MOSFET AUDIO AMPLIFIER(Low Noise and High Gain)

Hello guys!

This project is the design and implementation of a Low Power Audio amplifier using MOSFET's. The design is as simple as it could be and the components are easily available. I am writing this instructable as i myself experienced a lot of difficulty inn finding some useful material regarding the project and an easy method for the implementation.

Hope you enjoy reading the instructable and I am positive it will help you.

Step 1: Introduction

"An audio power amplifier (or power amp) is an electronic amplifier that strengthens low-power, inaudible electronic audio signals such as the signal from radio receiver or electric guitar pickup to a level that is strong enough for driving loudspeakers or headphones."

This includes both amplifiers used in home audio systems and musical instrument amplifiers like guitar amplifiers.

The audio amplifier was invented in 1909 by Lee De Forest when he invented the triode vacuum tube (or "valve" in British English). The triode was a three terminal device with a control grid that can modulate the flow of electrons from the filament to the plate. The triode vacuum amplifier was used to make the first AM radio. Early audio power amplifiers were based on vacuum tubes. Whereas, now-a-days transistor-based amplifiers are used that are lighter in weight, more reliable and require less maintenance than tube amplifiers. Applications for audio amplifiers include home audio systems, concert and theatrical sound reinforcement and public address systems. The sound card in a personal computer, every stereo system and every home theatre system contains one or several audio amplifiers. Other applications include instrument amplifiers such as guitar amplifiers, professional and amateur mobile radio and portable consumer products such as games and children’s toys. The amplifier presented here uses mosfets to achieve the desired specifications of an audio amplifier. Gain and power stage is employed in the design to attain the required gain and bandwidth.

Step 2: Design and Some Important Amplifier Stages

The specifications of the amplifier include:

> Power output 0.5 W.

> Bandwidth 100Hz-10KHz

GAIN OF THE CIRCUIT:
The first objective is to attain a considerable power gain that is sufficient to give a noise free audio signal at the output through speakers. To achieve this the following stages were employed in the amplifier:

1. Gain Stage: The gain stage uses a potential divider biased mosfet amplifier circuit. The potential divider biased circuit is shown in figure 1.

It simply amplifies the input signal and produces gain according to the equation (1).

Gain = [ ( R1 || R2)/ (rs+ R1 || R2 ) ] * (-gm) * ( rd || RD || RL) (1)

Here, R1 and R2 are the input resistances, rs is the source resistance, RD is the resistance between bias voltage and drain and RL is the load resistance.

gm is transconductance which is defined as the ratio of the change in drain current to the change in gate voltage.

It is given as

gm = Delta (ID) / delta (VGS) (2)

To produce the desired gain three potential divider biased circuits were cascaded in series and the total gain is the product of the gains of individual stages.

Total Gain = A1*A2*A3 (3)

Where, A1, A2 and A3 are the gains of first, second and third stage respectively.

The stages are isolated from each other with the help of interconnected capacitors that is RC coupling.

2. Power Stage: A push pull amplifier is an amplifier which has an output stage that can drive a current in either direction through through the load.

The output stage of a typical push pull amplifier consists of two identical BJTs or MOSFETs one sourcing current through the load while the other one sinking the current from the load. Push pull amplifiers are superior over single ended amplifiers (using a single transistor at the output for driving the load) in terms of distortion and performance. A single ended amplifier, how well it may be designed will surely introduce some distortion due to the non-linearity of its dynamic transfer characteristics.

Push pull amplifiers are commonly used in situations where low distortion, high efficiency and high output power are required.

The basic operation of a push pull amplifier is as follows:

"The signal to be amplified is first split into two identical signals 180° out of phase. Generally this splitting is done using an input coupling transformer. The input coupling transformer is so arranged that one signal in applied to the input of one transistor and the other signal is applied to the input of the other transistor. "

Advantages of push pull amplifier are low distortion, absence of magnetic saturation in the coupling transformer core, and cancellation of power supply ripples which results in the absence of hum while the disadvantages are the need of two identical transistors and the requirement of bulky and costly coupling transformers. A power gain stage was cascaded as the final stage of audio amplifier circuit.

