Coupling amplifiers
- Key People:
- Lei Jun
- An Wang
- Robert Morris Page
- Walter Schottky
- Related Topics:
- physics
- industry
- electronic system
The existence of more than one type of transistor gives the circuit designer additional freedom not available for vacuum tube circuits and allows many clever circuits to be constructed. This becomes apparent in the direct coupling of successive amplifier stages. There are many ways to couple a signal from one circuit to another. Each has its advantages and disadvantages. Consideration must be given to the voltage levels in the circuits. In cases where the voltage level at the collector of the first amplifier is different from that at the base of the second, a direct connection could not be used. A transformer could be employed for coupling, with its primary in the collector circuit of the first amplifier and its secondary in the base circuit of the second one. However, transformers often do not exhibit uniform behaviour over a wide range of frequencies, which can be a problem. Transformers also are expensive and bulky. Similarly, a capacitor could be inserted between the collector of the first amplifier and the base of the second. This works well for many applications, providing uniform coupling inexpensively over a wide frequency range. At low frequencies, capacitive coupling becomes ineffective, however.
The use of a p-n-p second amplifier allows direct connection between the amplifiers (see ). If properly designed, this arrangement provides useful amplifying properties from DC to quite high frequencies. Care is required to avoid any changes in the DC operating conditions of the first amplifier; such changes will cause an amplified change in the DC conditions of the second one. Changes in temperature, in particular, can cause changes in resistor values and changes in the amplification properties of transistors. These factors must be carefully taken into account. Judicious use of feedback from later parts of a circuit to earlier ones can be utilized to stabilize such circuits or to perform various other useful functions (see below Oscillation). In negative feedback, the feedback signal is of a sense opposite to the signal present at the point in the circuit where the feedback signal is applied. While this has the effect of reducing the overall gain of the circuit, it also corrects numerous small distortions that may have occurred in the signal. For example, if the amplifier does not amplify large signals as much as small ones, the feedback from larger signals will be less, as will the reduction in gain, and the larger signals will be increased in the output of the circuit. Thus the distortion is reduced.
Oscillation
If feedback is positive, the feedback signal reinforces the original one, and an amplifier can be made to oscillate, or generate an AC signal. Such signals are needed for many purposes and are created in numerous kinds of oscillator circuits. In a tunable oscillator, such as that required for a radio receiver, the parallel combination of an inductor and a capacitor is a tuned circuit: at one frequency, and only one, the inductive effects and the capacitive effects balance. At this frequency the voltage developed across the tuned circuit is a maximum. Positive feedback is provided by the inductor in the collector circuit, which is magnetically coupled to the inductor of the tuned circuit. The connections to these inductors are arranged so that, when the collector current increases, the voltage at the base also increases, thus causing the collector current to rise further. The action of the tuned circuit reverses this sequence after a time and causes the base voltage to start to fall. This reduces the collector current; the positive feedback then further reduces the base voltage, and so on.
The circuit is in fact an amplifier whose output provides the input signal. The tuned circuit affects the feedback process in such a way that the circuit responds to an input signal at only one frequency—namely, the frequency to which the inductor and capacitor are tuned. The variable capacitor provides a way to adjust the frequency of oscillation. The output signal is obtained from the emitter resistor, through which the current rises and falls in synchrony with the collector current.
Oscillators that produce a single, accurate frequency are often needed. Such an oscillator is used in electronic watches. Other circuits in the watch count the output signals from the oscillator to determine the passage of time. These oscillators use a quartz crystal instead of a tuned circuit to establish the operating frequency (see ).
Quartz has the useful properties of changing its dimensions slightly if an electric field is applied to it and, conversely, of producing a small electrical voltage when pressure is applied (the piezoelectric effect). In a quartz-crystal oscillator a small plate of quartz is provided with metal electrodes on its faces. Just as a bell rings when struck, the quartz plate also “rings,” but at a very high frequency, and produces an AC voltage between the electrodes at this mechanically resonant frequency. When such a crystal is used in an oscillator, positive feedback provides energy to the quartz crystal to keep it ringing, and the oscillator output frequency is precisely controlled by the quartz crystal.
Quartz is not the only crystalline material that exhibits a piezoelectric effect, but it is used in this application because its oscillation frequency can be quite insensitive to temperature changes. Quartz-controlled oscillators are able to produce output frequencies from about 10 kilohertz to more than 200 megahertz and, in carefully controlled environments, can have a precision of one part in 100 billion, though one part in 10 million is more common.
