common base transistor characteristics experiment readings

Input and output characteristics of common base configuration

Since transistor is a three terminal device, therefore, it can be connected in a circuit in three different ways:

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

Definition – In common base configuration , base terminal of transistor is common between the input and output circuits.

The graphs showing the relationship between different currents and voltages of a transistor are known as the characteristics of the transistor . The characteristics of a transistor are of two types:

• Input Characteristics.
• Output Characteristics.
• 1.1.1 Early effect or base width modulation
• 1.2.1 Active region
• 1.2.2 Saturation region
• 1.2.3 Cut off region
• 1.3 What is the output characteristics of common base configuration?
• 1.4 What is common base characteristics?
• 1.5 What is a common base configuration?

The graph between voltages and currents when the base terminal of a transistor is common to input and output circuit are known as common base transistor characteristics of a transistor .

The circuit diagram to study the input and output characteristics of common base configuration of a transistor is shown in the figure.

Characteristics of a transistor in common base configuration are of two types:

• input characteristics common base configuration. 
• output characteristics common base configuration .

Input Characteristics of common base configuration

The graph showing the variation of emitter current ( ‎I E ) with the variation of emitter-base voltage ( ‎V EB ) when a collector-base voltage ( ‎V CB )  is kept constant is known as input characteristics of a transistor.

It can be written as  ‎ V EB = f ( V CB  , ‎I E ) ‎

A set of input characteristics are shown in the figure.

A transistor in fact consists of two diodes in series i.e. a PN junction diode in contact with another and np junction diode. So when the emitter-base junction is forward biased, the variation of  ‎I E with  V EB is similar to the forward characteristics of a PN junction diode .

From the input characteristics, it is clear that there is a cut in or off set of threshold voltage below which there is no emitter current or the emitter current is very small. The value of threshold voltage = 0.1 V for Ge transistor and threshold voltage = 0.5 V for Si transistor.

Early effect or base width modulation

When a transistor is not biased, the width of the base region is W as shown in the figure. When the emitter-base junction is forward biased and the collector-base junction is Reverse Biased, emitter current ‎I E  increases with an increase in V EB at a fixed V CB .

However, when V CB increasing, the width of the depletion layer at the collector-base junction increases. As a result of this, the effective width of the base region decreases.. if the width of the depletion layer at the collector junction penetrating the base region is W’, then the effective width of the base region is equal to W – W’.

This change of the effective width of the base region by the reverse Bias Voltage of collector junction is known as early effect or base width modulation. Due to this effect, the gradient of the holes concentration in the base region increases with the increase in a reverse bias V CB for a given value of V EB . 

Thus, Emitter current I E increases with the increase in V CB . 

The Dynamic input resistance = Δ V CB / Δ I E .

Since a small change in V EB causes a large change in I E  so the Dynamic input resistance of junction Je is very small. At a certain reverse bias of collector junction, the depletion layer at the collector junction covers the whole base region so the effective width of the base reduced to zero. 

As a result of this, the potential barrier at the emitter junction decreases and hence large emitter current flows. This phenomenon is known as punch through . The value of the collector voltage at which punch through takes place is called punch-through voltage. 

Output Characteristics of common base configuration  

The variation of collector current I C with the collector-base voltage (V CB ) at constant emitter current ( I E ) is called output characteristics of the transistor in the common base configuration. It can be written as I c = F (V CB , I c ).

It is clear that the output characteristics of a transistor in the common base configuration are divided into three regions :

Active region

In this region, the emitter-base junction J E is forward biased and collector-base junction J C is reversed biased. If I E = 0, then the transistor behaves as a PN junction diode formed by base and collector part of the transistor. In this case, the collector current I C is equal to the reverse saturation current I CO . The magnitude of I CO is constant and independent of V CB.

Now let us suppose, small emitter current I E flows. Since only a few holes entering the base region from the emitter Recombine with the electrons in the base region, so most of the emitter current reaches the collector. Then the collector current, I C = I CO – α I E

Where α Is the fraction of total emitter current which represents the holes that have traveled from the emitter to the collector through the base.

The collector current I C depends only on the value of I E and independent of V CB.

Saturation region

In this region, both the junction i.e emitter-base junction ( J C ) and collected base junction ( J e ) is forward biased. Thus, for pnp transistor, the saturation region of the output characteristics lies to the left of V CB = 0 and above  I E =0 .

In fact, V CB Is slightly positive. As the collector junction is forward biased, so the collector current I C increases exponentially with V CB just like in PN junction diode. That is why I C increases with a small increase in V CB in this region. The holes from the Collector (P region) cross the junction to enter the base region under the condition of forwarding bias.

Thus, hole current flows from the collector region to the base region. This hole current flows just opposite to the hole current flows from the collector region to the base region. Hence, the net hole current fin the transistor decreases. If the forward bias is high, then the hole current from the collector  I C  becomes positive.

Cut off region

In this region, both the emitter-base junction and collector-base junction are reverse biased. This region lies I E =0 and to the right side of V CB =0 .