FREQUENCY RESPONSE OF THE CIRCUIT:

Capacitance plays a dominant role in shaping the time and frequency response of modern electronic circuits. An extensive and in-depth experimental investigation has been performed the role of various capacitors in small-signal MOSFET amplifier circuit.

Particular emphasis has been given to addressing basic issues involving capacitances in MOSFET amplifiers, rather than modifying the design. Three different enhancement n-channel MOSFETs (2N7000 model, referred hereafter to as MOS-1, MOS-2 and MOS-3) manufactured by Motorola Inc. have been used for the experiment. The study uncovers several important new features of the amplifiers. It indicates that in the design of small-signal MOS amplifiers, it should never be taken for granted that coupling and bypass capacitors act as short circuit and have no effect on the ac input and output voltages. In fact, they contribute to the voltage levels seen at both the input and the output port of the amplifier. When chosen judiciously for coupling and bypass operations they dictate the actual voltage gain of the amplifier at various frequencies of input signal.

The lower cut-off frequencies are governed by the values of coupling and bypass capacitors whereas the upper cut-off is a result of shunt capacitance. This shunt capacitance is the stray capacitance present between the junctions of the transistor.

The capacitance is given by the formula.

C = (Area * Ebsilon ) / distance (4)

The value of the capacitors is chosen such that the output bandwidth is between 100-10KHz and the signal above and below this frequency is attenuated.

Figures:

Figure.1 Potential Divider Biased MOSFET circuit

Figure.2 Power Amplifier Circuit using BJT

Figure.3 Frequency Response of MOSFET

Step 3: Software and Hardware Implementation

The circuit was designed and simulated on PROTEUS software as shown in figure 4. The same circuit was implemented on the PCB and same components were used.

All the resistors are rated for 1 Watt and capacitors for 50 volt to avoid damage.

The list of components used are listed below:

> R1, R5, R9 = 1MΩ

> R2, R6, R11 = 68Ω

> R3, R7, R10 = 230KΩ

> R4, R8, R12 = 1KΩ

> R13, R14 = 10KΩ

> C1, C2, C3, C4, C5 = 4.7µF

> C6, C7 = 1.5µF

> Q1, Q2, Q3 = 2N7000

> Q4 = TIP122

> Q5 = TIP127

The circuit simply consists three gain stages connected in cascade.

Gain stages are connected through RC coupling. RC coupling is the most widely used method of coupling in multistage amplifiers. In this case the Resistance R is the resistor connected at the source terminal and the capacitor C is connected between the amplifiers. It is also called a blocking capacitor, since it will block DC voltage. The input after passing through these stages reach the power stage. The power stage uses BJT transistors (one npn and one pnp). Loudspeaker is connected at the output of this stage and we get an amplified audio signal. Signal given to the circuit for simulation is 10mV sin wave and the output at the loudspeaker is 2.72 V sin wave.

FIGURES:

Figure.4 PROTEUS circuit

Figure.5 Gain Stage

Figure.6 Power Stage

Figure.7 Output of gain stage 1 (Gain = 7)

Figure.8 Output of gain stage 2 (Gain = 6.92)

Figure.9 Output of gain stage 3 (Gain = 6.35)

Figure.10 Output of three gain stages (Total Gain = 308)

Figure.11 Output at the loudspeaker

Step 4: PCB LAYOUT

The circuit shown in Figure 4 was implemented on the PCB.

Above are some snippets of the software design of the PCB

FIGURES:

Figure.12 PCB layout

Figure.13 PCB layout (pdf)

Figure.14 3D View (TOP VIEW)

Figure.15 3D View (BOTTOM VIEW)

Figure 16 Hardware (BOTTOM VIEW) Top view already present in the first image

Step 5: Conclusion

Utilizing the high gain and high input impedance of short channel power MOSFETs, a simple circuit has been devised to provide sufficient drive for amplifiers upto 0.5 watt output.

It offers performance which meets the criteria for high quality audio reproduction. Important applications include public address systems, theatrical and concert sound reinforcement systems and domestic systems such as stereo or home-theatre system.

Instrument amplifiers including guitar amplifiers and electric keyboard amplifiers also use audio amplifiers.