100 Watts VMOSFET Power Amplifier

Figure 1 

The circuit design aims to create a power amplifier that can produce a power output of 100 Watts using the functions of Vertical Metal Oxide Semiconductor Field Effect Transistor (VMOSFET).

• Vertical Metal Oxide Semiconductor Field Effect Transistor (VMOSFET) – a type of power MOSFET used for several applications where the requirements are medium powers and has an arranged structure with a V-groove
• BC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selections
• BC212/BC182 – a PNP complementary Silicon planar epitaxial transistor used in AF small signal amplifiers and drivers, as well as in low power universal applications
• 2N5460 – a P-Channel general purpose JFET amplifier primarily designed for low level audio applications with high impedance signal sources
• MPSA93 – a small signal PNP epitaxial planar transistor suited especially in video output stages of TV receivers and monitors and as line switch in telephone subsets and is the complement type of MPSA43
• MPSA43 – NPN high voltage transistor with low current and high voltage used for telephony, professional communication equipment, and telephony, and is the complement type of MPSA93
• 2SK134/2SK135 – a Silicon N-Channel MOSFET used for low frequency power amplifier with features such as being equipped with gate protection diodes, good complementary characteristics, enhancement mode, wide area of safe operation, high speed switching, excellent frequency response, and high power gain, and is the complement of 2SJ49
• 2SJ49/2SJ50 – a Silicon P-Channel power MOSFET designed for motor/lamp driver circuits and DC/DC converters with features such as built-in gate protection diode, low current, and low on-state resistance, and is the complementary of 2SK134
• Trimmer – a miniature variable component used to make fine adjustments to capacitance, inductance or resistance (potentiometers)

VMOSFETs were introduced to the market with a very good performance compared with other bipolar semiconductors. They are much easier and cheaper to manufacture in the design of amplifiers. They are used with a diversity of power MOS applications since have confidently established as power MOSFET electronics components. The power supply switching applications are in the range of medium power RF amplifiers. Because they have the ability to switch very quickly, they are utilized in several integrated circuits. VMOSFETs found its way to resolve the issues in power applications which prevented the use of FETs. With the new configuration, much high powers can now be handled unlike the previous bipolar transistors with the same cost and size.

The distinctive structure of the device is the reason for the great improvement in the family of MOSFET. The V groove in the structure is the most outstanding point with regards to the new device. This is also the means of operation of the transistor. In the structure, the source is at the top of the device while the drain is at the bottom. The current flows in the vertical direction rather than flowing horizontally as in the standard FET, which gives this device the name Vertical Metal Oxide Semiconductor. The source of the device is using two connections and the space where the current can flow is large enough to accommodate such value. This would allow handling of much higher powers as it reduces the ON resistance of the device, compared to standard FETs. The current flow in the P region is controlled by the metalized area over the V groove that comprises the gate of the transistor. Since the gate is designed in this manner, it signifies that the outstanding high input resistance is preserved which is typical in the MOS family of devices. Because of the complicated design than a conventional FET, the device is more expensive, which makes it a negative aspect of this type of transistor.

As the VMOSFET is used than the simple bipolar transistor, it provides good features relative to the output. Thermal stability, low distortion, and high speed, are some of the assets given by the VMOSFET. The source current comes from the transistor Q3 2N5463 while the attributes of the input in differential amplifier BC547 Q1 and Q2 are complemented by the trimmer TR1. The trimmer TR2 attached in Q6 BC182 regulates the bias current around 100 mA of the output stage. To protect the gates of VMOSFET from exceeding 14 V, the combination of resistors R17 to R19 and diodes D2 to D5 is used. This combination will create a gap in very thin layer of Silicon Dioxide that will insulate the gates. This type of protection is typically used with all the amplifiers employing VMOSFET.

The resistors R6, R8, and R18 regulate the gain of the power amplifier which is 32.6 in the negative feedback. Under all the conditions, the local feedback of the circuit is being used to stabilize the operation. The increase of temperature in the VMOSFET also increases its resistance since it has a positive factor of temperature. In the event of high resistance, the current through the transistor will also be reduced thus, resulting to reduce power. To ensure the stability and prevent the distortion of intermodulation, a separated power supply is used in the stage of the drive and output.

Figure 2 

In constructing integrated circuits, MOSFETs are very useful since they can be made very compact. Although MOSFETs can get damaged by static electricity at higher voltages, they still provide several advantages as compared to other transistors which include faster switching time than BJT, lower losses than BJT, very small switching current, and least effects of temperature. MOSFET drivers are applied in electronic motor control for different types of motors. Also, they are specifically used with long duty cycles, high operating frequency above 200 KHZ, lower output power, and wide load variations. The largest application of MOSFETs are the switched mode power supplies and in battery charging applications. In transducer drivers for high power devices such as light bulbs and motors, large current output with a small input is provided by MOSFETs. Since they are more non-linear than BJTs while producing less distortion, they can be utilized with Hi-Fi amplifiers.

MOSFET can function in two ways. The first is known as depletion mode wherein the channel shows its maximum conductance in the absence of a voltage on the gate. The second way that the MOSFET can function is known as enhancement mode wherein the device is not conducting even in the absence of a voltage on the gate because no channel is produced. A channel is being created with the application of a voltage to the gate. To generate better conductivity, greater voltage to the gate is required.

The circuit applications of the VMOSFET are similar to any general purpose FET or those of a bipolar transistor since the characteristics of a VMOSFET is very identical to an enhancement MOSFET. They are capable of carrying a large power gain that enables them to be used as linear amplifiers or as switches. The creation of this device is mainly accountable in the improvement of the power control capabilities of a single transistor.

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