Configuraciones del transistor bipolar

Hay tres tipos de configuraciones típicas en los amplificadores con transistores, cada una de ellas con características especiales que las hacen mejor para cierto tipo de aplicación. y se dice que el transistor no está conduciendo. Normalmente este caso se presenta cuando no hay corriente de base (Ib = 0)

Amplificador emisor común

Para que una señal esa amplificada tiene que ser una señal de corriente alterna.

No tiene sentido amplificar una señal de corriente continua, por que ésta no lleva ninguna información.

En un amplificador de transistores están involucradas los dos tipos de corrientes (alterna y continua).

La señal alterna es la señal a amplificar y la continua sirve para establecer el punto de operación del amplificador.

Este punto de operación permitirá que la señal amplificada no sea distorsionada.

Amplificador colector común

El amplificador seguidor emisor, también llamado colector común, es muy útil pues tiene una impedancia de entrada muy alta y una impedancia de salida baja.

Nota: La impedancia de entrada alta es una característica deseable en una amplificador pues, el dispositivo o circuito que lo alimenta no tiene que entregarle mucha corriente (y así cargarlo) cuando le pasa la señal que se desea amplificar.

Amplificador base común

Nota: Corriente de colector y corriente de emisor no son exactamente iguales, pero se toman como tal, debido a la pequeña diferencia que existe entre ellas, y que no afectan en casi nada a los circuitos hechos con transistores.

BJT

The Bipolar Junction Transistor (BJT) is an extremely common electronic device to all forms of electronic circuits. It can be used for a number of useful applications such as an amplifier, a switch, a buffer, an oscillator, a nonlinear circuit – so forth.

The BJT is made by P and N type semiconductor material, which should be familiar from the study of diodes. The BJT is a three terminal device (Fig 1).

Figure 1 – The BJT

The three terminals are Base, Collector and Emitter. The emitter terminal always has an arrow. The collector is always on the opposite side of the emitter and the base is the other remaining terminal on the left. Note that this is the conventional schematic diagram of a BJT transistor. Furthermore, there are two types of BJT transistors. They are the NPN type, and the PNP type. Figure 2 illustrates this:

(a) NPN (b) PNP

Figure 2 – The two types of BJT


The letters b, e, and c have been used for abbreviations for the base, emitter and collector terminals respectively. An NPN transistor is always drawn with the arrow pointing outwards whilst the PNP transistor always has the arrow pointing inwards. And of course, remember that the arrow is always the emitter terminal. So which type is the transistor in Fig 1? – It is NPN. The other diagrams shown in Figure 2 illustrate why the transistors are called either NPN or PNP. It’s simply due to the semiconductor material used for each terminal.

Now, lets take a more detailed look:

Figure 3.


The arrows show the direction of DC current flow for both the NPN and PNP cases. In both cases the base current (Ib) is a very small current in the order of microamps whilst the collector current (Ic) and emitter current (Ie) are larger and in the order of milliamps. Note that for the NPN transistor, the base current flows into the transistor but for the PNP transistor, the base current flows out the transistor. Also note Ic and Ie always flow in the same direction and in the direction of the (black) arrow, the same arrow that tells us whether the transistor is PNP or NPN.

Now for the voltages:

The voltage at the base is normally written as Vb.
The voltage at the collector is normally written as Vc.
The voltage at the emitter is normally written Ve.

That part was easy, but what about the voltage between the collector and the emitter? Is it written as Vce or Vec? The convention is that the first subscript letter is the voltage that you are measuring and the second subscript letter is the reference. That means, if:

Vc = 6V (The voltage at the collector is 6 volts)
Ve = 2V (The voltage at the emitter is 2 volts)

Then Vce is 4V because the voltage at the collector is 4V higher than the voltage at the emitter. Also, Vec = -4V because the voltage at the emitter (measuring point) is 4V lower than the voltage at the collector (reference point). This concept is important and If you’re a bit lost read it again. The following diagram should summarize. This is the convention used for measuring voltages between terminals of the NPN and PNP transistors. The reason for this is that in these examples the first subscript letter is usually of higher voltage than the second, hence all variables listed below will have positive values.

Figure 4.