What is the output characteristics of common base configuration?

What is common base characteristics?

What is a common base configuration.

In common base configuration , base terminal of transistor is common between the input and output circuits.

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Common Base Transistor Characteristics:

To investigate the Common Base Transistor Characteristics of a diode (a two-terminal device), several levels of forward or reverse bias voltage are applied and the resulting current levels are measured. The characteristics of the device are then derived by plotting the graph of current versus voltage. Because a transistor is a three-terminal device, there are three possible connection arrangements (configurations) for investigating its Common Base Transistor Characteristics. From each of these configurations, three sets of Common Base Transistor Characteristics may be derived.

Figure 4-20 shows a pnp transistor with its base terminal common to both the input (EB) voltage and the output (CB) voltage. The transistor is said to be connected in common base configuration. Voltmeters and ammeters are included to measure the input and output voltages and currents.

Input Characteristics of Common Base Configuration:

To determine the Input Characteristics of Common Base Configuration, the output (CB) voltage is maintained constant, and the input (EB) voltage is set at several convenient levels. For each level of input voltage, the input current I E is recorded. I E is then plotted versus V EB to give the Input Characteristics of Common Base Configuration shown in Fig. 4-21.

Because the EB junction is forward biased, the common-base input characteristics are essentially those of a forward biased pn-­junction. Figure 4-21 also shows that for a given level of input voltage, more input current flows when higher levels of CB voltage are used. This is because larger CB ( reverse bias ) voltages cause the depletion region at the CB junction to penetrate deeper into the base of the transistor, thus shortening the distance and reducing the resistance between the EB and CB depletion regions.

Output Characteristics of Common Base Configuration:

The emitter current I E is held constant at each of several fixed levels. For each fixed level of I E , the output voltage V CB  is adjusted in convenient steps, and the corresponding levels of collector current I C are recorded. In this way, a table of values is obtained from which a family of output characteristics may be plotted. In Fig. 4-22 the corresponding I C  and V CB  levels obtained when I E was held constant at 1 mA are plotted, and the resultant Common Base Transistor Characteristics is identified as I E = 1 mA. Similarly, other characteristics are plotted for I E levels of 2 mA, 3 mA, 4 mA, and 5 mA.

The Output Characteristics of Common Base Configuration is shown in Fig. 4-22 show that for each fixed level of I E , I C is almost equal to I E and appears to remain constant when V CB  is increased. In fact, there is a very small increase in I C with increasing V CB . This is because the increase in collector-to-base bias voltage (V CB ) expands the CB depletion region, and thus shortens the distance between the two depletion regions. With I E held constant, the increase in I C is so small that it is noticeable only for large variations in V CB .

As illustrated in Fig. 4-22, when V CB  is reduced to zero, I C still flows. Even when the externally applied bias voltage is zero, there is still a barrier voltage existing at the CB junction, and this assists the flow of I C . The charge carriers which constitute I C are minority carriers as they cross the CB junction. Thus, the reverse-bias voltage V CB  and the (unbiased) CB barrier voltage assist their movement across the junction. To stop the flow of charge carriers, the CB junction has to be forward biased.

Consequently, as shown in Fig. 4-22, I C  is reduced to zero only when V CB is increased positively.

The region of the graph for the forward-biased CB junction is known as the saturation region , (see Fig. 4-22). The region in which the junction is reverse biased is named the active region, and this is the normal operating region for the transistor.

If the reverse-bias voltage on the CB junction is allowed to exceed the maximum safe limit specified by the manufacturer, device breakdown may occur. Breakdown, illustrated by the dashed lines in Fig. 4-22, can be caused by the same effects that make diodes breakdown. Breakdown can also result from the CB depletion region penetrating into the base until it makes contact with the EB depletion region, (Fig. 4-23). This condition is known as punch-through, or reach-through, and very large currents can flow when it occurs, possibly destroying the device. The extension of the depletion region is produced by the increase in V CB . So, it is very important to maintain V CB  below the maximum safe limit specified by the device manufacturer. Typical maximum V CB levels range from 25 V to 80 V.

Current Gain in Common Base Configuration:

The current gain characteristics (also termed the forward transfer characteristics) are a graph of output current (I C ) versus input current (I E ). They are obtained experimentally by use of the circuit in Fig. 4-20. V CB  is held constant at a convenient level, and I C  is measured for various levels of I E . I C  is then plotted versus I E , and the resultant graph is identified by the V CB  level, (see Fig. 4-24).

The CB current gain characteristics can be derived from the CB output characteristics, as shown in Fig. 4-25. A vertical line is drawn through a selected V CB  value, and corresponding levels of I E and I C  are read along the line. The I C  levels are then plotted versus I E , and the characteristic is labelled with the V CB  used. Because almost all of I E  flows out of the collector terminal as I C , V CB  has only a small effect on the current gain characteristics.

