Do-it-yourself Unch on field-effect transistors. Low frequency amplifiers based on field effect transistors. Schematic diagram of an amplifier using field-effect transistors

The figure shows the circuit of a 50 W amplifier with MOSFET output transistors.
The first stage of the amplifier is a differential amplifier using transistors VT1 VT2.
The second amplifier stage consists of transistors VT3 VT4. The final stage of the amplifier consists of MOSFETs IRF530 and IRF9530. The amplifier output is connected through coil L1 to an 8 Ohm load.
The chain consisting of R15 and C5 is designed to reduce noise levels. Capacitors C6 and C7 are power filters. Resistance R6 is designed to regulate the quiescent current.

Note:
Use bipolar power supply +/-35V
L1 consists of 12 turns of insulated copper wire with a diameter of 1 mm.
C6 and C7 should be rated at 50V, the remaining electrolytic capacitors at 16V.
A heatsink for the MOSFETs is required. Dimensions 20x10x10 cm made of aluminum.
Source - http://www.circuitstoday.com/mosfet-amplifier-circuits

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Field-effect transistor (FET) amplifiers have a high input impedance. Typically, such amplifiers are used as the first stages of pre-amplifiers, DC amplifiers for measuring and other electronic equipment.
The use of amplifiers with high input impedance in the first cascades makes it possible to match signal sources with high internal resistance with subsequent more powerful amplifier cascades that have a low input impedance. Amplification stages using field-effect transistors are most often implemented using a common-source circuit.

Since the bias voltage between the gate and source is zero, the rest mode of the transistor VT is characterized by the position of point A on the drain-gate characteristic at U ZI = 0 (Fig. 15, b).
In this case, when an alternating harmonic (i.e., sinusoidal) voltage U 3G with amplitude U m3 arrives at the input of the amplifier, the positive and negative half-cycles of this voltage will be amplified unequally: with a negative half-cycle of the input voltage U 3G, the amplitude of the alternating component of the drain current I" mc will be greater, than with a positive half-cycle (I"" mc), since the slope of the drain-gate characteristic in the AB section is greater compared to the slope in the AC section: As a result, the shape of the alternating component of the drain current and the alternating voltage it creates on the load U OUT will differ from the shape input voltage, that is, distortion of the amplified signal will occur.
To reduce signal distortion when amplifying it, it is necessary to ensure that the field-effect transistor operates at a constant slope of its drain-gate characteristic, that is, in the linear section of this characteristic.
For this purpose, a resistor Ri is included in the source circuit (Fig. 16, a).

The drain current I C0 flowing through the resistor creates a voltage across it
U Ri = I C0 Ri, which is applied between the source and the gate, including the EAF formed between the gate and source areas, in the opposite direction. This leads to a decrease in the drain current and the operating mode will be characterized in this case by point A" (Fig. 16, b).

To prevent a decrease in the gain, a high-capacity capacitor C is connected in parallel with the resistor Ri, which eliminates the negative feedback on the alternating current formed by the alternating voltage across the resistor Ri. In the mode characterized by point A", the slope of the drain-gate characteristic when amplifying the alternating voltage remains approximately the same when amplifying the positive and negative half-cycles of the input voltage, as a result of which the distortion of the amplified signals will be insignificant
(sections A"B" and "A"C" are approximately equal).
If in quiescent mode the voltage between the gate and the source is denoted by U ZIO, and the drain current flowing through the PT is I C0, then the resistance of the resistor Ri (in ohms) can be calculated using the formula:
Ri =1000 U ZIO /I C0,
into which the drain current I C0 is substituted in milliamps.
The amplifier circuit shown in Fig. 15 uses a FET with a control p-n junction and a p-type channel. If a similar transistor is used as a PT, but with an n-type channel, the circuit remains the same, and only the polarity of the power supply connection changes.
Amplifiers made on field-effect MOS transistors with an induced or built-in channel have even greater input resistance. At constant current, the input impedance of such amplifiers can exceed 100 MΩ. Since their gate and drain voltages have the same polarity, to provide the required bias voltage in the gate circuit, you can use the voltage of the power supply G C by connecting it to a voltage divider connected at the input of the transistor in the manner shown in Fig. 17.

Common Drain Amplifiers

The circuit of a DC amplifier with a common drain is similar to that of an amplifier with a common collector. Figure 18a shows a circuit of an amplifier with a common drain on a PT with a control p-n junction and a p-type channel.