Heterojunction bipolar transistor

Bands in graded heterojunction NPN bipolar transistor. Barriers indicated for electrons to move from emitter to base, and for holes to be injected backward from base to emitter; Also, grading of bandgap in base assists electron transport in base region; Light colors indicate depleted regions

The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is common in modern ultrafast circuits, mostly RF systems Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. The figure shows that this difference in bandgap allows the barrier for holes to inject backward into the base, denoted in figure as Δφp, to be made large, while the barrier for electrons to inject into the base Δφn is made low. This barrier arrangement helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias, and thus reduces base current and increases emitter injection efficiency.

The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. In the more traditional BJT, also referred to as homojunction BJT, the efficiency of carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and base, which means the base must be lightly doped to obtain high injection efficiency, making its resistance relatively high. In addition, higher doping in the base can improve figures of merit like the Early voltage by lessening base narrowing.

The grading of composition in the base, for example, by progressively increasing the amount of germanium in a SiGe transistor, causes a gradient in bandgap in the neutral base, denoted in the figure by ΔφG, providing a "built-in" field that assists electron transport across the base. That drift component of transport aids the normal diffusive transport, increasing the frequency response of the transistor by shortening the transit time across the base.

Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by epitaxy techniques like MOCVD and MBE.

Active-mode PNP transistors in circuits

Structure and use of PNP transistor.

The diagram opposite is a schematic representation of a PNP transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from E to C, VEB must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different depending on the type of transistor and its biasing. This applied voltage causes the upper P-N junction to 'turn-on' allowing a flow of holes from the emitter into the base. In active mode, the electric field existing between the emitter and the collector (caused by VCE) causes the majority of these holes to cross the lower P-N junction into the collector to form the collector current IC. The remainder of the holes recombine with electrons, the majority carriers in the base, making a current through the base connection to form the base current, IB. As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents.

In the diagram, the arrows representing current point in the direction of conventional current – the flow of holes is in the same direction of the arrows because holes carry positive electric charge. In active mode, the ratio of the collector current to the base current is called the DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value. The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to as hfe. However, when there is no particular frequency range of interest, the symbol β is used

It should also be noted that the emitter current is related to VEB exponentially. At room temperature, an increase in VEB by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way.

Active-mode NPN transistors in circuits

Structure and use of NPN transistor. Arrow according to schematic.

The diagram opposite is a schematic representation of an NPN transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E, VBE must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different depending on the type of transistor and its biasing. This applied voltage causes the lower P-N junction to 'turn-on' allowing a flow of electrons from the emitter into the base. In active mode, the electric field existing between base and collector (caused by VCE) will cause the majority of these electrons to cross the upper P-N junction into the collector to form the collector current IC. The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, IB. As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents.

In the diagram, the arrows representing current point in the direction of conventional current – the flow of electrons is in the opposite direction of the arrows because electrons carry negative electric charge. In active mode, the ratio of the collector current to the base current is called the DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value (for example see op-amp). The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to as hfe. However, when there is no particular frequency range of interest, the symbol β is used
It should also be noted that the emitter current is related to VBE exponentially. At room temperature, an increase in VBE by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way.

AC Circuit Analysis

AC analysis is pretty tricky to get a grasp on an first but with a good understanding of the principals and a bit of practice it's a piece of cake.

I prefer to use the Hybrid pi model for AC analysis but there are other models equally as valid. The Hybrid p model for the transistor is shown below:

rp is used to model the input resistance of the transistor. It's actually modelling the AC resistance of the forward biased Base-Emitter PN junction. vp is just the voltage across rp. Also note that the AC base current, ib, flows through rp.

Lower case letters are always used for AC signals. For example, ib is the AC signal base current whereas Ib is the DC base bias current. The exception to this is Vp, which I always seem to write using a captial V, but that's just my personal choice.

The diamond shaped box with the arrow is a 'Voltage Controlled Current Source'. It's important that you understand what this is. It is a current source, whose current is controlled by a voltage which is somewhere else in the circuit. In this case, the current is controlled by the voltage across rp, which is Vp. gm is just a constant which determines how much of a change in current is caused by a certain change of the controlling voltage. The units of gm is amps per volt or, A/V.

An example,
for the voltage controlled current source shown above, a Vp of 5V peak to peak with a gm of 0.5A/V would result in a current flow of:

0.5A/V x 5V = 2.5A peak to peak.

