BIPOLAR TRANSISTORS
Here we are at the lesson that marks your launch into adult life, because starting with this lesson you will be able to fully and meaningfully begin soldering your first simple circuits. And for the successful assembly and adjustment of these designs, it is very important to clearly understand what function a particular electronic component performs in a circuit, what parameters need to be monitored, etc. The practical work in this lesson will be no less interesting and comes down to performing the proposed experiments with a bipolar transistor. I think that after this practical work, questions about the principles of transistor operation will disappear on their own. If there are still questions, it only speaks of your dedication and desire to deeply understand the essence of what is happening.
The large family of semiconductor devices called transistors includes two types: bipolar and field-effect (FET). The first ones, to somehow distinguish them from the second, are often called conventional transistors. Bipolar transistors are the most widely used. This is exactly where we will begin. The term transistor is formed from two English words: transfer and resistor. In a simplified form, a bipolar transistor is a semiconductor plate with three (like a layer cake) alternating regions of different electrical conductivity (Fig. 1), which form two p-n junctions. The two outer regions have electrical conductivity of one type, and the middle one has electrical conductivity of the other type. Each region has its own contact lead.
If hole conductivity prevails in the outer regions and electronic conductivity in the middle (Fig. 1, a), then such a device is called a p-n-p structure transistor. A transistor with an n-p-n structure, on the contrary, has regions with electronic conductivity on the edges, and a region with hole conductivity between them (Fig. 1, b).
If you mentally cover any of the outer regions of the transistors shown schematically in (Fig. 1). What do you get? The remaining two regions are nothing more than a junction diode. If you cover the other outer region, you will also get a diode. This means a transistor can be thought of as two junction diodes with one common region, connected facing each other.
The common (middle) region of the transistor is called the base, one outer region is the emitter, and the second outer region is the collector. These are the three electrodes of the transistor.
The difference in the symbols of transistors of different structures on schematic diagrams lies only in the direction of the emitter arrow: in p-n-p transistors, it points towards the base, and in n-p-n transistors, it points away from the base.
During transistor operation, its emitter injects (emits) holes (in a p-n-p transistor) or electrons (in an n-p-n transistor) into the base, and the collector collects these electrical charges injected into the base by the emitter. Electron-hole junctions in a transistor can be obtained in the same way as in junction diodes. For example, to make a p-n-p transistor, a thin plate of germanium with electronic conductivity is taken, and pieces of indium are melted onto its surface. Indium atoms diffuse (penetrate) into the body of the plate, forming two p-type regions in it — the emitter and collector, and between them remains a very thin (a few microns) layer of n-type semiconductor — the base. Transistors manufactured using this technology are called alloy-junction transistors.
Remember the names of the p-n junctions of a transistor: between the collector and the base is the collector junction, and between the emitter and the base is the emitter junction.
The schematic layout and design of an alloy-junction transistor are shown in (Fig. 2). The device is assembled on a metal disk less than 10 mm in diameter. A crystal holder, which is the internal lead of the base, is welded to the top of this disk, and its external wire lead is at the bottom. The internal leads of the collector and emitter are welded to wires that are soldered into glass insulators and serve as the external leads of these electrodes. An all-metal cap protects the device from mechanical damage and the influence of light. This is how the most common low-power, low-frequency transistors of the MP39, MP40, MP41, MP42 series and their variants are constructed.
There are other ways of manufacturing transistors, for example, the diffused-alloy method (Fig. 3). The collector of a transistor manufactured using this technology is a plate of the original semiconductor. Two small balls of impurity elements are melted onto the surface of the plate very close to each other. Heating to a strictly defined temperature causes the diffusion of impurity elements into the semiconductor plate. In this case, one ball (in Fig. 3 - the right one) forms a thin base region in the collector, and the second (in Fig. 3 - the left one) forms an emitter region. As a result, two p-n junctions are obtained in the original semiconductor plate, forming a p-n-p transistor. This technology is used, in particular, to manufacture the most mass-produced low-power high-frequency transistors of the P401-P403, P422, P423, GT308 series.
Currently, there is a transistor designation system (in Russian standards) according to which mass-produced devices have designations consisting of four elements, for example: GT109A, KT315V, GT403I.
- The first element characterizes the semiconductor material: G (or 1) for Germanium, K (or 2) for Silicon (Kremniy), and A (or 3) for Gallium Arsenide.
