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• Semiconductors

Bipolar Junction Transistors

• Introduction to Bipolar Junction Transistors (BJT)
• The Bipolar Junction Transistor (BJT) as a Switch
• Meter Check of a Transistor (BJT)
• Active-mode Operation (BJT)
• The Common-emitter Amplifier
• The Common-collector Amplifier
• The Cascode Amplifier
• Biasing Techniques (BJT)
• Transistor Biasing Calculations
• Input and Output Coupling
• Amplifier Impedances
• Current Mirror BJTs
• Transistor Ratings and Packages (BJT)

The final transistor amplifier configuration (Figure below) we need to study is the common-base amplifiers . This configuration is more complex than the other two and is less common due to its strange operating characteristics.

Common-base amplifier

Why is it Called a Common-base Amplifier?

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 below.

Common-base amplifier: Input between emitter and base, output between collector and base.

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.

Attenuation of Current in Common-base Amplifiers

Because the input current exceeds all other currents in the circuit, including the output current, the current gain of this amplifier is less than 1 (notice how Rload is 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 always a fraction less than 1. This fractional value for any transistor is called the alpha ratio, or α ratio.

Boosting Signal Voltage in Common-base Amplifiers

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 the figure below will vindicate that assumption.

Common-base circuit for DC SPICE analysis.

Common-base amplifier DC transfer functio n.

Notice in the figure above 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. 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) exceeds the power supply (15 volts) at saturation, due to the series-aiding effect of the input voltage source.

The second set of SPICE analyses with an AC signal source (and DC bias voltage) tells the same story: a high voltage gain

Example circuit

Common-base circuit for SPICE AC analysis.

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

The AC SPICE analysis in Table below at a single frequency of 2 kHz provides input and output voltages for gain calculation.

Common-base AC analysis at 2 kHz– netlist followed by output.

Voltage figures from the second analysis (Table above) show a voltage gain of 42.74 (4.274 V / 0.1 V), or 32.617 dB:

Here’s another view of the circuit in the figure below, summarizing the phase relations and DC offsets of various signals in the circuit just simulated.

Phase relationships and offsets for NPN common base amplifier.

. . . and for a PNP transistor: Figure below.

Phase relationships and offsets for PNP common base amplifier.

Predicting Voltage Gain

Predicting voltage gain for the common-base amplifier configuration is quite difficult, and involves approximations of transistor behavior that are difficult to measure directly. Unlike the other amplifier configurations, where voltage gain was either set by the ratio of two resistors (common-emitter) or fixed at an unchangeable value (common-collector), the voltage gain of the common-base amplifier depends largely on the amount of DC bias on the input signal. As it turns out, the internal transistor resistance between emitter and base plays a major role in determining voltage gain, and this resistance changes with different levels of current through the emitter.

While this phenomenon is difficult to explain, it is rather easy to demonstrate through the use of computer simulations. SPICE simulations on a common-base amplifier circuit (Figure previous), changing the DC bias voltage slightly (vbias in Figure below ) while keeping the AC signal amplitude and all other circuit parameters constant. As the voltage gain changes from one simulation to another, different output voltage amplitudes will be noted.

Although these analyses will all be conducted in the “transfer function” mode, each was first “proved” in the transient analysis mode (voltage plotted over time) to ensure that the entire wave was being faithfully reproduced and not “clipped” due to improper biasing. See “*.tran 0.02m 0.78m” in Figure below, the “commented out” transient analysis statement. Gain calculations cannot be based on waveforms that are distorted. SPICE can calculate the small-signal DC gain for us with the “.tf v(4) vin” statement. The output is v(4) and the input as vin .

SPICE net list: Common-base, transfer function (voltage gain) for various DC bias voltages. SPICE net list: Common-base amp current gain; Note .tf v(4) vin statement. Transfer function for DC current gain I(vin)/Iin; Note .tf I(vin) Iin statement.

