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# School Project Help.

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#### 313help

##### Newbie level 1
Hello,

I need a little advise for a little project.

I need to build a smart(with automatic gain control) amplifier with the following characteristic:

When the output voltage swing exceeds an external limit set by the user (Vref), amplifier decreases its gain such that the output voltage swing remains aroud Vref.

You do not say what frequency range this is mean't to work over. Like wise over what sort of dynamic limits its meant to work, i.e. stabilise an input voltage of 20V p-p at an output level of 10mV?
frank

Ozgunoral

### Ozgunoral

Points: 2
You do not say what frequency range this is mean't to work over. Like wise over what sort of dynamic limits its meant to work, i.e. stabilise an input voltage of 20V p-p at an output level of 10mV?
frank

Thank you for reminding.

The circuit is supposed to be operating up to 20 kHz and maximum gain should be 40.
Supply voltages are +5 and -5 volts.
Min Vref=100mV and maximum deviation can be 5%.

Silicon Chip magazine published a design of an audio compressor using Phillips SA571 intergrated circuit.

You can look at the SA571 datasheet. It shows how a compressor is made with a rectifier and a voltage controlled amplifier.

You can google for "voltage controlled amplifier" and "audio compressor".

You could do this with a DSP instead of analog electronics but that may not be easier.

Compressors :- have ATTACK time, time it takes for the compressor to reduce the gain by some specified amount (10 dB?). DECAY time, time for the gain to return to unity from some specified attenuation (10dB?). So when you clash your cymbals, the wave form is a very fast rising edge, which gets through the compressor until it starts its attenuation. The attack time should be as fast as possible. If the decay time is too long, during speech, there will be a sort of "breathing" background, as the level bounces about due to the loud bit that happened 3 secs ago!
To overcome the attack time problem, the main sound chain is delayed so that when the control signal can get . to its correct value before it is applied to the attenuation element along with the audio (delay line compressor).
One extremely linear attenuation is made from a chain (5?) of diode in series (a super diode?) , you use two super diodes back to back, and use it in a circuit with an impedance of 200 ohms or so. Passing a current of 0 -50mA through the diodes will get an attenuation of 60 dB at a signal level of 1V with i/ps of -120 dBs.
Frank

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