Resistor Ri is included in the source circuit, and the drain is directly connected to the negative pole of the power source. Therefore, the drain current, which depends on the input voltage, creates a voltage drop only across the resistor Ri. The operation of the cascade is illustrated by the graphs shown in Fig. 18b for the case when the input voltage has a sinusoidal shape. In the initial state, a drain current I C0 flows through the transistor, which creates a voltage U I0 (U OUT0) across the resistor R and. During the positive half-cycle of the input voltage, the reverse bias between the gate and source increases, which leads to a decrease in the drain current and the absolute value of the voltage across the resistor Ri. In the negative half-cycle of the input voltage, on the contrary, the gate bias voltage decreases, the drain current and the absolute value of the voltage across the resistor Ri increase. As a result, the output voltage removed from the resistor Ri, i.e., from the source of the PT (Fig. 18b), has the same shape as the input voltage.
In this regard, amplifiers with a common drain are called source followers (the source voltage repeats the input voltage in shape and value).

Low-frequency amplifiers are very popular among radio electronics enthusiasts. Unlike the previous scheme, this power amplifier based on field-effect transistors consists mainly of transistors and uses an output stage that, with a bipolar supply voltage of 30 volts, can provide up to 70 W output power on speakers with a resistance of 4 Ohms.

Schematic diagram of an amplifier using field-effect transistors

The amplifier is assembled on the basis of an operational amplifier TL071 (IO1) or any similar one, which creates the main amplification of the differential signal. The amplified low-frequency signal from the output of the op-amp, most of which flows through R3 to the midpoint. The remainder of the signal is sufficient to be directly amplified by the IRF9530 (T4) and IRF530 (T6) MOSFETs.

Transistors T2, T3 and their surrounding components serve to stabilize the operating point of the variable resistor, since it must be correctly set in the symmetry of each half-wave across the amplifier load.

All parts are assembled on a single-sided printed circuit board. Please note that three jumpers must be installed on the board.

Amplifier settings

Setting up the amplifier is best done by applying a sinusoidal signal to its input and connecting a load resistor with a value of 4 ohms. After this, resistor R12 is installed in such a way that the signal at the output of the amplifier is symmetrical, i.e. the shape and size of the positive and negative half-waves were the same at maximum volume.

