sábado, 26 de junio de 2010
The first SiGe device to go into production was the HBT in 1999. The performance of a Si bipolar (BJT) can be enhanced by the addition of SiGe to the base region of the transistor. Two schemes exist to enhance performance, a rectangular Ge base profile (HBT) and a linearly graded base. The linear base provides a built in electric field which accelerates electrons across the base region. The band structure for the HBT and the linear grade transistors are shown on the right.
The demonstrated cutoff frequencies (fT - the frequency where the gain for the transistor equals 1) for HBTs has now reached 360 GHz and this is predicted to increase as technology progresses. There is a downside to this, however, in that the breakdown voltage between the collector and the emitter also decreases as fT is increased.
Freddy R Vallenilla R
EES SEC 2
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 1:18
A varactor is also known as a variable capacitance diode or a varicap. It provides an electrically controllable capacitance, which can be used in tuned circuits. It is small and inexpensive, which makes its use advantageous in many applications. Its disadvantages compared to a manually controlled variable capacitor are a lower Q, nonlinearity, lower voltage rating and a more limited range. Background material on varactors can be found in the Reference.
Any PN junction has a junction capacitance that is a function of the voltage across the junction, as discussed in any account of PN junctions. The electric field in the depletion layer that is set up by the ionized donors and acceptors is responsible for the voltage difference that balances the applied voltage. A higher reverse bias widens the depletion layer, uncovering more fixed charge and raising the junction potential. The capacitance of the junction is C = Q(V)/V, and the incremental capacitance is c = dQ(V)/dV. The capacitance to be used in the formula for the resonant frequency is the incremental capacitance, where it is assumed that the voltage excursions dV are small compared to V. Finite voltages give rise to nonlinearities. Efforts may be made to reduce these nonlinearities in some cases.
The capacitance decreases as the reverse bias increases, according to the relation C = Co/(1 + V/Vo)n, where Co and Vo are constants. Vo is approximately the forward voltage of the diode. The exponent n depends on how the doping density of the semiconductors depend on distance away from the junction. For a graded junction (linear variation), n = 0.33. For an abrupt junction (constant doping density), n = 0.5. If the density jumps abruptly at the junction, then decreases (called hyperabrupt), n can be made as high as n = 2. I expect that the doping on one side of the junction is heavy, and the depletion layer is predominately on one side, but this is a constructional detail.
Availability of Varactors
For the experiments described below, I used some varactors that were furnished by a surplus house. These were in the TO-92 package that is so convenient for experiments, and came in matched sets of three. A look in the Digi-Key catalog revealed that although a variety of varactors from Zetex are available and inexpensive (p. 398), they are available only in the SOT-23 surface-mount package. This is another example of how things are becoming more difficult for those trying to learn about electronics.
The solution for this problem is offered by the "Surfboards" of Capital Advanced Technologies (p. 752). These are small boards, as shown at the right, with SIP pins (inline pins at 0.1" spacing) and pads suitable for surface-mount devices. Discrete devices, like our SOT-23 diodes, can be easily soldered to the 6000-series Surfboards. The 6103CA, which holds one device, is suitable. The connections are shown at the right. If you buy your Surfboards from Digi-Key, you will get instructions on how to use them. The methods described here can also be used with surface-mount transistors and components, which will also fit on the Surfboards. This seems to be a practical way to use surface-mount devices when you are compelled to do so.
