Description.

The circuit diagram shown here is of a 10V switching regulator based on the LM5007 from National Semiconductors. The LM5007 is an integrated step down switching regulator which has all necessary systems required for making a cost effective and reliable switching regulator circuit. The IC is available in MSOP-8, LLp-8 packages and has a lot of  built in  features like thermal shut down, under voltage lock out, duty cycle limiting, current limiting etc.

The output voltage of this regulator can be adjusted using the resistor R3 and R4. For the given values of R3 and R4 in the circuit diagram, the output voltage will be 10V. The equation governing the output voltage is Vout = 2.5 x (R3+R4)/R4. Resistor R1 sets the switch on time and C4 is the boost boot strap capacitor. Resistor R2 determines the variation of OFF time and C3 is a decoupling capacitor.

Circuit diagram.

Notes.

• The supply voltage can be anything between 12 to 72V DC.
• Output voltage can be adjusted using R3 and R4.
• C1  and C5 are polyester capacitors.
• C1 and C2 must be rated at least 100V.
• R5 and C5 forms a filter network.

As the applied electric field, E, across the GaAs material is increased, the charge carriers, that is electrons in this case, gain energy from the applied field. At the same time, through collisions (that is, optical phonon scattering), with the lattice, the electrons also lose a small portion of this energy. So long as the resultant balance is positive, the energy and drift velocity of the charge carriers increases with an increase in the applied field. However, at some point, the energy gained from the field becomes equal to the energy lost as the result of collisions. This result in the drift velocity approaching a limiting value referred to as the saturation velocity, v_sat

Since gallium arsenide is a multi-valley semiconductor, when the energy of lower valley electrons rises sufficiently, that is at electric fields greater than approximately 3500V/cm, electrons become hot. There is a region in the electron velocity-field characteristics where some of the ‘hot’ electrons populate an upper conduction band that is characterized by larger electron effective mass. The resultant effect is a reduction in the number of high mobility electrons and hence the drift velocity.

In this region the drift velocity is no longer proportional to the electric field, but instead passes through a maximum of about 2*10⁷ cm/sec with increasing field, and decreases to an electric field independent saturation value of about 1.4*10⁷ cm/sec.

The velocity-field characteristics illustrating the three regions of interest are shown in the figure below.

Electron Velocity vs Electric field

For convenience of comparison, characteristics for silicon are also illustrated. From the figure it can readily be noted that in low electric field regions, silicon has a much lower mobility than gallium arsenide. This increases monotonically until the drift velocity saturates at a value of about 1*10⁷ cm/sec.

Before going into details, it is better to know the basics on GaAs in VLSI technology. Click on the link below.

TAKE A LOOK : ULTRA-FAST SYSTEMS AND GaAs VLSI TECHNOLOGY

One of the important characteristics that is attributed to GaAs is its superior electron mobility brought about as the result of its energy band structure as shown in the figure below. Gallium arsenide is a direct gap material with valence band maximum and conduction band minimum coinciding in k space at the Brillouin zone centers. Valleys in the band structure that are narrow and sharply curved correspond to electrons with low effective mass state, while valleys that are wide with gentle curvature are characterized by larger effective masses. The curvature of the energy versus electron momentum profile determines the effective mass of electrons travelling through the crystal. The minimum point of gallium arsenide’s conduction band is near the zero point of crystal-lattice momentum, as opposed to silicon, where conduction band minimum occurs at high momentum. Now, mobility, µ, depends upon

• Concentration of impurity, N
• Temperature, T
• And is also inversely related to the electron effective mass, m.

Energy Band Strucure of GaAs

For GaAs, the effective mass of these electrons is 0.067 times the mass of free electron (that is, 0.067m_e, where m_e is the free electron rest mass). This means electrons travel faster in gallium arsenide than in silicon as the result of their superior electron mobility brought about by the shapes of their conduction bands. Electrons in the higher valleys have high mass and strong inter-valley scattering and therefore exhibit very low mobility, which is very similar to conduction electrons in silicon. Furthermore, gallium arsenide is a direct-gap semiconductor. Its conduction band minimum occurs at the same wave vector as the valence band maximum , which means little momentum change is necessary for the transition of an electron from the conduction band, to the valence band. Since the probability of photon emission with energy nearly equal to the band gap is somewhat high, GaAs makes an excellent light-emitting diode. Silicon on the other hand, is an indirect-gap semiconductor since the minimum associated with its conduction band is separated in momentum from the valence band minimum. Therefore it cannot be a light-emitting device.

Before going into details, it is better to know the basics on GaAs in VLSI technology. Click on the link below.

TAKE A LOOK : ULTRA-FAST SYSTEMS AND GaAs VLSI TECHNOLOGY

The whole concept of crystal orientation becomes important during

• The etching of the crystal
• Ion-implantation
• Passivation

This introduces an orientation dependency that influences the properties of GaAs field effect transistor. For example, during implantation, when a high energy ion enters a single crystal lattice at a critical angle to the major axis of the GaAs crystal, the ion is steered down the open directions of the lattice. This steering is called axial channelling. This implies that of a random equivalent direction is not used during ion implantation, the depth distribution will be greater than those predicted by range statistics which are used to establish penetration depth.

The channelling effect is not as dramatic in the <100> direction when compared with <110> direction. Many of the current GaAs wafers employ the <100> direction. It should be noted that the profile difference between the aligned <100> direction implant and any other direction of implant has a significant influence upon the threshold voltages of the fabricated devices.

Description.

This is just another 20W audio amplifier circuit , but this time based on the LM1875 audio amplifier IC from National Semiconductors. With a 25V dual power supply LM1875 can deliver 20W of audio power into a 4 ohm speaker. The LM1875 requires very less external components and has very low distortion. The IC is also packed with a lot good features like fast slew rate, wide supply voltage range, high output current, high output voltage swing, thermal protection etc. The IC is available in TO-220 plastic power package and is well suitable for a variety of applications like audio systems, servo amplifiers, home theatre systems etc.

Circuit diagram.

Notes.

• Assemble the circuit on a good quality PCB.
• Use +/-25V DC dual supply for powering the circuit.
• K1 can be 4 ohm, 20W speaker.
• A proper heat sink is necessary for the IC.
• F1 and F2 are 2A fuses.