Transistor amplifier: types, circuits, simple and complex
The simplest transistor amplifier can be a good tool for studying the properties of devices. Schemes and designs are quite simple, you can make the device yourself and check its operation, measure all parameters. Thanks to modern field-effect transistors, a miniature microphone amplifier can be made from just three elements. And connect it to a personal computer to improve the recording settings. Yes, and the interlocutors during conversations will be much better and clearer to hear your speech.
Low frequency (sound) amplifiers are available in almost all household appliances - music centers, televisions, radios, radio and even personal computers. But there are still RF amplifiers on transistors, lamps and microchips. Their difference is that ULF allows you to amplify the signal only audio frequency,which is perceived by the human ear. Amplifiers sound transistors can reproduce signals with frequencies in the range from 20 Hz to 20000 Hz.
Consequently, even the simplest device can amplify a signal in this range. And it does it as evenly as possible. The gain depends directly on the frequency of the input signal. The plot of these quantities is practically a straight line. If the amplifier input signal with a frequency out of range, the quality of work and the efficiency of the device will quickly decrease. ULF cascades are usually assembled on transistors operating in the low and mid-frequency ranges.
Sound Amplifier Performance Classes
All amplifying devices are divided into several classes, depending on what degree of flow during the period of operation of the current through the cascade:
- Class "A" - the current flows non-stop during the entire period of operation of the amplifier cascade.
- In the class of work "B" current flows for half the period.
- Class "AB" says that the current flows through the amplifier cascade for a time equal to 50-100% of the period.
- In mode “C”, an electrical current flows for less than half the time period of operation.
- Mode “D” ULF is used in amateur radio practice quite recently - a little more than 50 years. In most cases, these devices are implemented on the basis of digital elements and have a very high efficiency - over 90%.
The presence of distortion in various classes of low-frequency amplifiers
The working area of the transistor amplifier class "A" is characterized by relatively small nonlinear distortion. If the incoming signal emits pulses with a higher voltage, this causes the transistors to become saturated. In the output signal, higher ones (up to 10 or 11) begin to appear near each harmonic. Because of this, a metallic sound appears, characteristic of transistor amplifiers only.
If the power supply is unstable, the output signal will be amplitude modeled near the line frequency. The sound will become more rigid on the left side of the frequency response. But the better the stabilization of the amplifier power, the more difficult the design of the entire device becomes. ULF, working in class "A", have a relatively small efficiency - less than 20%.The reason is that the transistor is constantly open and the current through it flows continuously.
To increase (albeit minor) efficiency, you can use push-pull schemes. One drawback is that the half waves at the output signal become asymmetrical. If we translate from class “A” to “AV”, nonlinear distortions will increase by a factor of 3-4. But the efficiency of the entire circuit device will increase. ULF classes “AB” and “B” characterize the increase in distortion with a decrease in the signal level at the input. But even if you turn up the volume, it does not help completely get rid of the shortcomings.
Intermediate class work
Each class has several varieties. For example, there is a class of work amplifiers "A +". In it, the input transistors (low voltage) operate in the "A" mode. But high-voltage, installed in the output stages, work either in "B" or in "AV". Such amplifiers are much more economical than working in class "A". Noticeably fewer non-linear distortions — no more than 0.003%. Higher results can be achieved using bipolar transistors.The principle of operation of amplifiers on these elements will be discussed below.
But still there are a large number of higher harmonics in the output signal, causing the sound to become characteristic metallic. There are also amplifier circuits operating in the AA class. In them nonlinear distortions are even less - up to 0.0005%. But the main disadvantage of transistor amplifiers is still there - a characteristic metallic sound.
It cannot be said that they are alternative, just some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction, increasingly prefer tube designs. Lamp amplifiers have such advantages:
- Very low level of non-linear distortion in the output signal.
- Higher harmonics are less than in transistor designs.
But there is one huge disadvantage that outweighs all the advantages - you definitely need to install a device for approval. The fact is that the lamp cascade has a very large resistance - several thousand ohms. But resistance winding speakers - 8 or 4 ohms. To coordinate them, you need to install a transformer.