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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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Transistor Common Base Configuration – a Hidden Champion

The transistor common base configuration is just as simple as the other two (common collector, common emitter) configurations, but far less known and used – which is a regrettable mistake, because it is absolutely awesome and has little-known characteristics which we’ll look into right away!

Operating principle

At first look, the common base configuration seems “impossible” (ground at the base???) and even if it worked, the benefits wouldn’t be obvious. The base configuration circuit diagram above is the most commonly shown. It suggests that some voltage will be applied to the transistor emitter which should cause a voltage change at the collector output. But since the base is grounded the transistor will never conduct and hence the collector voltage will never change. So how does this circuit work?

What most of these simple diagrams don’t tell you is that they actually assume that the “input” voltage drops below 0V so that the potential difference between base and emitter becomes positive and the transistor conducts current which causes a voltage drop at “output” (collector). Since negative voltages aren’t practical, all we need to do is “lift” the base voltage higher than the input signal.

Assuming an input signal between 0V and 1V, the simplest working configuration consists of biasing the transistor base to a voltage higher than the input signal voltage.

In the diagram above the input signal (green) is a sine wave with 0,5V amplitude and creates an almost rectangular output signal (red) between 0V and 5V with pretty steep slopes.

What is there to be learned about the common base configuration from this experiment?

• It has steep amplification characteristics
• It amplifies voltage (a lot)
• It doesn’t change the phase (unlike ie. common emitter configuration)

For an amplifier, the resistor values are rather low and the power consumption is probably high. Let’s try this again, with a few changes. We’re going to bias the base with a low-impedance voltage source (0,8V) and a larger resistor (10KΩ) at the collector. There’s also a small resistor between emitter and input signal just to limit the maximum current through the transistor.

The output signal has again a rectangular shape with even steeper slopes; so increasing the collector resistor increases amplification. The 0,8V at the base and the very low resistor at the emitter (1Ω) are worth a closer look. This configuration potentially creates a large current which could destroy the transistor, so clearly this isn’t a real-world circuit, but it hints at another characteristic of the common base configuration: is has a low input impedance. So let’s summarise what we know so far about the common base configuration:

• It doesn’t change the phase
• It has a low input impedance
• It has a high output impedance

Applications

Wikipedia mentions three fields of application: high frequency amplification, digital circuits and low impedance amplification such as in microphones.

The common base configuration has a low input capacitance and isn’t affected much by the Miller effect which caps high frequencies. The easiest way to understand the Miller effect is to imagine the NPN (or PNP) transistor as a semiconductor sandwich stacked with three layers.

In a way, this looks like two capacitors in series with the collector, base and emitter bridged by those capacitors. While low, the transistor’s own capacitance will attenuate high frequencies, reduce amplification and increase leaking. Common base configuration doesn’t suffer from this effect so much because the base-emitter current is already high while the base-emitter voltage is low: there isn’t much potential difference left to bridge for the intrinsic capacitance.

The next application is digital circuits, if the steep amplification slopes didn’t give this application already away. The utility in discrete circuits is kind-of obvious; the clear separation between “high” and “low” voltage levels, the low power requirements for driving the output signal and the low input capacitance (see Miller effect) make the common base configuration an interesting choice for fast switching digital circuits.

The last application is circuits where a low input impedance is required. Counter-intuitively, this is a great use-case for buffer circuits. A buffer circuit is meant to feed the output signal of one circuit to the input of another circuit without drawing much power from the first circuit. A classic example is the power amplification of an LC oscillator which produces a weak sine signal. We want to grab that sine and amplify its power, eg. because we want to send it into an antenna.

If we connect the antenna directly to the oscillator (the wavy circle on the left), then signals the antenna catches from the environment (there is plenty of EM radiation around) will disturb the oscillator leading to a dirty signal. Even if we directly connect the power stage (the transistor at the right of the diagram) directly to the oscillator, it will draw too much power from the oscillator and dampen its oscillation. The buffer (the transistor in the middle of the diagram) has a high input impedance (1MΩ), draws very little power from the oscillator and thus doesn’t interfere with it.

But low interference can be achieved not only with high input impedance (drawing little power), but also with low input impedance when used correctly. The trick here is to make the buffer a part of the input circuit (eg. the oscillator). Since the buffer has a low impedance, it would look just like a connecting wire with a slightly higher resistance.

The input signal in this configuration is a sine wave that oscillates between -0,5V and +0,5V and is coupled through a transformer to a common base configuration. The base is biased in such a way that the collector (output) rests at 2,5V (half the operating voltage). This configuration interferes very little with the input signal; the transformer can have an almost arbitrarily low impedance. As the common base configuration is driven by input current and not input voltage, the transformer can be a step down transformer which creates very little load on the oscillator. By making the transformer part of the input circuit (eg. the transformer replaces the coil in the LC part), it can replace a wire in the input circuit and drive all its current through that transformer.

Wikipedia mentions that the configuration combo of transformer and common base is used with microphones which seems to make sense, as the circuit doesn’t draw any power from the oscillating membrane and thus doesn’t impede its movement.

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Transistor Characteristics Experiment Readings

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