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Overview The circuit shows the peak level of the audio input using MOSFET Op-Amp. Circuit Explanation
Audio level meters are designed to allow the optimum setting of input level for various sound functions. It utilizes a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) to amplify the input signals for recording purposes. MOSFET Op-Amp approximate more closely the ideal op-amp than bipolar ICs where large signals must be handled at room temperature over a limited bandwidth with a much higher input impedance. Operational amplifiers is utilized because they are very easy to handle, cheap, easy to obtain and they offer a very high level of performance. The use of an analog meter (like tape recorder meter), is to assess the peak audio reaction in the circuit. Using the op-amp (CA3140) as non-inverting amplifier, the output stays the same as the input signal. The detector part comprises a semiconductor diode providing full wave output through the op-amp to enhance the linearity of operation of the detector-amplifier combination. The meter will obtain a gradual decay time but quick response time to point toward the peak readings. 1N4148 provides half wave rectification of the input signal while the output is evenly received by the capacitor which charges to the peak input waveform then discharges through the meter and resistor. The meter to be used should have a value of 50uA – to 250uA to cause the needle to deflect to its maximum position. Increasing the value of the capacitor would permit a longer discharge time. MOSFET is preferred in this circuit because JFETs gate current and transconductance is greater than the MOSFET which may not be applicable onto the circuit. Source:www.zen22142.zen.co.uk/Circuits/Audio/palm.htm
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