A simple transistor amplifier can be a good tool for studying the properties of devices. The circuits and designs are quite simple; you can make the device yourself and check its operation, take measurements of all parameters. Thanks to modern field-effect transistors, it is possible to make a miniature microphone amplifier from literally three elements. And connect it to a personal computer to improve sound recording parameters. And the interlocutors during conversations will hear your speech much better and more clearly.
Frequency characteristics
Low (audio) frequency amplifiers are found in almost all household appliances - stereo systems, televisions, radios, tape recorders, and even personal computers. But there are also RF amplifiers based on transistors, lamps and microcircuits. The difference between them is that the ULF allows you to amplify the signal only at the audio frequency that is perceived by the human ear. Transistor audio amplifiers allow you to reproduce signals with frequencies in the range from 20 Hz to 20,000 Hz.
Consequently, even the simplest device can amplify the signal in this range. And it does this as evenly as possible. The gain depends directly on the frequency of the input signal. The graph of these quantities is almost a straight line. If a signal with a frequency outside the range is applied to the amplifier input, the quality of operation and efficiency of the device will quickly decrease. ULF cascades are assembled, as a rule, using transistors operating in the low and mid-frequency ranges.
Classes of operation of audio amplifiers
All amplifying devices are divided into several classes, depending on the degree of current flow through the cascade during the period of operation:
1. Class “A” - current flows non-stop during the entire period of operation of the amplifier stage.
2. In work class "B" current flows for half a period.
3. Class “AB” indicates that current flows through the amplifier stage for a time equal to 50-100% of the period.
4. In mode “C”, electric current flows for less than half the operating time.
5. ULF mode “D” has been used in amateur radio practice quite recently - a little over 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 a class “A” transistor amplifier is characterized by fairly small nonlinear distortions. If the incoming signal spits out higher voltage pulses, this causes the transistors to become saturated. In the output signal, higher ones begin to appear near each harmonic (up to 10 or 11). Because of this, a metallic sound appears, characteristic only of transistor amplifiers.
If the power supply is unstable, the output signal will be modeled in amplitude near the network frequency. The sound will become harsher on the left side of the frequency response. But the better the stabilization of the amplifier's power supply, the more complex the design of the entire device becomes. ULFs operating in class “A” have a relatively low efficiency - less than 20%. The reason is that the transistor is constantly open and current flows through it constantly.
To increase (albeit slightly) efficiency, you can use push-pull circuits. One drawback is that the half-waves of the output signal become asymmetrical. If you transfer from class “A” to “AB”, nonlinear distortions will increase by 3-4 times. But the efficiency of the entire device circuit will still increase. ULF classes “AB” and “B” characterize the increase in distortion as the signal level at the input decreases. But even if you turn up the volume, this will not help completely get rid of the shortcomings.
Work in intermediate classes
Each class has several varieties. For example, there is a class of amplifiers “A+”. In it, the input transistors (low voltage) operate in mode “A”. But high-voltage ones installed in the output stages operate either in “B” or “AB”. Such amplifiers are much more economical than those operating in class “A”. There is a noticeably lower number of nonlinear distortions - no higher than 0.003%. Better results can be achieved using bipolar transistors. The operating principle of amplifiers based on these elements will be discussed below.
But there is still a large number of higher harmonics in the output signal, causing the sound to become characteristically metallic. There are also amplifier circuits operating in class “AA”. In them, nonlinear distortions are even less - up to 0.0005%. But the main drawback of transistor amplifiers still exists - the characteristic metallic sound.
"Alternative" designs
This is not to say that they are alternative, but some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction increasingly prefer tube designs. Tube amplifiers have the following advantages:
1. Very low level of nonlinear distortion in the output signal.
2. There are fewer higher harmonics than in transistor designs.
But there is one huge disadvantage that outweighs all the advantages - you definitely need to install a device for coordination. The fact is that the tube stage has a very high resistance - several thousand Ohms. But the speaker winding resistance is 8 or 4 Ohms. To coordinate them, you need to install a transformer.
Of course, this is not a very 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 circuit is a hybrid one - which uses single-ended amplifiers that are not affected by negative feedback. Moreover, all these cascades operate in ULF class “A” mode. In other words, a power amplifier on a transistor is used as a repeater.
Moreover, the efficiency of such devices is quite high - about 50%. But you should not focus only on efficiency and power indicators - they do not indicate the high quality of sound reproduction by the amplifier. The linearity of the characteristics and their quality are much more important. Therefore, you need to pay attention primarily to them, and not to power.
Single-ended ULF circuit on a transistor
The simplest amplifier, built according to a common emitter circuit, operates in class “A”. The circuit uses a semiconductor element with an n-p-n structure. A resistance R3 is installed in the collector circuit, limiting the flow of current. The collector circuit is connected to the positive power wire, and the emitter circuit is connected to the negative wire. If you use semiconductor transistors with a p-n-p structure, the circuit will be exactly the same, you just need to change the polarity.
Using a decoupling capacitor C1, it is possible to separate the alternating input signal from the direct current source. In this case, the capacitor is not an obstacle to the flow of alternating current along the base-emitter path. The internal resistance of the emitter-base junction together with resistors R1 and R2 represent the simplest supply voltage divider. Typically, resistor R2 has a resistance of 1-1.5 kOhm - 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 HF amplifiers on transistors are made according to similar circuits, only they work a little differently.
In this case, the emitter voltage is 9 V and the drop in the “E-B” section of the circuit is 0.7 V (which is typical for transistors on silicon crystals). If we consider an amplifier based on germanium transistors, then in this case the voltage drop in the “E-B” section will be equal to 0.3 V. The current in the collector circuit will be equal to that flowing in the emitter. You can calculate it by dividing the emitter voltage by the resistance R2 - 9V/1 kOhm = 9 mA. To calculate the value of the base current, you need to divide 9 mA by the gain h21 - 9 mA/150 = 60 μA. ULF designs usually use bipolar transistors. Its operating principle is different from field ones.