It's also worth mentioning that from previous subjects, you learned that the resistance of an ideal current source is infinity. Hence whilst the input resistance to the circuit is rp, the output resistance of the circuit is infinity. In practice this is not true and the output resistance is just very large. This effect is modeled by another resistor ro connected between collector and emitter at the output but is not shown on the above diagram. It can be omitted as a simplification to the model.

So if gm is a constant for a given amplifier circuit, how do you work out it's value. There is a relationship relating gm to the collector current, it is:

gm=Ic/Vt

VT is another constant which is given by:

Vt=kT/q=25mV Aprox

This equation should be familiar from diode study. Hence:

gm=Ic/Vt=40,Ic(10)

So all you have to do to find gm is to calculate the DC current Ic and multiply by 40, remembering the units should be in A/V.

rp is easy to calculate as well, it is given by:

rp=b/gm(11)

Another useful thing to mention is that the Voltage controlled current source, "gm.Vp" is sometimes expressed in another form:

gm.Vp=b/rp.Vp=b.Ib(12) , where equation 11 has been rearranged and substituted in for gm and then Vp / rp is of course just equal to ib --> You can see this from ohms law looking at figure 9. The b.ib model for the voltage controlled current source comes in useful for the 'common collector' and 'common emitter with emitter resistor' amplifiers.

So that's the Hybrid p model, all you have to do is replace the transistor in your circuit with this model, and do some analysis.

Transistor DC Parameters

There are some important equations we need to look at first. Recall that Kirchoffs Current Law (KCL) states that the sum of all currents entering a node (a point) must equal the sum of all currents leaving the node. By taking a look at Fig 3 we can see then that for both the NPN and PNP transistors:

Ie = Ic + Ib (1) i.e. Current flowing into the transistor (Ic and Ib) equals current flowing out of the transistor (Ie) for the NPN, and Current in (Ie) equals current out (Ic and Ib) for the PNP.

There is a parameter called b (Beta) for every transistor, which is a constant. The value of b for transistors is normally between 50 – 500. Equation 2 states that the collector current is b times bigger than the base current. Hence b is simply a ratio between collector and base current. Recall that the base current is relatively small and the collector current is relatively large.

Ic = b.Ib (2).

For a transistor with a b =100 and Ic=1mA, then from equation 2, Ib = 10uA. Run through this in your head to make sure…

We can now substitute equation 2 into equation 1: The highlight shows the substitution

Ie = b.Ib + Ib, which simplifies to:

Ie = (b+1)Ib. (3)

We can now substitute equation (2) into equation (3) to obtain:

Ib = Ic/b (equation 2 rearranged)

Ie = (b+1).Ic/b , and rearrange to obtain:

Ic=b/(b+1).Ie

We now define a new parameter a (alpha) where

a=b/(b+1) (4)

Hence:

Ic = a.Ie. (5)

And that’s it. I highly recommend you go through the mathematics yourself and verify every step that I have done. Only after you do this will you fully understand.

In summary you should definitely try to remember the first two following equations as they crop up all the time. It’s also handy to remember the third one.

Ic = b.Ib
Ie = (b+1)Ib
a = b/b+1

E.g. For the transistor with b = 100, a = 100/(100+1) = 0.99
Hence from equation 5 you can see that Ic » Ie. This is true for all transistors with high b. Now take a look at the NPN transistor in Figure 3 again. The reason Ic is only approximately equal to Ie is because of the small base current that adds in to make Ie just a little bigger.

h-PARAMETER MODEL

Generalized h-parameter model of an NPN BJT.replace x with e, b or c for CE, CB and CC topologies respectively.

Another model commonly used to analyze BJT circuits is the "h-parameter" model, closely related to the hybrid-pi model and the y-parameter two-port, but using input current and output voltage as independent variables, rather than input and output voltages. This two-port network is particularly suited to BJTs as it lends itself easily to the analysis of circuit behaviour, and may be used to develop further accurate models. As shown, the term "x" in the model represents a different BJT lead depending on the topology used. For common-emitter mode the various symbols take on the specific values as:

• x = 'e' because it is a common-emitter topology


• Terminal 1 = Base


• Terminal 2 = Collector


• Terminal 3 = Emitter


• iin = Base current (ib)


• io = Collector current (ic)


• Vin = Base-to-emitter voltage (VBE)


• Vo = Collector-to-emitter voltage (VCE)

and the h-parameters are given by –

• hix = hie – The input impedance of the transistor (corresponding to the emitter resistance re).


