Posts Tagged ‘amplifier’

Telephone Amplifier

April 7th, 2010 by Thomson | No Comments | Filed in Coreelectronics

A telephone amplifier enables more than one person to follow a telephone conversation. The unit described here, in common with all normal units of this type, requires no direct connection to the telephone. Instead, the special pick-up coil has a built-in rubber suction cap that enables it to be easily attached to the telephone base. This produces a very weak signal from the magnetic field radiated by an inductive component inside the telephone, but satisfactory results can be obtained if it is fed to low noise, high gain amplifier. It would of course be possible to use a much simpler circuit if a direct connection to the telephone were to be made, but this would make installation more difficult and it is ILLEGAL to make a direct connection to a Post Office telephone anyway.

Telephone amplifier Telephone Amplifier

The preamplifier stage of the unit is based on IC1 which is a low noise op amp having a FET input stage. This is used in the conventional inverting audio amplifier mode and the negative feedback network, R1, 4, sets the voltage gain at about 40dB. (100 times C11 reduces the gain slightly at high frequencies in order to obtain an improved signal to noise ratio.

C4 couples the output from the preamplifier to volume control, RV1, and from here the signal is coupled to the power amplifier by C5. The output stage uses the TBA820M, a class B amplifier which will give an output power of a few hundred milliwatts rms. The closed loop voltage gain of the device is determined by the value of R6, about 25dB. (180 times) with the specified value. This gives the required very high overall gain in conjunction with the preamplifier’s gain. C7, R7 and C8 are needed in order to maintain stability.

The quiescent current consumption of the unit is only about 5mA, but this rises to as much as 50mA or so at high volume levels. The best position for the pick-up coil on the telephone base (not the handset) can be located with a little experimentation.

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VMOS 10 Watt Amplifier

April 6th, 2010 by Thomson | No Comments | Filed in Coreelectronics

At first sight this circuit may seem to be a straightforward Class B design having an emitter follower, complementary output pair and Darlington Pair common emitter drive stage. However, the output devices are, in fact, complementary VMOS transistors used in the source follower mode (the FET equivalent of the emitter follower).

VMOS 10 watt amplifier free projects VMOS 10 Watt Amplifier

R1 and R2 are used to bias the unit to give the optimum quiescent output potential and they provide overall negative feedback, which improves the quality of reproduction. D1 and C4 are boot strapping components, enabling the gate drive voltage to Q3 to go above the positive supply potential, giving improved efficiency to the circuit. R3 is the main collector load for Q2 and PR1 is used to give a standing bias on the output transistors that gives a quiescent current consumption of about 25mA. The thermal compensation circuitry normally used is totally unnecessary in this circuit, since VMOS devices do not suffer from thermal runaway. In fact the quiescent bias current will drop slightly as the output devices heat up, but not sufficiently to give rise to significant crossover distortion.

C2 and C5 provide DC blocking at the input and output respectively, while C1 is a supply decoupling component. C3 gives a degree of high frequency attenuation and aids the stability of the circuit.

Although the current in the driver stage, only about 1mA, may seem to be totally inadequate, it is in fact more than sufficient since the VMOS devices have extremely high input impedances and consume no significant, input current. This is one of their main advantages over bipolar devices. One disadvantage in this particular application is lower efficiency due to the higher threshold voltages and on resistance of VMOS transistors in comparison to bipolar devices. However, the circuit will give an output of 10W rms using a supply voltrage of about 33V or so (with a current drain of up to about 600mA). An input of about 500mV rms is needed for maximum output.

Note: The output devices do not have internal zener protection diodes and the appropriate handling precautions should be taken.

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Simple amplifier

March 17th, 2010 by Thomson | No Comments | Filed in Coreelectronics

The term “amplifier” covers a very wide range, from a one transistor preamp to an ultra sophisticated high power hi-fi system. There is little doubt that the circuit shown here is very simple. The output is in the order 250mW – quite sufficient for most purposes and comparable to that of the average transistor radio. The distortion level is rather high, being about 5%.

The amplifier is also reasonably sensitive and will give full output with an input of about 50mV. Input impedance is about 50kR.

free electronic projects simple amplifier 300x227 Simple amplifier

The slider from the volume control is connected to the, base of Q1 via a DC blocking capacitor. Q1 is connected as a conventional common emitter amplifier with R2 provides the base bias and R3 acting as the collector load. This stage is directly connected to the second transistor which is a PNP type. In this way the current passing through Q1 provides the bias for the second transistor. The output of the second transistor is connected directly to the speech coil of the loudspeaker. This is not normally good practice since the standing current in the output transistor continually biases the coil either slightly in or out from its usual operating point. However if a large speaker is used, as it should be, this has very little effect and, since we are not aiming at hi-fi, it does not matter.

The tone control comprises C2 and RV2 which are connected between the collector and base of Q1 At high resistance settings RV2 has little effect but on minimum settings the 100nF feeds back the high frequencies out of phase, thus cancelling them.

For this circuit to work properly, R3 must be selected with great care. The value shown here of 39 ohms is a typical one and, although it may be used for initial setting up to ensure the circuit is operating, the value should be found by experiment. If it is too low there will be severe distortion at higher volume settings. If it is too high the current drain will be excessive even though the quality of reproduction will be good.

It is very important that Q2 is fitted with a heat sink as it will get very not.

The speaker impedance is not all that critical and in the prototype speakers with an impedance as low as 8 ohms and as high as 80 ohms all worked well, although changing the speaker impedance will also necessitate a change in the value of R3.

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RIAA stereo preamplifier

March 13th, 2010 by Thomson | No Comments | Filed in Coreelectronics

Records are cut with a frequency response such that when they are replayed with a magnetic pickup and a preamplifier with RIAA equalization (Recording Industry Association of America) the reproduced sound will be as similar to the original as possible.

The disc is cut at constant amplitude, except from 500Hz to 2120Hz where it is cut at constant velocity. When this disc is replayed with a magnetic pickup, the relative output voltage rises with frequency, due to the fact that the magnetically generated voltage is proportional to the velocity of the stylus as it moves sideways in the groove. To restore the original sound quality, a preamplifier with a frequency response that, gives decreasing output with increasing frequency is required. This response curve is known as the RIAA equalization and it is tailored accurately to fit the cutting and replay processes. The signal level from a magnetic pickup is low, generally 20mVpp and so a low noise pre- amplifier is needed.

RIAA sterio preamplifier left RIAA stereo preamplifier

RIAA sterio preamplifier right 300x181 RIAA stereo preamplifier

The circuit shows a realization of this requirement. The low noise amplifier is the LM381 -made by National Semiconductors. A DC bias control is included (RV1, RV2), and the feedback components generate the RIAA curve. Use screened cable for the wiring to the pickup, keep the circuit away from transformers (and the pickup and its wiring) and connect all the earths together, near to the IC.

RIAA sterio preamplifier graph 300x263 RIAA stereo preamplifier

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