VELOCITY TRANSDUCERS BASIC INFORMATION AND TUTORIALS

Signal conditioning techniques make it possible to derive all motion measurements displacement, velocity, or acceleration—from a measurement of any one of the three. Nevertheless, it is sometimes advantageous to measure velocity directly, particularly in the cases of short-stroke rectilinear motion or high-speed shaft rotation.

The analog transducers frequently used to meet these two requirements are

- Magnet-and-coil velocity transducers

- Tachometer generators

A third category of velocity transducers, Counter-type velocity transducers, is simple to implement and is directly compatible with digital controllers.

The operation of magnet-and-coil velocity transducers is based on Faraday’s law of induction. For a solenoidal coil with a high length-to-diameter ratio made of closely spaced turns of fine wire, the voltage induced into the coil is proportional to the velocity of the magnet.

Magnet-and-coil velocity transducers are available with strokes ranging from less than 10 mm to approximately 0.5 m.

A tachometer generator is, as the name implies, a small AC or DC generator whose output voltage is directly proportional to the angular velocity of its rotor, which is driven by the controlled output shaft. Tachometer generators are available for shaft speeds of 5000 r/min, or greater, but the output may be nonlinear and there may be an unacceptable output voltage ripple at low speeds.

AC tachometer generators are less expensive and easier to maintain thanDC tachometer generators, but DC tachometer generators are directly compatible with analog controllers and the polarity of the output is a direct indication of the direction of rotation.

The output of an AC tachometer generator must be demodulated (i.e., rectified and filtered), and the demodulatormust be phase sensitive in order to indicate direction of rotation. Counter-type velocity transducers operate on the principle of counting electrical pulses for a fixed amount of time, then converting the count per unit time to velocity.

Counter-type velocity transducers rely on the use of a proximity sensor (pickup) or an incremental encoder. Proximity sensors may be one of the following types:

- Electro-optic

- Variable reluctance

- Hall effect

- Inductance

- Capacitance

Since a digital controller necessarily includes a very accurate electronic clock, both pulse counting and conversion to velocity can be implemented in software (i.e., made a part of the controller program). Hardware implementation of pulse counting may be necessary if time-intensive counting would divert the controller from other necessary control functions.

A special-purpose IC, known as a quadrature decoder/counter interface, can perform the decoding and counting functions and transmit the count to the controller as a data word.

AVALANCHE TRANSISTORS BASIC INFORMATION AND TUTORIALS

Avalanche transistors have probably been known since the earliest days of silicon transistors but I have never heard of them being implemented with germanium devices, though some readers may know otherwise.

One important use for them was in creating extremely fast, narrow pulses to drive the sampling gate in a sampling oscilloscope.

Such oscilloscopes provided, in the late 1950s, the then incredible bandwidth of 2 GHz, at a time when other oscilloscopes were struggling, with distributed amplifiers and special cathode ray tubes, to make a bandwidth of 85 MHz.

Admittedly those early sampling oscilloscopes were plagued by possible aliased responses and, inconveniently, needed a separate external trigger, but they were steadily developed over the years, providing, by the 1970s, a bandwidth of 10–14 GHz.

The latest digital sampling oscilloscopes provide bandwidths of up to 50 GHz, although like their analog predecessors they are limited to displaying repetitive waveforms, making them inappropriate for some of the more difficult oscilloscope applications, such as glitch capture.

The basic avalanche transistor circuit is very simple, and a version published in the late 1970s (Ref. 1) apparently produced a 1 Mpulse/sec pulse train with a peak amplitude of 11 V, a halfamplitude pulse width of 250 ps and a risetime of 130 ps.

This with a 2N2369, an unremarkable switching transistor with a 500 MHz ft and a Cobo of 4 pF. The waveform, reproduced in the article, was naturally captured on a sampling oscilloscope.


Interest in avalanche circuits seems to have flagged a little after the 1970s, or perhaps it is that the limited number of specialised uses for which they are appropriate resulted in the spotlight always resting elsewhere.

Another problem is the absence of transistor types specifically designed and characterised for this application. But this situation has recently changed, due to the interest in high-power laser diodes capable of producing extremely narrow pulses for ranging and other purposes, in Pockel cell drivers, and in streak cameras, etc.

Two transistors specifically characterised for avalanche pulse operation, types ZTX413 and ZTX415 (Ref. 2), have recently appeared, together with an application note (Ref. 3) for the latter.

TYPICAL CONSTRUCTION OF A P-N-P BIPOLAR TRANSISTOR BASIC INFORMATION

1. A silicon dioxide (SiO2) layer is grown on a p-doped silicon wafer.

2. A positive photoresist layer is applied to the SiO2.

3. A photomask is created with opaque and clear areas, patterning the clear areas in locations where windows in the SiO2 are to be formed. The photomask image is transferred onto the positive photoresist, which becomes polymerized in the areas where it is not exposed to the UV light (opaque areas in the photomask).