I used the following tools: a 12W fine-tip soldering iron (Weller WM120); .025" dia. 60/40 rosin-core solder; fine-point tweezers; a round toothpick; clear household cement; a 10X magnifier for inspection; and, finally, a bright light. Remove the diode by pulling off the clear tape on the carrier. The SOT-23 package is seriously tiny! Make sure you can recognize top and bottom. Lay the 6103CA face-up. Put a small drop of cement on the end of the toothpick, and deposit a tiny amount at the point where the package will be attached. Then place the package on the dot of cement with the tweezers, with its feet in the proper places, and press down. This holds the package while it is being soldered, and is a step that should not be omitted. The tip of the soldering iron should be tinned. Touch the solder to the tip so that a small drop is left hanging on the tip. Now, very carefully bring the drop into contact with the pad of the package and the foil of the 6103CA board at one of the feet of the SOT-23. Capillary attraction will soon cause the solder to spread in the usual way. Press down lightly to ensure that the package is seated. This all takes only a second. Examine the joints with a 10X magnifier to make sure that the feet are entangled in the solder. It's a good idea to put a label on the back of the 6103 to identify the part, since the SOT-23 package is too small for identification.
The basic circuit for testing a varactor is shown at the right. The key is the 1M resistor that isolates the DC voltage source from the circuit attached to the varactor. The 0.1μF capacitor blocks the DC bias voltage. I happened to have a 10μH inductor at hand, one of those that looks like a fat resistor, and has a reasonably high Q. The RF signal generator was coupled through a 220pF capacitor, and set for an unmodulated output. Because of stray capacitances, we cannot accurately measure the capacitance of the varactor with this circuit, but we can certainly see its action.
A capacitance meter did not give satisfactory results, so another method closely related to the actual application of the varactor was used. While observing the voltage across the tuned circuit with an oscilloscope, I varied the frequency looking for a maximum. From the resonant frequency, I then calculated the capacitance using the usual formula.
The results of a series of measurements is shown at the left. The capacitance varied from about 170 pF at 8 V to 750 pF at 0.5V, a satisfactory range. If you plot the frequency vs. the voltage, the result is almost linear, showing that the varactor is of the hyperabrupt type, since n = 2 will give frequency proportional to voltage. I also determined that the MPN3404 that I found in the varactor drawer was probably not a varactor. It was not described, but was listed, in the Motorola data book.
Further experiments will be described, with applications, when I obtain some more varactors.
Freddy R Vallenilla R
EES SEC 2
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 1:13
Etiquetas: 2010-1 CAF Freddy Vallenilla
When used as an AC signal amplifier, the transistors Base biasing voltage is applied so that it operates within its "Active" region and the linear part of the output characteristics curves are used. However, both the NPN & PNP type bipolar transistors can be made to operate as an "ON/OFF" type solid state switch for controlling high power devices such as motors, solenoids or lamps. If the circuit uses the Transistor as a Switch, then the biasing is arranged to operate in the output characteristics curves seen previously in the areas known as the "Saturation" and "Cut-off" regions as shown below.
The pink shaded area at the bottom represents the "Cut-off" region. Here the operating conditions of the transistor are zero input base current (Ib), zero output collector current (Ic) and maximum collector voltage (Vce) which results in a large depletion layer and no current flows through the device. The transistor is switched "Fully-OFF". The lighter blue area to the left represents the "Saturation" region. Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current flow and minimum collector emitter voltage which results in the depletion layer being as small as possible and maximum current flows through the device. The transistor is switched "Fully-ON". Then we can summarize this as:
- 1. Cut-off Region - Both junctions are Reverse-biased, Base current is zero or very small resulting in zero Collector current flowing, the device is switched fully "OFF".
- 2. Saturation Region - Both junctions are Forward-biased, Base current is high enough to give a Collector-Emitter voltage of 0v resulting in maximum Collector current flowing, the device is switched fully "ON".
An example of an NPN Transistor as a switch being used to operate a relay is given below. With inductive loads such as relays or solenoids a flywheel diode is placed across the load to dissipate the back EMF generated by the inductive load when the transistor switches "OFF" and so protect the transistor from damage. If the load is of a very high current or voltage nature, such as motors, heaters etc, then the load current can be controlled via a suitable relay as shown.
