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There are several different kinds of amplifiers used with transducers. The particular type of amplifier depends on the nature of the signal to be amplified. We may classify amplifiers in several categories, as follows: low-level ac amplifiers, power ac amplifiers, dc amplifiers, and switching (pulse) amplifiers.
LOW-LEVEL AC AMPLIFIERS The most important consideration in a low-level amplifier (also commonly called a preamplifier) is the noise figure. Operational amplifiers are often used as preamplifiers (ill. 26-1). These devices can provide gain, in practice, of more than 60 dB. ill. 26-1. A preamplifier using an operational amplifier. The input Impedance and gain are determined by the values of the resistors. The main problem with operational amplifiers in applications where extreme sensitivity is needed is the noise figure. Where noise must be kept to a minimum, it's common practice to increase the negative feedback in the operational amplifier circuit, thereby reducing the gain, and precede the operational amplifier with a low- noise field-effect-transistor (FET) circuit. An example of such an amplifier is shown in ill. 26-2. The lowest noise figure is generally obtained with the gallium-arsenide (GaAs) FET. If broadband operation is needed, the FET circuit may be untuned, although low- pass or high-pass networks at the input and output may help re duce the overall noise figure. ill. 26-2. A preamplifier using an FET. The values of C1, C2 and C3 depend on the frequency range desired. Examples of FET low-level amplifiers using low-pass and high-pass networks are shown in Figs. 26-3A and B. The cutoff frequency of the network at A should be well above the highest operating frequency. A value of about 1.5 times the highest operating frequency will permit a linear response in the operating range. Similarly, the cutoff frequency of the high-pass network (B) should be well below the lowest operating frequency; an optimal value might be about 0.67 times the lowest expected operating frequency. ill. 26-3. Examples of FET amplifiers using selective networks. At A, low pass; at B, high pass; at C, bandpass. If the operating frequency range is quite narrow, tuned circuits may be incorporated at the input and /or output of the FET preamplifier stage. These circuits may be simple parallel-resonant LC networks (ill. 26-3C) or they may be Chebyshev or Butterworth bandpass networks. The primary problem with parallel-resonant LC networks at both the input and output is a tendency toward oscillation of the amplifier stage. A resistor in series with the inductance of the tuned circuit will reduce this tendency. Careful attention to the physical layout of the circuit, minimizing interwiring capacitance, is important. An example of a low-level, highly sensitive ac amplifier is shown in ill. 26-4. This circuit is used in conjunction with an ordinary photovoltaic (solar) cell for the purpose of receiving modulated-light signals. The FET amplifier incorporates a low-pass filter having a cutoff of approximately 4.5 kHz at the input, restricting the response to the communications baseband range. A high-pass filter, with a cutoff of about 200 Hz, helps to filter out the 120-Hz ac “hum” emitted by light bulbs operating from the utility mains. The high-pass filter is at the input of the FET stage, and the low-pass filter is at the output. Pot cores are used for the coils. ill. 26-4. Au FET amplifier incorporating both low-pass and high-pass networks. This amplifier is used with a solar cell for reception of audio-modulated light signals. POWER AC AMPLIFIERS In a power amplifier the design depends on the amount of power needed and on whether or not linearity is important. In a radio- frequency transmitter, e.g., the output power may be as low as 1 or 2 W or as high as 1 million W or so. Linearity is not important in a radio-frequency transmitter insofar as the rf waveform itself is concerned, but linearity may be essential for the modulation envelope. Power amplifiers are classified according to the proportion of the input cycle during which current flows in the output (collector or drain). In class A amplifiers, current flows in the output circuit 100 percent of the time. Furthermore, the instantaneous output is linearly proportional to the instantaneous input throughout the cycle. A class A amplifier is biased so that, under conditions of zero input signal, the device is at the middle of the linear portion of the base/gate-voltage vs. collector/drain-current curve (ill. 26-5). A class AB1 amplifier has collector or drain current over 100 percent of the input cycle, just as the class A amplifier does. But the bias is set not at the middle of the characteristic curve but closer to the cutoff or pinchoff condition. Thus, the