Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-115 Low-pass filter Load
Current
Frequency Frequency response High-pass filter Load
Current
Frequency Frequency response Band-pass filter Load Band-pass filter Load
Current
Frequency Frequency response operation is the most efficient. Because the transistor does not conduct except during a small portion of the input signal, this is the most efficient class of amplifier. The distortion of the Class C amplifier is greater (poor fidelity) than the Class A, AB, and B amplifiers because a small portion of the input signal is reproduced on the output. Class C amplifiers are used when the output signal is used for only small portions of time. Methods of Coupling Coupling is used to transfer a signal from one stage on an amplifier to another stage. Regardless of whether an amplifier is a single stage or one in a series of stages, there must be a method for the signal to enter and leave the circuit. Coupling is the process of transferring the energy between circuits.
There are a number of ways for making this transfer and to discuss these methods in detail goes beyond the scope of this handbook. However, four methods are listed below with a brief description of their operation. Direct Coupling Direct coupling is the connection of the output of one stage directly to the input of the next stage. Direct coupling provides a good frequency response because no frequency-sensitive components, such as capacitors and inductors, are used. Yet this method is not used very often due to the complex power supply requirements and the impedance matching problems. RC Coupling RC coupling is the most common method of coupling and 12-116 Band-pass filter Load Band-pass filter Load
Current
Frequency Frequency response uses a coupling capacitor and signal developing resistors. [Figure 12-242] In this circuit, R1 acts as a load resistor for Q1 and develops the output signal for that stage. The capacitor C1 blocks the DC bias signal and passes the AC output signal. R2 then becomes the load over which the passes AC signal is developed as an input to the base of Q2. This arrangement allows for the bias voltage of each stage to be blocked, while the AC signal is passed to the next stage. Impedance Coupling Impedance coupling uses a coil as a load for the first stage but otherwise functions just as an RC coupling.
[Figure 12-243] This method is similar to the RC coupling method. The difference is that R1 is replaced with inductor L1 as the output load. The amount of signal developed on the output load depends on the inductive reactance of the coil. In order for the inductive reactance to be high, the inductance must be large; the frequency must be high or both. Therefore, load inductors should have relatively large amounts of inductance and are most effective at high frequencies. Transformer Coupling Transformer coupling uses a transformer to couple the signal from one stage to the next. [Figure 12-244] The transformer action of T1 couples the signal from the first stage to the second stage. The primary coil of T1 acts as a load for the output of the first stage while the secondary coil acts as the developing impedance for the second stage Q2. Transformer coupling is very efficient and the transformer can aid in impedance matching.
Feedback Feedback occurs when a small portion of the output signal is sent back to the input signal to the amplifier. There are two types of feedback in amplifiers: 1. Positive (regenerative) 2. Negative (degenerative) The main difference between these two signals is whether the feedback signal adds to the input signal or if the feedback signal diminishes the input signal. When the feedback is positive, the signal being returned to the input is in phase with the input signal and thus interferes constructively. Figure 12-245 illustrates this concept applied in the amplified circuit through a block diagram. Notice that the feedback signal is in phase with the input signal, which regenerates the input signal. This results in an output signal with amplitude greater than would have been without the constructive, positive feedback. This type of positive feedback is what causes an audio system to squeal.
negative or degenerative feedback occurs. In this case, the feedback signal is out of phase with the input signal. This causes destructive interference and degenerates the input signal. The result is a lower amplitude output signal than would have occurred without the feedback. Operational Amplifiers (OP AMP) An operational amplifier (OP AMP) is designed to be used with other circuit components and performs either computing functions or filtering. [Figure 12-246] Operational amplifiers are usually high-gain amplifiers with the amount of gain governed by the amount of feedback. Operational amplifiers were originally developed for analog computers and used to perform mathematical functions.
Today many devices use the operational amplifier for DC amplifiers, AC amplifiers, comparators, oscillators, and filter circuits. The widespread use is due to the fact that the OP AMP is a versatile device, small, and inexpensive. Built into the integrated chip, the operational amp is used as a basic building block of larger circuits. There are two inputs to the operational amplifier, inverting (−) and non-inverting (+), and there is one output. The polarity 12-117 NPN Input Class A amplifier Output VCC VEE + − NPN Input Class AB amplifier Output VCC VEE + − NPN Input Class B amplifier Output VCC VEE + − of a signal applied to the inverting input (−) is reversed at the output. A signal applied to the non-inverting (+) input retains its polarity on the output. To be classified as an operational amplifier, the circuit must have certain characteristics: 1. Very high gain 2. Very high input impedance 3. Very high output impedance This type of a circuit can be made up of discrete components, such as resistors and transistors. However, the most common form of an operational amplifier is found in the integrated circuit. This integrated circuit or chip contains the various stages of the operational amplifier and can be treated as if it were a single stage.
Applications The number of applications for OP AMPs is too numerous to detail in this handbook. However, the technician occasionally comes across these devices in modern aircraft and should be able to recognize their general purpose in a circuit. Some of the basic applications are: 1. Go/no-go detectors 2. Square wave circuits 3. Non-inverting amplifier 4. Inverting amplifier 5. Half-wave rectifier