The accuracy is poor for input voltages under 1 V, however for less stringent applications, it can be  and  effective.

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Because the differential voltage on the LM101 is within the low mV range, the 2N5457 JFET will have linear resistance over several decades of resistance giving a good electronic gain control.

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Parts list :
R1,R2,R5:10 K
R3 100K
R4 : 1K ( Variable resistor)
D1 : 1N4148
Q1 : 2N3905
IC1, IC2 : CA 3080

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This is the schematic diagram of power booster circuit is capable of driving medium loads. The circuit as presented works with a NE5535 chip.

Other amps might be replaced only if R1 values are changed due to the Icc current needed by the amplifier. R1 needs to be calculated with the following expression :

R1 = 600 mW 
      Icc

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This amplifier will pull in all distant FM stations clearly. Active FM amplifier circuit is configured as a common-emitter tuned RF pre-amplifier wired around the VHF / UHF transistor MFE201. There are several other types of transistors that will probably work as well, such as NTE107, 2SC2570, etc. but this is untested.

Adjust capacitor trimmers C1 and C2 for maximum gain. Input coil L1 is made up of 4 turns of 20SWG enamelled copper wire over the 5mm diameter former. It is tapped at the first turn from ground lead side. Coil L2 is just like L1, but has only 3 turns. Pin configuration is shown in the diagram.

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Parts list :
R1 : 100K
R2 : 47 ohm
R3 : 4,7K
C1 :0.47uF
C2, C3 : 470uF
VR1 : 50K
Transistor : 2N3565 and 2N3706

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On figure 1 is a simple, inverting amplifier with class AB output signal amplitude value 1 kV, which uses two identical amplifying cells. Frequency response amplifier gain full located in the region from DC to 20 kHz.

You can provide gain and higher frequencies, but at a lower value of gain. Ratio of resistors R1 and R2 set the gain. This scheme eliminates the need for multiple components to the level shifts, which are commonly used in standard construction of the scheme. Positive and negative cells gain controlled in opposite phase.

Resistors R4 and R5, connected to the source of 15 V and -15 V, provide the required voltage shift guarantee always on the state of the output transistors. Fine tuning the value of resistors R4 and R5 can eliminate distortion output signal of the “step”. Zener D1 and D2 provides reverse bias photodiodes magnitude 6.2 V.

Resistors R10, R11, R12, and R13 form a local negative feedback to the output transistors. You must install four STW8N80 N-channel MOSFET transistor with a suitable heat sink to prevent overheating. The scheme does not require active protection against short circuits. One pair of 125 mA fuse high-voltage power lines is enough to protect circuits from damage.

Above shows the response to the scheme on a square wave of 10 kHz. The signal has no distortion at the top of the pulse and the front and rear edges are virtually identical. Below pic shown on the scheme in response to sine wave frequency of 20 kHz. Signal in both figures has an amplitude of 1 kV.

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These units can get acquainted with one embodiment of the amplifier is Class D. Controller ATtiny15L AVR family firm Atmel is perfectly suited for this purpose, because it contains a ten-digit analog-digital converter (ADC) and a timer with a pulse-width modulator (PWM).

ADC reference voltage is selected voltage (register bits ADMUX REFS1 = 0, REFS0 = 0). Signal input of the controller – one of the inputs of the ADC, in this case, the input – 3 (RS 4). PWM output is – OS timer output 1 (PB 1). Auxiliary outputs: RV0 and PB2 commute positive and negative wave signal -, respectively.

For pre-amplification signal required linear gain stage with gain K = 10, and the voltage at its output in the absence of a signal should be half the supply voltage, so the values ​​of resistors R1 and R2 are equal, and the operational amplifier (op amp) TL071 has a large input resistance.

The program uses two interrupts: the timer overflow T1 and at the end of the cycle of the ADC. ADC is configured to equalize the result to the right. At interruption of the ADC result is read into the working registers and shifted one digit to the right, thus, zero discharge of the high byte is the test switching the positive and negative half-waves. Overflow timer is updated contents of the register PWM.

For the signal required to control the bridge output amplifier is designed gate-4 and not (561LA7). The amplifier incorporates composite transistors KT972A, KT973A.

Adjustment of the amplifier should start with the pre-amplifier assembly, first of all, make sure that the output of op amp there is a voltage close to 2.5 V when powered by 5V

The next stage – the firmware of the microcontroller. To flash microcontroller programmer used Tiny15L PonyProg. It is convenient because it requires no additional power source (powered by the COM port of your computer) and does not contain microcontrollers. After programming the controller serve a test signal of 1 kHz from the generator to the input of op amp so that its output amplitude was less than 2 V. The outputs of the microcontroller RV0 and PB2 must be square wave with the same frequency as in PB1 – PWM signal with a frequency of approximately 100 kHz .

Require high sound quality from the amplifier should not be, the controller used in it are not designed to handle the audio signal.

The firmware file is named Amp_t15.hex , the listing file (for fans of programming) Amp_t15.asm .

For writing the program used the standard software from Atmel – AVRStudio version 4.12, the debugging was done using version 3.14 VMLAB.

Download the archive with the files in the format of the scheme PCAD-2002 and firmware of the controller can be here
source :http://www.rlocman.ru

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An integral part of modern multimedia computer is the presence of two active audio speakers (mounted on a monitor or set next to it). Due to a number of physical limitations of the two small computer speakers can not achieve a good reproduction of the full range of sound frequencies. Especially collapses and distorted bass. Therefore, to obtain high-quality background music is usually used yet a third column – the subwoofer.

In high quality active speakers, such as company Altec Lansing, a built-in audio amplifier has an output labeled “SUB” (model ACS40, Fig. 5.1). It is designed to connect low-frequency active column.

This circuit uses IC K174UN14 from Russia. Its gain depends on the ratio of resistors R8-R9. Chain of elements R10-C6 band of amplified frequencies limits that increases the stability of work, excluding the occurrence of generation at frequencies above 100 kHz. Signals from the output of “SUB” come to the mixer, collected from field-effect transistors (VT1, VT2). Provides isolation between the mixer outputs and amplifies the signal by about 3 times. Resistor R6 sets the desired level of sound in the low column BA1 with respect to the lateral columns, and the main (overall) volume control and tone control is usually performed in the main amplifier on the body in a side column or from the program.

This scheme has the following main amplifier specifications:
1) output power at 4 ohm 4 W;
2) The band of amplified frequencies 60 … 120 000 Hz;
3) current consumption at Vin = 0 to 35 mA;
4) The supply voltage can range from 9 … 15 V;
5) THD of less than 0.5%.

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Chip operates at supply voltage range 1.8-6V. THD – 0.7%. Frequency – 40-40000Hz. Resistor R1 is used to adjust the volume.

Parts list :
R1 : 50K variable resistor
C1, C2, C3, C9 : 10uF/10V
C4, C5 : 3.3uF/10V
C6, C7, C8 : 0.1uF
C10, C11, C12 : 220uF/10V
DA1 : CXA1622P

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