Switching and timing
Using transistors
Transistors in amplifier circuits are used as linear devices; i.e., the input signal and the larger output signal are nearly exact replicas of each other. Transistors and other semiconductor devices may also be used as switches. In such applications the base or gate of a transistor, depending on the type of transistor in use, is employed as a control element to switch on or off the current between the emitter and collector or the source and drain. The purpose may be as simple as lighting an indicator lamp, or it may be of a much more complex nature.
An example of a moderately sophisticated application is in a backup, or “uninterruptible,” power source for a computer. Such equipment consists of a storage battery (which is normally kept charged by rectifying the power coming from the AC power line), a circuit for converting the battery power into AC, and the necessary control circuits. The control circuits monitor the voltage supplied from the power line. If this voltage varies significantly either upward or downward from its normal values, the control circuit causes the power supply lines to the computer to be switched from the incoming power line to an alternate source of AC derived from the battery.
Batteries are usually low-voltage DC sources. Consequently, their energy has to be converted to AC and applied to a transformer so as to raise the voltage to the proper level for operating the computer. The conversion from DC to AC, known as inversion, is often done with high-power transistors operated as switches. The battery is connected to the primary coil of the transformer through the transistors, first in one polarity and then in the other, at a frequency identical to the normal power-line frequency—usually 50 or 60 hertz.
The same result could in principle be obtained by operating the transistors as an oscillator powered by the battery and supplying a smoothly varying AC voltage to the transformer rather than the square pulses obtained via the switching process. This is a much less efficient procedure, however. A transistor operated as a switch is quite efficient, because in its “off” condition very little current flows at a relatively high voltage (a slight leakage through the reverse-biased collector junction), while in the “on” condition the collector-emitter voltage is very low, even though the current is large. In both conditions, the power lost is the product of the voltage and the current. Given this fact, the loss is small, because at any instant either the voltage or the current is small.
Using thyristors
Thyristors are another important class of semiconductor devices used in switching applications. The simplest of these devices is the controlled rectifier (see ), made of silicon. It may be regarded as two transistors connected to each other.
The device will start to conduct if a suitable amount of gate current is applied, but otherwise it will not. The gate current is the equivalent of the base current for the n-p-n transistor; the resulting larger collector current is the base current for the p-n-p transistor. The p-n-p transistor has an unusually wide base region, so its gain is small, especially at low currents. Its collector current augments the initial gate current, however. This positive feedback increases the current levels throughout the thyristor, increasing the gain of the p-n-p transistor, and at a certain point the combined currents through the n-p-n and p-n-p transistors are sufficient to maintain conduction through the device even if the gate current is removed. The transistors drive each other into a saturated condition such that the thyristor conducts a large current with a very low voltage drop, typically about one volt. The device remains in this conducting state for an arbitrary period and cannot be turned off under control of the gate. Conduction will cease if the anode polarity becomes negative with respect to the cathode.
Thyristors are thus well suited for operation in AC rather than DC circuits. They can be switched on during the appropriate half-cycle of voltage (anode positive) and will automatically switch off when the polarity reverses. A single thyristor can be used as a rectifier to produce a variable DC output from a fixed AC input. Adjustment of the DC output is made by modifying the time at which the gate current is applied after the AC voltage crosses zero and becomes the right polarity for conduction. Two thyristors connected in antiparallel (i.e., the anode of each is connected to the cathode of the other) form an AC switch, one thyristor being able to conduct on one half-cycle and the other on the alternate half-cycle. The amount of AC power delivered to the load may be adjusted to any level between zero and full power by appropriate timing of the gate signals to the two thyristors.
Thyristors are designed to handle both small and large amounts of power; the largest ones can withstand up to 5,000 volts in the “off” state and can conduct up to 2,000 amperes in the “on” state. Such a device is contained in an enclosure approximately 150 mm (6 inches) in diameter and about 30 mm (1 inch) thick fitted with external air- or water-cooling means. The power loss in the thyristor in such cases may be as much as 4 kilowatts, but the total amount of power handled may be up to 1,000 times as large. The efficiency is thus very high.
Other types of thyristors include those in which the gate is able to turn off the thyristor and those that can be switched on in either direction of current flow. The latter finds wide use in light-duty applications—for example, in variable-speed home appliances and light dimmers.
Thyristors have many applications in industrial equipment where substantial amounts of power must be controlled electronically. These applications range from transmission of electric power over long distances, which is more efficient if done as DC rather than AC, to control of heating elements in furnaces and supplying power for electronic equipment. The very large thyristors mentioned earlier are employed in power conversion for DC transmission, both from AC to DC and vice versa.