- The second element is the letter T, standing for Transistor.
- The third element is a three-digit number (101 to 999) indicating the device's purpose according to a classification table.
- The fourth element is a letter indicating the variety (revision) within this series.
Here are some examples: GT109A is a germanium low-power low-frequency transistor, variety A; KT315V is a silicon low-power high-frequency transistor, variety V. Alongside this system, the old designation system continues to operate (e.g., P27, P401, MP39), since these transistors were developed before the modern marking was introduced.
The appearance of some bipolar transistors most widely used by radio amateurs is shown in (Fig. 4). The low-power low-frequency GT109 transistor is only 3.4 mm in diameter, designed for miniature radios and hearing aids. KT315 transistors are produced in plastic cases and are intended for amplifying and generating high-frequency oscillations. P213 is a powerful low-frequency germanium transistor widely used in audio frequency amplifier output stages. They heat up during operation, so they are usually mounted on special heat-dissipating radiators (heatsinks). KT904 is an ultra-high-frequency, high-power silicon transistor with a screw base for mounting on a radiator.
Connection Schemes and Main Parameters of Bipolar Transistors
So, a bipolar transistor, regardless of its structure, is a three-electrode device. Its electrodes are the emitter, collector, and base. To use a transistor as a voltage, current, or power amplifier, the input signal to be amplified can be applied to any two electrodes, and the amplified signal can be taken from two electrodes.
In this case, one of the electrodes will necessarily be common. It determines the name of the transistor connection method: Common Emitter (CE) circuit, Common Collector (CC) circuit, Common Base (CB) circuit.
- Connecting a p-n-p transistor in a CE scheme is shown in (Fig. 5, a). The power supply voltage is applied to the collector through resistor R_k (the load), and to the emitter — through a common grounded wire. The input signal is fed through a coupling capacitor C_c to the base and emitter, and the amplified signal is taken from the emitter and collector. The emitter is common to the input and output circuits. Depending on its amplifying properties, a transistor in a CE scheme can provide a 10 to 200-fold voltage gain and a 20 to 100-fold current gain. This is the most popular connection method among radio amateurs. Its main disadvantage is a relatively low input resistance (500-1000 Ohms).
- Connecting a transistor in a CC scheme is shown in (Fig. 5, b). The input signal is applied to the base and emitter through the emitter resistor R_e. The output signal is taken from the same resistor. The collector is common to both circuits here (for AC). A CC stage gives a voltage gain of slightly less than one. The current gain is roughly the same as in a CE scheme. However, its input resistance is very high (10 - 500 kOhms). This stage essentially repeats the input voltage, which is why transistors connected in this scheme are called emitter followers.
- Now about connecting a transistor in a CB scheme (Fig. 5, c). In this case, the base is grounded for alternating current through capacitor C_b. The input signal is applied to the emitter and base, and the amplified signal is taken from the collector and grounded base. The base is the common electrode. This stage gives a current gain of less than one, but a voltage gain identical to the CE scheme (10-200). Due to its very low input resistance (30-100 Ohms), it is mainly used in high-frequency oscillators and superregenerative stages.
The quality and amplifying properties of bipolar transistors are evaluated by several electrical parameters. From a practical point of view, you should primarily be interested in three main parameters:
1. Reverse collector current (I_cbo) - this is the uncontrolled current through the collector p-n junction created by minority charge carriers. It characterizes the quality of the transistor: the lower the numerical value of I_cbo, the higher the quality. For low-power transistors, it should not exceed 30 μA. Transistors with large I_cbo values are unstable in operation.
2. Static forward current transfer ratio (h21e or hFE) characterizes the amplifying properties. The letter 'e' indicates that it is measured in a Common Emitter circuit. It is the ratio of DC collector current to DC base current. The larger the value of h21e, the greater the signal amplification the transistor can provide.
3. Transition frequency (Cutoff frequency, f_T), expressed in kilohertz or megahertz, allows judging the possibility of using a transistor to amplify oscillations of certain frequencies.
Briefly about the Field-Effect Transistor (FET)
In this semiconductor device, the working current is controlled not by the current in the input (base) circuit, as in a bipolar transistor, but by the effect of an electric field on charge carriers. Hence the name field-effect transistor.