Of course, this is not a big drawback - there are also transistor devices that use transformers to match the output stage and the speaker system. Some experts argue that the most effective scheme is a hybrid - in which single-ended amplifiers are used that are not covered by negative feedback. And all these cascades function in the ULF class “A” mode. In other words, a power amplifier at a transistor is used as a repeater.
Moreover, the efficiency of such devices is quite high - about 50%. But you shouldn’t focus only on efficiency and power indicators - they do not speak about high quality sound reproduction by an amplifier. Of much greater importance are the linearity of the characteristics and their quality. Therefore, you need to pay attention primarily to them, and not to power.
Single-ended ULF transistor circuit
The simplest amplifier, built according to the scheme with a common emitter, works in class "A". The circuit uses a semiconductor element with an n-p-n structure. In the collector circuit installed resistance R3, limiting the flow of current.The collector circuit is connected to the positive power wire, and the emitter circuit is connected to the negative one. In the case of using semiconductor transistors with the p-n-p structure, the circuit will be exactly the same, only it will be necessary to change the polarity.
With the help of coupling capacitor C1, it is possible to separate the variable input signal from the DC source. In this case, the capacitor is not a barrier to the flow of alternating current along the base-emitter path. The internal resistance of the emitter-base junction together with the resistors R1 and R2 represent the simplest voltage divider. Typically, resistor R2 has a resistance of 1-1.5 kΩ - the most typical values for such circuits. In this case, the supply voltage is divided exactly in half. And if you power the circuit with a voltage of 20 volts, you can see that the value of the current gain h21 will be 150. It should be noted that the HF amplifiers on transistors are performed according to similar schemes, they only work a little differently.
At the same time, the emitter voltage is 9 V and the drop in the section of the EB circuit is 0.7 V (which is typical of transistors on silicon crystals). If we consider the amplifier on germanium transistors, then in this case the voltage drop in the section “EB” will be equal to 0.3 V.The current in the collector circuit will be equal to that flowing in the emitter. It is possible to calculate by dividing the emitter voltage by the resistance R2 - 9V / 1 kΩ = 9 mA. To calculate the base current value, it is necessary to divide 9 mA by the gain h21 - 9mA / 150 = 60 μA. In the construction of ULF bipolar transistors are usually used. The principle of operation is different from the field ones.
On the resistor R1, it is now possible to calculate the drop value - this is the difference between the base and power voltages. In this case, the base voltage can be recognized by the formula - the sum of the characteristics of the emitter and the “EB” transition. When powered from a source of 20 volts: 20 - 9.7 = 10.3. From this it is possible to calculate the value of resistance R1 = 10.3 V / 60 μA = 172 kΩ. In the circuit there is a capacitance C2, which is necessary for the realization of a circuit along which the variable component of the emitter current can pass.
If you do not install the capacitor C2, the variable component will be very limited. Because of this, such a transistor audio amplifier will have a very low current gain h21. It should be noted that in the above calculations, the base and collector currents were assumed to be equal.And the base current was taken by the one that flows into the circuit from the emitter. It occurs only under the condition that a bias voltage is applied to the output of the transistor base.
But keep in mind that absolutely always, regardless of the presence of bias, a collector leakage current flows through the base circuit. In circuits with a common emitter, the leakage current is increased at least 150 times. But usually this value is taken into account only when calculating amplifiers on germanium transistors. In the case of silicon, in which the current of the “K-B” circuit is very small, this value is simply neglected.
A field-effect transistor amplifier, presented in the diagram, has many analogues. Including using bipolar transistors. Therefore, we can consider as a similar example the design of a sound amplifier assembled according to a scheme with a common emitter. The photo shows a circuit made according to the circuit with a common source. On the input and output circuits collected R-C-connection, so that the device operates in the class "A" amplifier mode.
The alternating current from the signal source is separated from the DC supply voltage by the capacitor C1.A field-effect transistor amplifier must have a gate potential that will be lower than a similar source characteristic. In the presented circuit, the gate is connected to the common wire through a resistor R1. Its resistance is very large - usually 100-1000 kΩ resistors are used in the construction. Such a large resistance is chosen so that the input signal is not shunted.