4. The resist is developed, and the unpolymerized areas dissolve, forming a window that exposes the SiO2.

5. The silicon dioxide is etched away in the photoresist windows, exposing the silicon wafer.

6. The photoresist is removed.

7. Using phosphorus as the dopant, an n-type region in the p-type silicon base is created by diffusion.

8. A new layer of silicon dioxide is grown on the surface of the n-region, and steps (2) through (6) are repeated to create a new window in the SiO2.

9. A second diffusion creates the p-type region in the n-type base by using boron as the dopant.

10. Silicon dioxide (SiO2) is again grown over the exposed silicon wafer, and the photoresist is applied over the SiO2.

11. The photomask, containing the two clearances for the emitter and base, is placed over the positive photoresist, and steps (2) through (6) are repeated.

12. The structure is now ready for metallization. An aluminum film is deposited over the entire surface, followed by a coating of positive photoresist.

13. The photomask, with the emitter and base areas opaque, is placed over the photoresist and exposed to UV light.

14. The photoresist is developed, leaving the resist over the emitter and base areas.

15. The exposed metallization is etched away, followed by the removal of the resist over the emitter and base areas.

16. A passivation layer of silicon nitride is applied to the circuitry, leaving the bond pads exposed.

17. The silicon planar bipolar transistor is now complete.

METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR BASIC DEFINITION AND TUTORIALS

Another type of field effect transistor (FET) is the metal-oxide semiconductor field-effect transistor (MOSFET). It operates on the same principle as the JFET transistor but uses the input voltage, applied across a built-in capacitor, to control the source-to-drain electron flow.

A MOSFET typically consists of a source and drain (n-type regions) embedded in a p-type material (Fig. 1.20). The gate terminal is connected to a metal (aluminum) layer that is separated from the p-type material by a silicon dioxide (SiO2) insulator.

This combination of metal, silicon dioxide (insulation), and p-type semiconductor layers forms a decoupling capacitor. The gate region is located between the source and drain regions, with a fourth region located under the gate, called the substrate.

The substrate is either internally connected to the source or is used as an external terminal.

The flow of electrons from the source to the drain is controlled by whether the gate has a positive or negative voltage. If the input voltage applied to the gate is positive, free electrons will be attracted from the n-regions and the pregion to the underside of the silicon dioxide layer, at the gate region.

The abundance of electrons under the gate forms an n-channel between the two nregions, thus providing a conductive path for the current to flow from the source to the drain. In this case, the MOSFET is said to be on.

If the input voltage at the gate is negative, the electrons in the p-region under the gate are repelled, and no n channel is formed. Since the resistance in the p-region between the two n-regions is infinite, no current will flow, thus turning the MOSFET off.

Although the MOSFET used in the above description was of an n-p-n type, a p-n-p type MOSFET can also be constructed, but its voltage polarities are reversed.

BIPOLAR JUNCTION TRANSISTOR AS SWITCH BASIC AND TUTORIALS

The bipolar junction transistor or BJT as it is more commonly known can be considered in digital terms as a simple single-pole switch. It physically consists of three layers of semiconductor (which can be either N-type or P-type) of which two transistor types exist - NPN or PNP.

We shall consider the operation of the NPN device since this device is used mainly in bipolar digital switching circuits. The symbol for the NPN transistor is shown in Fig. 9.1 and is connected as a simple switch.

The transistor consists of three terminals: base (b); emitter (e); and collector (c). Notice that the arrow on this type of transistor is pointing out from the emitter which indicates the direction of current flow. For the PNP the arrow points in.

A simple rule for remembering the direction of the arrow is that with an NPN transistor the arrow is Not Pointing iN?

Fig. 9.1 A transistor switch


The input to the circuit in Fig. 9.1 is connected to the base terminal via the resistor R b whilst the output is taken from the collector. Several text books are available that discuss the operation of a bipolar transistor in detail.

However, for this simple BJT switch, and other BJT applications to follow, we just need to know the following.

1. To turn the transistor on a voltage at the base with respect to the emitter of greater than 0.7 V is needed. Under this condition a large collector current, I, flows through the transistor. The amount of current that flows is related to the base current, I b, by I~ =hf~Ib, where hfe is called the current gain and is typically 100. In this condition the transistor is in the on state, called saturation, and the voltage across the collector to emitter is approximately 0.2 V and is called V~.

2. To turn the transistor off the voltage at the base with respect to the emitter has to be less than 0.7 V. The collector current that flows is now zero (or more accurately a very small current called the leakage current). The transistor in this off state is called cut-off and the voltage across it is the supply voltage, Vcc, which is usually 5 V.

