ampliﬁer.

A quick SPICE simulation (Figure 4.26) of the circuit in Figure 4.25 will verify our qualita tive conclusions about this ampliﬁer circuit.

At the beginning of the simulation in Figure 4.26 where the current source (solar cell) is outputting zero current, the transistor is in cutoff mode and the full 15 volts from the battery is shown at the ampliﬁer output (between nodes 2 and 0). As the solar cell’s current begins to increase, the output voltage proportionally decreases, until the transistor reaches saturation at 30 µA of base current (3 mA of collector current). Notice how the output voltage trace on the graph is perfectly linear (1 volt steps from 15 volts to 1 volt) until the point of saturation,

where it never quite reaches zero. This is the effect mentioned earlier, where a saturated ransistor can never achieve exactly zero voltage drop between collector and emitter due to internal junction effects. What we do see is a sharp output voltage decrease from 1 volt to 0.2261 volts as the input current increases from 28 µA to 30 µA, and then a continuing decrease in output voltage from then on (albeit in progressively smaller steps). The lowest the output voltage ever gets in this simulation is 0.1299 volts, asymptotically approaching zero.

The common-collector ampliﬁer

Our next transistor conﬁguration to study is a bit simpler for gain calculations. Called the common-collector conﬁguration, its schematic diagram is shown in Figure 4.39.

It is called the common-collector conﬁguration because (ignoring the power supply battery) both the signal source and the load share the collector lead as a common connection point as in Figure 4.40

It should be apparent that the load resistor in the common-collector ampliﬁer circuit receives both the base and collector currents, being placed in series with the emitter. Since the emitter lead of a transistor is the one handling the most current (the sum of base and collector currents, since base and collector currents always mesh together to form the emitter current),it would be reasonable to presume that this ampliﬁer will have a very large current gain. This

presumption is indeed correct: the current gain for a common-collector ampliﬁer is quite large, larger than any other transistor ampliﬁer conﬁguration. However, this is not necessarily what sets it apart from other ampliﬁer designs.

Let’s proceed immediately to a SPICE analysis of this ampliﬁer circuit, and you will be able to immediately see what is unique about this ampliﬁer. The circuit is in Figure 4.41. The netlist is in Figure 4.42.

Unlike the common-emitter ampliﬁer from the previous section, the common-collector produces an output voltage in direct rather than inverse proportion to the rising input voltage. See Figure 4.42. As the input voltage increases, so does the output voltage. Moreover, a close

examination reveals that the output voltage is nearly identical to the input voltage, lagging behind by about 0.7 volts.

The common-base ampliﬁer

The ﬁnal transistor ampliﬁer conﬁguration (Figure 4.52) we need to study is the common-base. This conﬁguration is more complex than the other two, and is less common due to its strange operating characteristics. It is called the common-base conﬁguration because (DC power source aside), the signal source and the load share the base of the transistor as a common connection point shown in
Figure 4.53.

Perhaps the most striking characteristic of this conﬁguration is that the input signal source must carry the full emitter current of the transistor, as indicated by the heavy arrows in the ﬁrst illustration. As we know, the emitter current is greater than any other current in the transistor, being the sum of base and collector currents. In the last two ampliﬁer conﬁgurations, the signal source was connected to the base lead of the transistor, thus handling the least current possible.

Because the input current exceeds all other currents in the circuit, including the output

current, the current gain of this ampliﬁer is actually less than 1 (notice how Rloadis Figure 4.53: Commonbase amplifier: Input between emitter

and base, output between collector and base.

connected to the collector, thus carrying slightly less current than the signal source). In other words, it attenuates current rather than amplifying it. With common-emitter and common-collector ampliﬁer conﬁgurations, the transistor parameter most closely associated with gain was β. In the common-base circuit, we follow another basic transistor parameter: the ratio between collector current and emitter current, which is a fraction always less than 1. This fractional value for any transistor is called the alpha ratio, or α ratio. Since it obviously can’t boost signal current, it only seems reasonable to expect it to boost signal voltage. A SPICE simulation of the circuit in Figure 4.54 will vindicate that assumption.

Notice in Figure 4.55 that the output voltage goes from practically nothing (cutoff) to 15.75 volts (saturation) with the input voltage being swept over a range of 0.6 volts to 1.2 volts. In fact, the output voltage plot doesn’t show a rise until about 0.7 volts at the input, and cuts off (ﬂattens) at about 1.12 volts input. This represents a rather large voltage gain with an output voltage span of 15.75 volts and an input voltage span of only 0.42 volts: a gain ratio of 37.5 or 31.48 dB. Notice also how the output voltage (measured across Rload) actually exceeds the power supply (15 volts) at saturation, due to the series-aiding effect of the input voltage source

A second set of SPICE analyses (circuit in Figure 4.56) with an AC signal source (and DC bias voltage) tells the same story: a high voltage gain.

As you can see, the input and output waveforms in Figure 4.57 are in phase with each other. This tells us that the common-base ampliﬁer is non-inverting.