• hrx = hre – Represents the dependence of the transistor's IB–VBE curve on the value of VCE. It is usually very small and is often neglected (assumed to be zero).


• hfx = hfe – The current-gain of the transistor. This parameter is often specified as hFE or the DC current-gain (βDC) in datasheets.


• hox = hoe – The output impedance of transistor. This term is usually specified as an admittance and has to be inverted to convert it to an impedance.

As shown, the h-parameters have lower-case subscripts and hence signify AC conditions or analyses. For DC conditions they are specified in upper-case. For the CE topology, an approximate h-parameter model is commonly used which further simplifies the circuit analysis. For this the hoe and hre parameters are neglected (that is, they are set to infinity and zero, respectively). It should also be noted that the h-parameter model as shown is suited to low-frequency, small-signal analysis. For high-frequency analyses the inter-electrode capacitances that are important at high frequencies must be added.

Electronic amplifier

An electronic amplifier is a device for increasing the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier may be considered as modulating the output of the power supply.

Types of amplifier

Amplifiers can be specified according to their input and output properties. They have some kind of gain, or multiplication factor relating the magnitude of the output signal to the input signal. The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage and power. In many cases, with input and output in the same units, gain will be unitless (although often expressed in decibels); for others this is not necessarily so. For example, a transconductance amplifier has a gain with units of conductance (output current per input voltage). The power gain of an amplifier depends on the source and load impedances used as well as its voltage gain; while an RF amplifier may have its impedances optimized for power transfer, audio and instrumentation amplifiers are normally employed with amplifier input and output impedances optimized for least loading and highest quality. So an amplifier that is said to have a gain of 20dB might have a voltage gain of ten times and an available power gain of much more than 20dB (100 times power ratio), yet be delivering a much lower power gain if, for example, the input is a 600 ohm microphone and the output is a 47 kilohm power amplifier's input socket.

In most cases an amplifier should be linear; that is, the gain should be constant for any combination of input and output signal. If the gain is not constant, e.g., by clipping the output signal at the limits of its capabilities, the output signal will be distorted. There are however cases where variable gain is useful.

BJT Small Signal Analysis

• re transistor model – employs a diode and controlled current source to duplicate the behavior of a transistor in the region of interest.
  
• The re and hybrid models will be used to analyze small-signal AC analysis of standard transistor network configurations.
  

Ex: Common-base, common-emitter and common-collector configurations.

• The network analyzed represent the majority of those appearing in practice today.
  

AC equivalent of a network is obtained by:

1. Setting all DC sources to zero
2. Replacing all capacitors by s/c equiv.
3. Redraw the network in more convenient and logical form

Common-Emitter (CE) Fixed-Bias Configuration

The input (Vi) is applied to the base and the output (Vo) is from the collector.
The Common-Emitter is characterized as having high input impedance and low output impedance with a high voltage and current gain.

Removing DC effects of VCC and Capacitors

re Model

Phase Relationship

The phase relationship between input and output is 180 degrees.
The negative sign used in the voltage gain formulas indicates the inversion.

CE – Voltage-Divider Bias Configuration

re Model

Phase Relationship

A CE amplifier configuration will always have a phase relationship between input and output is 180 degrees. This is independent of the DC bias.

Common-Base (CB) Configuration

The input (Vi) is applied to the emitter and the output (Vo) is from the collector.
The Common-Base is characterized as having low input impedance and high output impedance with a current gain less than 1 and a very high voltage gain.

re Model

Collector DC Feedback Configuration

The network has a dc feedback resistor for increased stability, yet the capacitor C3 will shift portions of the feedback resistance to the input and output sections of the network in the ac domain. The portion of RF shifted to the input or output side will be determined by the desired ac input and output resistance levels.

re Model

Approximate Hybrid Equivalent Circuit

The h-parameters can be derived from the re model:

hie = bre hib = re
hfe = b hfb = -a
hoe = 1/ro

The h-parameters are also found in the specification sheet for the transistor.

Approximate Common-Emitter Equivalent Circuit

Hybrid equivalent model re equivalent model

Approximate Common-Base Equivalent Circuit

Hybrid equivalent model re equivalent model