This resistance almost does not pass an electric current, as a result of which the potential of the gate (in the absence of a signal at the input) is the same as that of the earth. At the source, the potential is higher than that of the earth, only due to the drop in voltage across the resistance R2. From this it is clear that at the gate the potential is lower than at the source. Namely, this is required for the normal functioning of the transistor. It is necessary to pay attention to the fact that C2 and R3 in this amplifier circuit have the same purpose as in the above construction. And the input signal is shifted relative to the output by 180 degrees.
ULF with output transformer
You can make such an amplifier with your own hands for home use. It is carried out according to the scheme working in the class "A".The design is the same as discussed above - with a common emitter. One feature is that you must use a transformer for matching. This is a disadvantage of this transistor audio amplifier.
The collector circuit of the transistor is loaded by the primary winding, which develops the output signal transmitted through the secondary to the speakers. The resistors R1 and R3 assembled voltage divider, which allows you to select the operating point of the transistor. With this chain, the supply of bias voltage to the base is provided. All other components have the same purpose as in the above schemes.
Push-pull audio amplifier
This is not to say that this is a simple transistor amplifier, since its operation is a bit more complicated than that of those considered earlier. In push-pull ULF, the input signal is split into two half-waves, different in phase. And each of these half-waves is amplified by its cascade performed on a transistor. After the amplification of each half-wave has occurred, both signals are connected and fed to the speakers. Such complex transformations can cause signal distortion, since the dynamic and frequency properties of two, even of the same type, transistors will be different.
As a result, the output of the amplifier significantly reduces the sound quality. When a push-pull amplifier in class "A" is in operation, it is not possible to reproduce the complex signal qualitatively. The reason is that an increased current flows continuously along the amplifier arms, half-waves are asymmetrical, and phase distortions occur. The sound becomes less legible, and when heated, the distortion of the signal is further enhanced, especially at low and ultra low frequencies.
The amplifier of the LF on the transistor, made using a transformer, despite the fact that the design may have small dimensions, is still imperfect. Transformers are still heavy and bulky, so it's best to get rid of them. Much more efficient is the circuit made on complementary semiconductor elements with different types of conductivity. Most of the modern ULF is performed according to such schemes and work in class "B".
Two powerful transistors used in the design work according to the emitter follower circuit (common collector). In this case, the input voltage is transmitted to the output without loss and amplification.If there is no signal at the input, then the transistors are on the verge of switching on, but they are still disabled. When a harmonic signal is applied to the input, the positive half-wave of the first transistor opens, and the second one is in the cut-off mode at this time.
Therefore, only positive half waves can pass through the load. But the negative ones open the second transistor and completely lock the first one. At the same time, only negative half-waves are loaded. As a result, the power amplified signal is output from the device. Such a transistor amplifier circuit is quite effective and capable of ensuring stable operation, high-quality sound reproduction.
ULF circuit on a single transistor
Having studied all the above features, you can assemble the amplifier with your own hands on a simple element base. The transistor can be used domestic KT315 or any of its foreign counterparts - for example, BC107. As a load, you need to use headphones, the resistance of which is 2000-3000 Ohms. At the base of the transistor, it is necessary to apply the bias voltage through a resistor of 1 Mom and a decoupling capacitor of 10 μF.The power supply of the circuit can be carried out from a source of 4.5-9 volts, current - 0.3-0.5 A.
If the resistance R1 is not connected, then there will be no current in the base and collector. But when connected, the voltage reaches a level of 0.7 V and allows a current of about 4 μA to flow. In this case, the current gain will be about 250. From here you can make a simple calculation of the amplifier transistors and find out the collector current - it is equal to 1 mA. By collecting this amplifier circuit on a transistor, you can test it. Connect the output to the output - headphones.
Touch the amplifier input with your finger - a characteristic noise should appear. If it is not there, then, most likely, the structure is assembled incorrectly. Recheck all connections and element ratings. To make the demonstration clearer, connect a sound source to the ULF input - output from a player or phone. Listen to music and enjoy the sound quality.