The schematic structure and design of a junction field-effect transistor (JFET) are shown in (Fig. 6). The base is an n-type silicon plate containing a thin p-type region. The plate is called the gate, and the p-type region is the channel. One end of the channel is the source, and the other is the drain. If a positive pole of a battery is connected to the source and a negative pole to the drain, a current (drain current, I_d) will appear in the channel. When a positive closing voltage acts on the gate relative to the source, the depletion region of the p-n junction expands (dashed lines in Fig. 6). This narrows the channel, increases its resistance, and reduces the drain current.
In a simplified form, this is how p-channel JFETs work (e.g., KP102, KP103). An n-channel FET works on the same principle, but the polarities of the gate bias and drain voltage are reversed. An FET is also a three-electrode device. It can be connected in three ways: Common Drain (CD), Common Source (CS), and Common Gate (CG).
An amplifier stage based on a field-effect transistor has a very high input resistance, measured in megohms. This is the main advantage of FETs over bipolar transistors.
The amplifying properties of an FET are characterized by its transconductance (S) — the ratio of the change in drain current to the change in gate voltage. The numerical value is expressed in milliamperes per volt (mA/V).
Another parameter is the cutoff voltage (U_gs(off)) — the reverse voltage at the gate-channel p-n junction at which the current through this junction decreases to zero.
Experiments with a Transistor
At the beginning of the lesson, I said that a bipolar transistor can be thought of as two planar diodes connected facing each other. This is easy to verify in experiments, for which you will need any used but working germanium low-frequency p-n-p transistor (like MP39, MP42, etc.), a 4.5 V battery, and a 2.5 V flashlight bulb.
Between the collector and base of the transistor, connect the battery and the bulb in series. If the positive pole of the battery is connected (through the bulb) to the collector, and the negative to the base (Fig. 1, a), the bulb should light up. With a different polarity of battery connection (Fig. 1, b), the bulb will not light up. In the first case, you applied a forward voltage to the collector p-n junction. The junction opened, its resistance was low, and a forward current flowed. In the reverse direction, the junction was closed, its resistance was high, and only a tiny reverse collector current (I_cbo) flowed, which could not heat the bulb filament.
The next experiment illustrates one of the transistor's operating modes according to the circuit shown in (Fig. 2). Connect the battery and the bulb in series between the emitter and collector. The positive pole goes to the emitter, the negative to the collector. The bulb won't light up. Connect the base to the emitter with a wire jumper (dashed line). It still won't light up. Now, remove the jumper and instead connect a resistor R_b (200-300 Ohms) and a 1.5 V AA battery so that the minus is on the base and the plus is on the emitter. Now the bulb should light up! Swap the polarity of the AA battery. The bulb goes out.
When you applied forward voltage from the AA cell to the emitter junction, it opened, and a forward current flowed, which in turn opened the second junction — the collector. The transistor opened, and a collector current I_c, which is many times greater than the emitter-base current, flowed through the emitter-base-collector circuit. This current heated the bulb. When you reversed the polarity, the voltage closed the emitter junction, the collector junction closed along with it, the current stopped, and the bulb went out. In these experiments, the transistor was in one of two states: open or closed. Switching was triggered by the base voltage U_b. This is called the switching mode or key mode, widely used in digital automation.
To illustrate the transistor working in amplification mode, we can do the following experiment (Fig. 3). In the collector circuit, include an electromagnetic earphone B1, and between the base and the minus of the power source, place a resistor R_b (200 - 250 kOhms). Connect a second earphone B2 to the base-emitter path through a coupling capacitor C_c (0.1 - 0.5 μF). You've got a simple amplifier! If your friend speaks softly into B2, you will hear it loudly in B1.
What are the functions of R_b and C_c? Through resistor R_b, a small negative voltage, called bias voltage, is applied to the base, which opens the transistor and ensures it operates in the linear amplification mode. Without it, the transistor would act like a diode and cut off half of the AC signal, causing distortion. Capacitor C_c is a coupling element that passes audio frequency AC signals but blocks DC current from the base circuit.
You must remember: for a transistor to work in amplification mode, a constant DC bias voltage must be applied to its base (relative to the emitter) along with the AC signal to be amplified. This opens the transistor and eliminates "crossover" type distortion.
For germanium transistors, the bias should be 0.1-0.2 V, and for silicon transistors 0.5-0.7 V.
Moving on to the next lesson!
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