Transistor Switching Circuit
The circuit resembles that of the Common Emitter circuit we looked at in the previous tutorials. The difference this time is that to operate the transistor as a switch the transistor needs to be turned either fully "OFF" (Cut-off) or fully "ON" (Saturated). An ideal transistor switch would have an infinite resistance when turned "OFF" resulting in zero current flow and zero resistance when turned "ON", resulting in maximum current flow. In practice when turned "OFF", small leakage currents flow through the transistor and when fully "ON" the device has a low resistance value causing a small saturation voltage (Vce) across it. In both the Cut-off and Saturation regions the power dissipated by the transistor is at its minimum.
To make the Base current flow, the Base input terminal must be made more positive than the Emitter by increasing it above the 0.7 volts needed for a silicon device. By varying the Base-Emitter voltage Vbe, the Base current is altered and which in turn controls the amount of Collector current flowing through the transistor as previously discussed. When maximum Collector current flows the transistor is said to beSaturated. The value of the Base resistor determines how much input voltage is required and corresponding Base current to switch the transistor fully "ON".
For example, using the transistor values from the previous tutorials of: β = 200, Ic = 4mA and Ib = 20uA, find the value of the Base resistor (Rb) required to switch the load "ON" when the input terminal voltage exceeds 2.5v.
Again using the same values, find the minimum Base current required to turn the transistor fully "ON" (Saturated) for a load that requires 200mA of current.
Transistor switches are used for a wide variety of applications such as interfacing large current or high voltage devices like motors, relays or lamps to low voltage digital logic IC's or gates like AND Gates or ORGates. Here, the output from a digital logic gate is only +5v but the device to be controlled may require a 12 or even 24 volts supply. Or the load such as a DC Motor may need to have its speed controlled using a series of pulses (Pulse Width Modulation) and transistor switches will allow us to do this faster and more easily than with conventional mechanical switches.
Digital Logic Transistor Switch
The base resistor, Rb is required to limit the output current of the logic gate.
Sometimes the DC current gain of the bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors are used. Here, one small input transistor is used to switch "ON" or "OFF" a much larger current handling output transistor. To maximise the signal gain the two transistors are connected in a "Complementary Gain Compounding Configuration" or what is generally called a "Darlington Configuration" where the amplification factor is the product of the two individual transistors.
Darlington Transistors simply contain two individual bipolar NPN or PNP type transistors connected together so that the current gain of the first transistor is multiplied with that of the current gain of the second transistor to produce a device which acts like a single transistor with a very high current gain. The overall current gain Beta (β) or Hfe value of a Darlington device is the product of the two individual gains of the transistors and is given as:
So Darlington Transistors with very high β values and high Collector currents are possible compared to a single transistor. An example of the two basic types of Darlington transistor are given below.
Darlington Transistor Configurations
The above NPN Darlington transistor configuration shows the Collectors of the two transistors connected together with the Emitter of the first transistor connected to the Base of the second transistor therefore, the Emitter current of the first transistor becomes the Base current of the second transistor. The first or "input" transistor receives an input signal, amplifies it and uses it to drive the second or "output" transistors which amplifies it again resulting in a very high current gain. As well as its high increased current and voltage switching capabilities, another advantage of a Darlington transistor is in its high switching speeds making them ideal for use in Inverter circuits and DC motor or stepper motor control applications.
One difference to consider when using Darlington transistors over the conventional single bipolar transistor type is that the Base-Emitter input voltage Vbe needs to be higher at approx 1.4v for silicon devices, due to the series connection of the two PN junctions.
Then to summarise when using a Transistor as a Switch.
- Transistor switches can be used to switch and control lamps, relays or even motors.
- When using bipolar transistors as switches they must be fully "OFF" or fully "ON".
- Transistors that are fully "ON" are said to be in their Saturation region.
- Transistors that are fully "OFF" are said to be in their Cut-off region.
- In a transistor switch a small Base current controls a much larger Collector current.
- When using transistors to switch inductive relay loads a "Flywheel Diode" is required.
- When large currents or voltages need to be controlled, Darlington Transistors are used.