SCHOTTKY TTL (74S/54S) BASIC INFORMATION

The Schottky TTL offers a speed that is about twice that offered by the high-power TTL for the same power consumption. Figure 5.19 shows the internal schematic of a Schottky TTL NAND gate.

The circuit shown is that of one of the four gates inside a quad two-input NAND (type 74S00 or 54S00). The circuit, as we can see, is nearly the same as that of the high-power TTL NAND gate.

The transistors used in the circuit are all Schottky transistors with the exception of Q5. A Schottky Q5 would serve no purpose, with Q4 being a Schottky transistor. A Schottky transistor is nothing but a conventional bipolar transistor with a Schottky diode connected between its base and collector terminals.

The Schottky diode with its metal–semiconductor junction not only is faster but also offers a lower forward voltage drop of 0.4V as against 0.7V for a P–N junction diode for the same value of forward current. The presence of a Schottky diode does not allow the transistor to go to deep saturation.

The moment the collector voltage of the transistor tends to go below about 0.3 V, the Schottky diode becomes forward biased and bypasses part of the base current through it. The collector voltage is thus not allowed to go to the saturation value of 0.1V and gets clamped around 0.3 V.

While the power consumption of a Schottky TTL gate is almost the same as that of a high-power TTL gate owing to nearly the same values of the resistors used in the circuit, the Schottky TTL offers a higher speed on account of the use of Schottky transistors.

Characteristic Features
Characteristic features of this family are summarized as follows: VIH = 2V; VIL =0.8 V; IIH =50 micro A;
IIL =2 mA; VOH=2.7 V; VOL =0.5 V; IOH =1 mA; IOL =20 mA; VCC =4.75–5.25V (74-series) and 4.5–5.5V (54-series); propagation delay (for a load resistance of 280 , a load capacitance of 15 pF, VCC =5V and an ambient temperature of 25 °C)=5 ns (max.) for LOW-to-HIGH and 4.5 ns (max.) for HIGH to-LOW output transitions; worst-case noise margin=0.3 V; fan-out=10; ICCH (for all four gates)=16 mA; ICCL (for all four gates)=36 mA; operating temperature range=0–70 °C (74- series) and −55 to +125 °C (54-series); speed–power product=57 pJ; maximum flip-flop toggle frequency=125 MHz.

COMMON COLLECTOR TRANSISTOR CONFIGURATION BASICS AND TUTORIALS

WHAT IS COMMON COLLECTOR TRANSISTOR CONFIGURATION

Figure 1 is a practical example of a common-collector transistor amplifier. Note that the output is taken off of the emitter instead of the collector (as in the common-emitter configuration).

A common-collector amplifier is not capable of voltage gain. In fact, there is a very slight loss of voltage amplitude between input and output.

However, for all practical purposes, we can consider the voltage gain at unity. Common-collector amplifiers are noninverting, meaning the output signal is in phase with the input signal.

Essentially, the output signal is an exact duplicate of the input signal. For this reason, common-collector amplifiers are often called emitter-follower amplifiers, because the emitter voltage follows the base voltage.

Common-collector amplifiers are current amplifiers. The current gain for the circuit illustrated in Fig. 1 is the parallel resistance value of R1 and R2, divided by the resistance value of R3. R1 and R2 are both 20 Kohms in value, so their parallel resistance value is 10 Kohms.

This 10 Kohms divided by 1 Kohm (the value of R3) gives us a current gain of 10 for this circuit. Because the voltage gain is considered to be unity (1), the power gain for a common-collector amplifier is considered equal to the current gain (10, in this particular case).

The input impedance of common-collector amplifiers is typically higher than the other transistor configurations. It is the parallel resistive effect of R1, R2, and the product of the value of R3 times the beta value.

Because beta times the R3 value is usually much higher than that of R1 or R2, you can closely estimate the input impedance by simply considering it to be the parallel resistance of R1 and R2. In this case, the input impedance would be about 10 Kohms.

The traditional method of calculating the output impedance of common-collector amplifiers is to divide the value of R3 by the transistor’s beta value. Although this method is still appropriate, a closer estimate can probably be obtained by considering the output impedance of most transistors to be about 80 ohms.

This 80-ohm output impedance should be viewed as being in parallel with R3, giving us a calculated output impedance of about 74 ohms (80 ohms in parallel with 1000 ohms). Resistors R1 and R2 have the same function within a common-collector amplifier as previously discussed with common-emitter amplifiers.

The high negative feedback produced by R3 provides excellent temperature stability and immunity from transistor variables. The circuit illustrated in Fig. 1 can be a valuable building block toward future projects.