At the command line, spice -b filename.cir produces a printed output due to the .tf statement: transfer_function, output_impedance, and input_impedance. The abbreviated output listing is from runs with vbias at 0.85, 0.90, 0.95, 1.00 V as recorded in Table below.

SPICE output: Common-base transfer function.

A trend should be evident in the Table above. With increases in DC bias voltage, voltage gain (transfer_function) increases as well. We can see that the voltage gain is increasing because each subsequent simulation (vbias= 0.85, 0.8753, 0.90, 0.95, 1.00 V) produces greater gain (transfer_function= 37.6, 39.4 40.8, 42.7, 44.0), respectively. The changes are largely due to minuscule variations in bias voltage.

The last three lines of Table above (right) show the I(v1)/Iin current gain of 0.99. (The last two lines look invalid.) This makes sense for β=100; α= β/(β+1), α=0.99=100/(100-1). The combination of low current gain (always less than 1) and somewhat unpredictable voltage gain conspire against the common-base design, relegating it to a few practical applications.

Those few applications include radio frequency amplifiers. The grounded base helps shield the input at the emitter from the collector output, preventing instability in RF amplifiers. The common base configuration is usable at higher frequencies than the common emitter or common collector. See “Class C common-base 750 mW RF power amplifier” Ch 9 . For a more elaborate circuit see “Class A common-base small-signal high gain amplifier” Ch 9 .

• Common-base transistor amplifiers are so-called because the input and output voltage points share the base lead of the transistor in common with each other, not considering any power supplies.
• The current gain of a common-base amplifier is always less than 1. The voltage gain is a function of input and output resistances, and also the internal resistance of the emitter-base junction, which is subject to change with variations in DC bias voltage. Suffice to say that the voltage gain of a common-base amplifier can be very high.
• The ratio of a transistor’s collector current to emitter current is called α. The α value for any transistor is always less than unity, or in other words, less than 1.

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Common Base Amplifier

• Boris Poupet
• bpoupet@hotmail.fr
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Presenting the Common Base Amplifier

In this article, we present the last topology of amplifiers for bipolar transistors known as the Common Base Amplifier (CBA). In Figure 1 below, the electric diagram of a CBA is presented, no particular bias circuit or decoupling capacitors are shown here.

Some specifications need to be highlighted for CBAs :

• The base is linked to the ground of the circuit, hence the name “Common Base”.
• The input signal is delivered to the emitter branch of the bipolar transistor.
• The output signal is taken to the collector branch of the bipolar transistor.

Further in the article, we will see that in many ways the CBA behaves in opposition with respect to the Common Collector Amplifier (CCA).

Equivalent circuit

An equivalent circuit of Figure 1 can be drawn considering the collector branch to be an ideal current source and the p/n junction between the collector and emitter branches to behave like a small diode resistance r e =25 mV/I out .

It can already be anticipated from Figure 2 that since I in =I out +I B (from Kirchhoff’s laws), the current gain A I =I out /I in of a CBA configuration is A I =1-(I B /I out )<1 . Therefore, the current gain of a CBA configuration is strictly lower than 1, so this type of amplifier cannot amplify currents . However, we will see further in the tutorial that the voltage gain is high.

Current gain

We have already seen in the previous paragraph that the current gain A I is strictly lower than one. To get the exact formula of A i , we consider as mentioned previously that I in =I out +I B . Moreover, we define I out =β×I B with β the transistor’s current gain. Notice that here I B ≠I out in contrary to the previous amplifier configurations CEA and CCA.

The output current satisfies I out =A I ×I in =β×I B , when isolating A I , it comes :

Dividing the numerator and denominator by I B , the term I out /I B =β appears and we get the exact expression of the current gain for a CBA configuration :

As an example, a bipolar transistor of gain β=200 has a current gain A I =200/201=0.995≅1 . Hence, the current gain of a CBA configuration can always be approximated by 1 without committing too much error. Since the current gain is equal to 1, the output current I out follows the input current I in , hence the other name commonly given to this configuration current follower/buffer .

Input resistance

As seen from the input in the emitter branch, the total input resistance is R E //r e where the symbol “//” denotes the fact that the emitter and small diode resistor are in parallel.