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 1:04
Etiquetas: II 2010-1 Sin nombre
- RF mixer and detector diode
- Power rectifier
- Power OR circuits
- Solar cell applications
- Clamp diode - especially with its use in LS TTL
Schottky diode as an RF mixer and detector diode
Schottky diode as a power rectifier diode
Schottky diode in power OR circuits
Schottky diode in solar cell applications
Schottky diode as a clamp diode
Schottky diode summary
Freddy R Vallenilla R
EES SEC 2
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 1:03
The PNP Transistor is the exact opposite to the NPN Transistor device we looked at in the previous tutorial. Basically, in this type of transistor construction the two diodes are reversed with respect to the NPN type, with the arrow, which also defines the Emitter terminal this time pointing inwards in the transistor symbol. Also, all the polarities are reversed which means that PNP Transistors "sink" current as opposed to the NPN transistor which "sources" current. Then, PNP Transistors use a small output base current and a negative base voltage to control a much larger emitter-collector current. The construction of a PNP transistor consists of two P-type semiconductor materials either side of the N-type material as shown below.
A PNP Transistor Configuration
|Note: Conventional current flow.|
The PNP Transistor has very similar characteristics to their NPN bipolar cousins, except that the polarities (or biasing) of the current and voltage directions are reversed for any one of the possible three configurations looked at in the first tutorial, Common Base, Common Emitter and Common Collector. Generally, PNP Transistors require a negative (-ve) voltage at their Collector terminal with the flow of current through the emitter-collector terminals being Holes as opposed to Electrons for the NPN types. Because the movement of holes across the depletion layer tends to be slower than for electrons, PNP transistors are generally more slower than their equivalent NPN counterparts when operating.
To cause the Base current to flow in a PNP transistor the Base needs to be more negative than the Emitter (current must leave the base) by approx 0.7 volts for a silicon device or 0.3 volts for a germanium device with the formulas used to calculate the Base resistor, Base current or Collector current are the same as those used for an equivalent NPN transistor and is given as.
Generally, the PNP transistor can replace NPN transistors in electronic circuits, the only difference is the polarities of the voltages, and the directions of the current flow. PNP Transistors can also be used as switching devices and an example of a PNP transistor switch is shown below.
A PNP Transistor Circuit
The Output Characteristics Curves for a PNP transistor look very similar to those for an equivalent NPN transistor except that they are rotated by 180o to take account of the reverse polarity voltages and currents, (the currents flowing out of the Base and Collector in a PNP transistor are negative).
You may think what is the point of having a PNP Transistor, when there are plenty of NPN Transistors available?. Well, having two different types of transistors PNP & NPN, can be an advantage when designing amplifier circuits such as Class B Amplifiers that use "Complementary" or "Matched Pair" transistors or for reversible H-Bridge motor control circuits. A pair of corresponding NPN and PNP transistors with near identical characteristics to each other are called Complementary Transistors for example, a TIP3055 (NPN), TIP2955 (PNP) are good examples of complementary or matched pair silicon power transistors. They have a DC current gain, Beta, (Ic / Ib) matched to within 10% and high Collector current of about 15A making them suitable for general motor control or robotic applications.
Identifying the PNP Transistor
We saw in the first tutorial of this Transistors section, that transistors are basically made up of two Diodes connected together back-to-back. We can use this analogy to determine whether a transistor is of the type PNP or NPN by testing its Resistance between the three different leads, Emitter, Base and Collector. By testing each pair of transistor leads in both directions will result in six tests in total with the expected resistance values in Ohm's given below.
- 1. Emitter-Base Terminals - The Emitter to Base should act like a normal diode and conduct one way only.
- 2. Collector-Base Terminals - The Collector-Base junction should act like a normal diode and conduct one way only.
- 3. Emitter-Collector Terminals - The Emitter-Collector should not conduct in either direction.
Transistor Resistance Values for the PNP transistor and NPN transistor types
|Between Transistor Terminals||PNP||NPN|
Freddy R Vallenilla R
EES SEC 2
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 1:02