Simple Microphone Amplifier Circuit

This is a circuit diagram of a device which can hear faint sounds and those sounds which any normal microphone can not hear. This is a very useful circuit and one can use this circuit for many tasks this circuit can be used as a hearing aid. Use direct coupling and do not use bulky coupling or bypass capacitors. The circuit using only a few components the size of the circuit is very small so one can easily fit it in a small box. This is the figure of the circuit;

Capacitor 1 bypasses ac negative feedback due to which the gain is not affected. Q1, Q2 and Q3 are bc 337. Resistor 1 is used for volume control. Transistor 1, transistor 2 and transistor 3 all are direct coupled amps. Their operation is stabilized by direct current (DC) negative feed back through resistor 4.

Simple OP90 Single Op Amp Full-Wave Rectifier

This is a design circuit for a full-wave rectifier circuit that provides the absolute value of input signals up to ±2.5 V even though operated from a single 5 V supply. This is the figure of the circuit;

The amplifier acts as a unity-gain inverter for negative inputs. The op amp output is forced by positive signal to ground. The 1N914 diode becomes reversed-biased and the signal passes through R1 and R2 to the output. Load impedance cause an asymmetric output since output impedance is dependent on input polarity. This can be corrected by reducing R2 for constant load impedance. A second OP90 can buffer varying or heavy loads. The output of the full-wave rectifier with a 4 Vp-p, 10 Hz input signal also shown in figure below. [Circuit's schematic diagram source: analog.com]