However, the emitter resistance R E is always much higher than the small diode resistance r e , hence it comes :

The input resistance of a CBA configuration is therefore equal to the small diode resistance r e between the emitter and collector branches, this value of impedance is very small.

Output resistance

On a real CBA configuration, a load R L is placed in parallel with the collector resistance R C . The output resistance is thus given by R out =R C //R L . If the load is chosen such as R L >>R C , the output resistance simplifies to R out =R C .

Voltage gain

It is considered in the following, such as proved previously that A I ≅1 . The voltage gain of a CBA configuration is thus given by the ratio A V =V out /V in where V out =R C ×I out and V in =(R E //r e )×I in . It comes afterwards that :

Since the collector resistance satisfies R C >>r e , the voltage gain of a CBA configuration is very high. We can moreover highlight that the voltage gain of a CBA configuration is the same as for Common Emitter Amplifiers except that the sign is here positive : the output voltage signal is in phase with the input voltage signal . This formula is valid if the load R L is considered to satisfy R L >>R C . However, in the general case, the expression of the voltage gain is :

Example : Voltage, Current and Power gains

In this section, we consider a real CBA configuration presented in Figure 3 with a voltage divider network to bias the base which is composed of two resistor R 1 and R 2 . Moreover, a load R L is in parallel with the collector resistance R C . Notice that a decoupling capacitor is added between the base and ground to make this diagram correct, but for the sake of simplicity, its value is not given and won’t be taken into account for the following calculations. Finally, the bipolar transistor’s current gain is β=100 .

The current gain of this CBA configuration is simply given by :

Before determining the voltage gain of this configuration, the first step is to calculate the total input and output resistances, and for that, we need the value of the small diode resistance.

The voltage drop in the collector resistance R C is given by :

Therefore, the current across the collector resistance is I C =V C /R C =0.97 V/5 kΩ= 194  μA . From this value, it comes that the small diode resistance is r e =25 mV/194 μA= 129 Ω .

The input resistance is hence given by :

Since in this configuration R L <R C , the parallel resistance R L //R C needs to be considered as the total output resistance :

Finally, the voltage gain is determined from Equation 4 :

In theory, the voltage can here be amplified by a factor 34.65. However, as seen during the Introduction to Electronic Amplifiers tutorial, the output voltage is limited by the power supply votage. Therefore, the output voltage here reaches only 2×V supply =20 V peak to peak instead of 2×34.65=69.3 V peak to peak and a rather important saturation effect will be observed.

It is interesting to check if our calculations are correct in determining both input and output currents :

• The input current is given by V in /R in =1 V/114.2 Ω, thus I in =8.76 mA .
• The output current is given by V out /R out =34.65 V/4 kΩ, thus I out =8.66 mA .

We can see that the input and output currents are approximately equal and the ratio I out /I in =0.99 gives again the current gain previously calculated.

If we consider that the output voltage can indeed be amplified by a factor 34.65, the power gain A P of this configuration is given by A I ×A V =0.99×34.65 , thus A P =34.3 . However, since R L =4×R C , only a fourth of the power is delivered to the load : A P,load =8.57 .

Using the simplified expression from Equation 3 R C /r e =5 kΩ/129 Ω gives a voltage gain A V =38.8 . The simplified value of the current gain is A I =1 which in turn gives a power gain A P =38.8 instead of 34.3 for the real value. The error E P  for the power gain is therefore :

In this tutorial, we dealt with many aspects of one of the three elementary topology of amplifier known as the Common Base Amplifier (CBA). We have seen that such a configuration cannot amplify currents since its current gain is approximately equal and strictly lower than 1, hence the name “current buffer/follower” often given to CBAs. However, we have seen through theory and an example that the voltage signal can be highly amplified and its voltage gain is only limited by the power supplied in the collector branch. As opposition to the Common Collector Amplifier , the input resistance of a CBA configuration is low and its output impedance is high. This feature makes CBAs very useful to interpose between low load inputs and high load outputs such as in radio frequency circuits. Finally, we have seen through an example how to practically calculate the voltage, current and power gains of a CBA configuration.