output cycle is somewhat distorted and not directly proportional to the input over the whole cycle. Figure 26-5 shows the approximate bias point for class AB1 operation. A class AB2 amplifier is biased still further toward the cutoff or pinchoff condition (ill. 26-5). For a small part of the cycle no current flows in the collector or drain. This proportion of the cycle may vary from less than 1 percent to almost half the cycle. In class B operation, the bipolar or field-effect transistor is biased at the cutoff or pinchoff point under zero-input-signal conditions (ill. 26-5). Current flows in the collector or drain for 50 percent of the cycle. ill. 26-5. Bias points for various classes of power-amplifier operation. In class C operation, bias is considerably beyond cutoff or pinchoff (ill. 26-5). Current flows during less than 50 percent of the cycle. The proportion may be as small as just a few percent. Figure 26-6 illustrates, qualitatively, the output waveforms for these various classes of power-amplifier operation, assuming a perfect sine-wave input. ill. 26-6. Output waveforms for class A (A), class AB1 (B), class AB2 (C), class B (D), and class C (E) operation. The main advantage of the class A amplifier is that it draws very little power from the source (input). This is especially true of the FET class A amplifier. Class AB1, class AB2, class B, and class C amplifiers require progressively more and more input power. The class AB2, class B, and class C amplifiers are used only as secondary amplifiers driven by a preamplifier. DC AMPLIFIERS Amplifiers for dc may be broadly classified as either low level or high level. Such amplifiers may operate either in current mode or in voltage mode. Generally, bipolar transistors are used as cur rent amplifiers, and field-effect transistors as voltage amplifiers, although it's possible to use a particular device either for current amplification or voltage amplification. ill. 26-7. A bipolar-transistor dc current amplifier. A bipolar-transistor current amplifier for dc is shown in ill. 26-7. The output current is a linear function of the input current within a given range, but if the input current exceeds a certain maximum, the output current will increase less and less rapidly as the condition of saturation is approached (ill. 26-8). Under zero-input-current conditions the transistor is biased exactly at cutoff. ill. 26-8. Output-vs.-input curve for do current or voltage amplifiers. An FET dc voltage amplifier is shown in ill. 26-9. As with the current amplifier, the output is directly and linearly proportional to the input. Under conditions of zero input voltage the FET is, pinched off. This is accomplished by means of a large-value resistor connected from the gate of the FET to a source of negative voltage. You may recognize the circuit at ill. 26-9 as a FET voltmeter (without the meter!). ill. 26-9. An FET dc voltage amplifier. As the input voltage increases, the output voltage increases in linear proportion up to a certain point. Then the output voltage increases less and less rapidly, and finally it levels off (ill. 26-8). There is essentially no difference in the circuit configuration for high-level current or voltage amplifiers for dc as compared with low-level amplifiers. The only real difference is in the size of the bipolar transistor or FET. Direct-current amplifiers may be cascaded to obtain enhanced gain. Operational amplifiers are often used for amplification of dc,, especially in conjunction with such devices as servomotors. The general configuration for a dc amplifier using an operational amplifier is shown in ill. 26-10. ill. 26-10. A dc amplifier using an operational amplifier. SWITCHING (PULSE) AMPLIFIERS Switching amplifiers are similar to low-level ac amplifiers, except that the switching amplifier must have broad bandwidth. This is necessary because of the rapid rise and decay of square pulses. The switching amplifier may or may not be driven into saturation (full conduction) during the conducting part of the cycle. Switching amplifiers are sometimes referred to as class D amplifiers if they are saturated in the “on” condition and cut off or pinched off during the “off” condition. A simple switching amplifier is shown in ill. 26-11. It consists of two bipolar transistors biased slightly beyond cutoff. The first transistor, Q1, is biased so that a small positive-going pulse (on the order of 0.2 V or more) will cause it to conduct. The output voltage is somewhat larger, about 2 V, and this pulse drives Q2 into saturation. An operational amplifier may be substituted for Q1 if high sensitivity is needed. ill. 26-11. A simple switching amplifier. This circuit is used to amplify voltage pulses at moderate to high speeds. PREV: Proximity Sensors |
Updated: Thursday, March 19, 2009 0:58 PST