The post explains a simple pre-amplifier circuit which can be quickly made using a couple of transistors (BJTs) and a few resistors. 

The idea was requested by Mr. Raveesh.

Technical Specifications

I am working in LIC of India and electronics is my hobby since so many years. Often I will be browsing your website and found many useful projects. I require a favour from you.

I have a FM transmitter module which works on 5 volts DC with provision to connect from Computer through USB or from audio out from any other device through 3.5 mm audio jack.

The module works great in computer USB mode with great signal strength, quality and coverage. But when I connect the same through audio input jack from DTH set top box the signal strength becomes weak even with full volume in both set top box and FM module. I think the audio signal level from set top box is not sufficient for the FM module.

Please suggest me a good quality stereo audio small signal pre-amplifier circuit which can work from 5 or 6 volts single supply, that would not load the set top box, preferably using good low noise op-amp with detailed circuit and parts label.

Sincerely hope you will help

With regards


The Design

A simple pre-amplifier circuit can be very easily built by assembling a couple of transistors and some resistors as shown in the following figure:

Pre-Amplifier Circuit

The circuit is a simple two transistor pre-amplifier using a feedback loop for enhancing the amplification.

Any music as we know is in the form of a consistently varying frequency, therefore when such a varying input is applied across the indicated C1 end terminals, the same is delivered across the base T1 and ground.

The higher amplitudes are processed normally and is reproduced with a potential that's approximately equal to the supply voltage, however for the lower misc amplitudes T2 is allowed to conduct at the higher ratio which is allowed to pass to its emitter. At this time when the actual enhancement of the music is implemented by transferring this accumulated higher potential back to the base of T1 which correspondingly saturates at a much optimal rate.

This push pull action ultimately results in an overall amplification of an insignificantly small music or data input into a significantly larger output.

This simple circuit enables boosting extremely small or minimal frequencies to an appreciably bigger outputs which can be then used for feeding lager amplifiers.

The discussed circuit was actually popularly used in old cassette type playback recorders in their preamp stages for boosting the minute signals from the tape head so that the output from this small amplifier became compatible for the attached high power amplifier.

Parts List

R1 = 22K
R2 = 220 ohms
R3 =100k
R4 = 4K7
R5 = 1K
C1 = 1uF/25V
C2 = 10uF/25V
T1/T2 = BC547

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