As a general conclusion, we have seen during this tutorial three elementary configurations of bipolar transistor-based amplifier : the Common Emitter Amplifier (CEA), the Common Collector Amplifier (CCA) and the Common Base Amplifier .

We summarize and give in the following a comparison of these different configurations:

• In absolute value, the voltage gain is the same for CEA and CBA configurations. However the CEA shifts the signal of a 180° phase since it has a sign “-“, therefore, the CEA inverts the signal.
• The input resistance is : approximately the same for CEA and CCA configurations.
• The output resistance is : the same for CEA and CBA configurations.
• The voltage gain is : high for CEA and CBA, ≅1 for CCA.
• The current gain is : high for CEA and CCA, ≅1 for CBA.
• The power gain is : very high for CEA, high for CBA, medium for CCA.

Finally, due to their different characteristics, the applications of these three configurations are also different :

• The CEA, due to its high voltage and current gains is used as a universal amplifier.
• The CCA, due to its high input and low output resistances is used as a step-down impedance adapter. It is also used as a current amplifier and an oscillator.
• The CBA, due to its low input and high output resistances is used as a step-up impedance adapter. It is also used as a voltage amplifier, an oscillator or a high frequency amplifier thanks to its good behavior in frequency.

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Common Base configuration, input and output characteristics

 common base configuration:.

Two terminals are needed for input and two terminals are needed for output, so one terminal is taken as common for both input and output.  Based on the terminal which is taken as common there are three types of configurations. They are

• Common base configuration
• Common emitter configuration
• Common collector configuration

In common base configuration base terminal taken as common for both input and output. Input is applied between emitter and base terminal and output is taken between collector and base terminal.

Common emitter configuration and Common collector configuration are widely used and common base configuration is the least used. 

Circuit diagram of Common Base NPN and PNP transistor:

In the Common base circuit for NPN and PNP the input is given between emitter and base terminals and output is taken from collector and base terminals. The input voltage is denoted as V BE and the output voltage is denoted as V CE . In all the configuration the base emitter junction is always forward biased and the collector base junction is reverse biased.

In the common base configuration of NPN circuit emitter is N type base is of P type and collector is of N type. The emitter base terminals are forward biased so the majority charge carriers in the emitter that is the electrons gets repelled by the negative applied voltage and in the same way the majority charge carriers in the base that is the holes gets repelled by the positive applied voltage. 

When free electrons from emitter move to the base the free electrons and the free holes combine with each other but since the base is very thin only some free electrons gets combined with the holes and the majority of the electrons are attracted towards the collector because of the positive terminal voltage connected to the collector. Thus the current flow through the output terminal.

Thus the emitter current is the sum of the base current and the collector current.

I E = I B +I C

In common base configuration, the input impedance is low and the output impedance is high and the overall power gain is low when compared with other configuration.

Input characteristics:

Input characteristics are the relationship between the input current and input voltage with constant output voltage. In common base configuration input current is emitter current I E and the input voltage is base emitter voltage VBE. The curve is plotted between I E and V BE keeping V CB as constant.

The V BE is increased keeping V CB constant, initially at zero and the input current I E is noted, similarly the V CB value is increased and kept constant and V BE is increased and the input current I E is noted.

Input side is forward biased so the input resistance is small so for a small increase in V BE there is rapid increase in the emitter current I E . As the output voltage V CB is increased the width of the depletion layer between emitter base decreases and the cut in voltage is reduced so the curve drifts to the left side.

Output characteristics:

Output characteristics are the relationship between output current I C and output voltage V CB keeping input current I E constant. When the input current I E is zero it is in cut off region. In saturation region both emitter base junction and collector base junction are forward biased. 

In active region I E is gradually increased and kept constant and output voltage V CB is increased further and the output current I C almost remains constant. So in active region curve is almost flat. Output voltage causes only a very little change in output current.

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