On resistor R1, you can now calculate the drop value - this is the difference between the base and supply voltages. In this case, the base voltage can be found using the formula - the sum of the characteristics of the emitter and the “E-B” transition. When powered from a 20 Volt source: 20 - 9.7 = 10.3. From here you can calculate the resistance value R1 = 10.3 V/60 μA = 172 kOhm. The circuit contains capacitance C2, which is necessary to implement a circuit through which the alternating component of the emitter current can pass.
If you do not install capacitor C2, the variable component will be very limited. Because of this, such a transistor-based audio amplifier will have a very low current gain h21. It is necessary to pay attention to the fact that in the above calculations the base and collector currents were assumed to be equal. Moreover, the base current was taken to be the one that flows into the circuit from the emitter. It occurs only if a bias voltage is applied to the base output of the transistor.
But it must be taken into account that collector leakage current absolutely always flows through the base circuit, regardless of the presence of bias. In common emitter circuits, the leakage current is amplified by at least 150 times. But usually this value is taken into account only when calculating amplifiers based on germanium transistors. In the case of using silicon, in which the current of the “K-B” circuit is very small, this value is simply neglected.
Amplifiers based on MOS transistors
The field-effect transistor amplifier shown in the diagram has many analogues. Including using bipolar transistors. Therefore, we can consider, as a similar example, the design of an audio amplifier assembled according to a circuit with a common emitter. The photo shows a circuit made according to a common source circuit. R-C connections are assembled on the input and output circuits so that the device operates in class “A” amplifier mode.
The alternating current from the signal source is separated from the direct supply voltage by capacitor C1. The field-effect transistor amplifier must necessarily have a gate potential that will be lower than the same source characteristic. In the diagram shown, the gate is connected to the common wire via resistor R1. Its resistance is very high - resistors of 100-1000 kOhm are usually used in designs. Such a large resistance is chosen so that the input signal is not shunted.
This resistance almost does not allow electric current to pass through, as a result of which the gate potential (in the absence of a signal at the input) is the same as that of the ground. At the source, the potential turns out to be higher than that of the ground, only due to the voltage drop across resistance R2. From this it is clear that the gate has a lower potential than the source. And this is exactly what 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 design discussed above. And the input signal is shifted relative to the output signal by 180 degrees.
ULF with transformer at the output
You can make such an amplifier with your own hands for home use. It is carried out according to the scheme that works in class “A”. The design is the same as those discussed above - with a common emitter. One feature is that you need to use a transformer for matching. This is a disadvantage of such a transistor-based audio amplifier.
The collector circuit of the transistor is loaded by the primary winding, which develops an output signal transmitted through the secondary to the speakers. A voltage divider is assembled on resistors R1 and R3, which allows you to select the operating point of the transistor. This circuit supplies bias voltage to the base. All other components have the same purpose as the circuits discussed above.
Push-pull audio amplifier
It cannot be said that this is a simple transistor amplifier, since its operation is a little more complicated than those discussed earlier. In push-pull ULFs, the input signal is split into two half-waves, different in phase. And each of these half-waves is amplified by its own cascade, made on a transistor. After each half-wave has been amplified, both signals are combined and sent to the speakers. Such complex transformations can cause signal distortion, since the dynamic and frequency properties of two transistors, even of the same type, will be different.
As a result, the sound quality at the amplifier output is significantly reduced. When a push-pull amplifier operates in class “A”, it is not possible to reproduce a complex signal with high quality. The reason is that increased current constantly flows through the amplifier's shoulders, the half-waves are asymmetrical, and phase distortions occur. The sound becomes less intelligible, and when heated, signal distortion increases even more, especially at low and ultra-low frequencies.
Transformerless ULF
A transistor-based bass amplifier 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 better to get rid of them. A circuit made on complementary semiconductor elements with different types of conductivity turns out to be much more effective. Most modern ULFs are made precisely according to such schemes and operate in class “B”.
The two powerful transistors used in the design operate according to an emitter follower circuit (common collector). In this case, the input voltage is transmitted to the output without loss or gain. If there is no signal at the input, then the transistors are on the verge of turning on, but are still turned off. When a harmonic signal is applied to the input, the first transistor opens with a positive half-wave, and the second one is in cutoff mode at this time.
Consequently, only positive half-waves can pass through the load. But the negative ones open the second transistor and completely turn off the first. In this case, only negative half-waves appear in the load. As a result, the signal amplified in power appears at the output of the device. Such an amplifier circuit using transistors is quite effective and can provide stable operation and high-quality sound reproduction.
ULF circuit on one transistor
Having studied all the features described above, you can assemble the amplifier with your own hands using a simple element base. The transistor can be used domestic KT315 or any of its foreign analogues - for example BC107. As a load, you need to use headphones with a resistance of 2000-3000 Ohms. A bias voltage must be applied to the base of the transistor through a 1 MΩ resistor and a 10 μF decoupling capacitor. The circuit can be powered from a source with a voltage of 4.5-9 Volts, a current of 0.3-0.5 A.
If 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 using transistors and find out the collector current - it turns out to be equal to 1 mA. Having assembled this transistor amplifier circuit, you can test it. Connect a load 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 was assembled incorrectly. Double-check all connections and element ratings. To make the demonstration more clear, connect a sound source to the ULF input - the output from the player or phone. Listen to music and evaluate the sound quality.

