An Audio amplifier needs to give an output waveform exactly similar to the input waveform even though the output is amplified. Op Amps are widely used to amplify signals mainly in the Preamplifier stage. The input can be signal from an audio device or from a microphone. The Pre Amplifier stage uses either a Non inverting amplifier or Inverting Amplifier. This note will help you to design an amplifier based on JFET input Op Amp TL0 71.
Most of the Audio amplifier circuits use dual power supply because these amplifiers are designed for amplifying alternating signals. (Voltage of the signals swings between positive and negative with reference to ground). So here the two amplifier designs based on the dual power supply are given . The dual power supply can be easily obtained from two batteries as shown below.
Non inverting amplifier
The diagram of the Non inverting amplifier works on dual power supply is shown below. The signal is given to the Non inverting input (+) and the output waveform is in exact phase with the input wave form. To give a negative feed back, resistor R1 is connected between the output and inverting (-) input. This amplifier is known as Unity gain since it has the gain of 1. To give a gain more than 1, the feed back voltage is reduced by the resistor R2.
Resistors R1 and R2 act as a potential divider to reduce the feed back voltage so that the OpAmp can amplify the signal. The degree of amplification or Gain of the Op Amp is determined by the ratio of R1 and R2 according to the formula
Gain = (R1/R2) + 1
The value chosen for R1 is 100 K and for R2 as 10 K. So the gain is
Gain = (100000 / 10000) +1 = 11.
So when the input signal is 10 mV, then the output voltage will be
10 x 11 = 110 mV.
Here the signal is given to the Inverting (-) input so that the output signal is 180 degree out of phase. This Signal inversion will not affect the working of the audio amplifier. Resistor R2 functions as the feed back resistor and Resistor R1 as the input resistor. Hence the gain is set by the ratio of R2 and R1.
Gain = – (R2 / R1). The minus sign indicates that the signal in inverted at the output.
Value for R1 is selected as 10 K and for R2 as 100 K as in the previous design. So the gain of Inverting amplifier is
Gain = – (100,000 / 10,000) = -10.
So when the input signal has 10 mV, the output will have -500 mV.
One important nature of this inverting amplifier is its Virtual earth. That is the voltage at the inverting input is virtually equal to the voltage at the non inverting input. This property has great significance in Audio mixers since many signals can be given to the virtual inverting input without out any interference.