Specifications
Maximum RMS power:
at RH = 4 Ohm, W 60
at RH = 8 Ohm, W 32
Operating frequency range. Hz 15...100 000
Harmonic distortion factor:
at f = 1 kHz, Pout = 60 W, RH = 4 Ohm, % 0.15
at f = 1 kHz, Pout = 32 W, RH = 8 Ohm, % 0.08
Gain, dB 25...40
Input impedance, kOhm 47

Settings

It is unlikely that any experienced experimenter will have difficulty achieving satisfactory results when constructing an amplifier according to this circuit. The main problems that should be considered are incorrect installation of elements and damage to MOSFETs due to improper handling or excitation of the circuit. The following troubleshooting checklist is suggested as a guide for the experimenter:
1. When assembling the PCB, first install the passive elements and make sure the polarity of the electrolytic capacitors is correct. Then install transistors VT1 ... VT4. Finally, install the MOSFETs, avoiding static charge by shorting the leads to ground at the same time and using a grounded soldering iron. Check the assembled board for correct installation of the elements. To do this, it will be useful to use the arrangement of elements shown in Fig. 2 Check the circuit boards for any solder shorts on the tracks and, if there are any, remove them. Check solder joints visually and electrically with a multimeter and redo if necessary.
2. Now the supply voltage can be applied to the amplifier and the quiescent current of the output stage can be set (50...100 mA). Potentiometer R12 is first set to the minimum quiescent current (to failure counterclockwise on the board topology Fig. 2). An ammeter with a measurement limit of 1 A is turned on in the positive branch of the power supply. By rotating the slider of resistor R12, the ammeter reading is 50...100 mA. Setting the quiescent current can be done without connecting a load. However, if a load speaker is included in the circuit, it must be protected by a DC overload fuse. When the quiescent current is set, an acceptable output offset voltage should be less than 100 mV.
Excessive or erratic changes in the quiescent current when adjusting R12 indicate the occurrence of generation in the circuit or incorrect connection of the elements. You should adhere to the recommendations described earlier (series connection of resistors in the gate circuit, minimizing the length of connecting conductors, common grounding). In addition, power decoupling capacitors must be installed in close proximity to the amplifier's output stage and the load grounding point. To avoid overheating of powerful transistors, quiescent current regulation should be performed with MOS transistors installed on the heat sink.
3.After establishing the quiescent current, the ammeter must be removed
from the positive supply circuit and to the amplifier input there can be
a working signal has been given. The input signal level to obtain full rated power must be as follows:
UBX = 150 mV (RH = 4 Ohm, Ki = 100);
UBX= 160 mV (RH = 8 Ohm, Ki = 100);
UBX = 770 mV (RH = 4 Ohm, Ki = 20);
UBX = 800 mV (RH = 8 Ohm, Ki = 20).
"Cutting" at the peaks of the output signal when operating at rated power indicates poor supply voltage regulation and can be corrected by reducing the amplitude of the input signal and reducing the amplifier's rated characteristics.
The amplifier's frequency response can be tested in the frequency range 15 Hz... 100 kHz using an audio test kit or a generator and an oscilloscope. Distortion of the output signal at high frequencies indicates the reactive nature of the load, and to restore the signal shape, it will be necessary to select the inductance value of the output inductor L1. The amplitude-frequency response at high frequencies can be equalized using a compensation capacitor connected in parallel with R6. The low-frequency part of the amplitude-frequency characteristic is corrected by elements R7, C2.
4. The presence of background (hum) most likely occurs in the circuit
when the gain is set too high. High input pickup
impedance is minimized by using shielded
cable grounded directly at the signal source. Low-frequency power ripples entering the input stage with the power supply
amplifier, can be eliminated by the capacitor SZ. Additional
background attenuation is carried out by a differential cascade
on transistors VT1, VT2 of the preamplifier. However, if the background source is the supply voltage, then the value of SZ, R5 can be selected to suppress the ripple amplitude.
5. If the output stage transistors fail due to a short circuit in the load or due to high-frequency generation, both MOS transistors must be replaced, and it is unlikely that other elements will fail. When installing a circuit of new devices, the setup procedure must be repeated.

Power supply diagram

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Amplifier circuit with changes: