In a superhet, the stages preceding the first mixer must be designed so they provide reasonable gain, but produce as little noise as possible. They must also be capable of handling strong signals without desensitization (losing gain), also known as overloading.
A tunable FET preamplifier for use in a radio receiver.
All preamplifiers operate in class A, and most employ FETs. An FET has a high input impedance that is ideally suited to weak-signal work. Above figure shows a simple RF preamplifier circuit. Input tuning reduces noise and provides some selectivity. This circuit produces 5 dB to 10 dB gain, depending on the frequency and the choice of FET.
It is important that the preamplifier be linear, and that it remain linear in the presence of strong input signals. Nonlinearity results in unwanted mixing in RF amplifiers. The mixing products produce intermodulation distortion (IMD), or intermod. That can wreak havoc in a receiver, producing numerous false signals. It also degrades the S/N ratio by generating hash, the result of complex mixing of many false signals over a wide range of frequencies.
The Front End
At low and medium frequencies, there is considerable atmospheric noise, and the design of a frontend circuit is simple. Above 30 MHz, atmospheric noise diminishes, and the main factor that limits the sensitivity is noise generated within the receiver. For this reason, front-end design becomes increasingly critical as the frequency rises through the VHF, UHF, and microwave spectra.
The front end, like a preamplifier, must be as linear as possible; the greater the degree of nonlinearity, the more susceptible the circuit is to the generation of mixing products. The front end should also have the greatest possible dynamic range.
The preselector provides a bandpass response that improves the S/N ratio, and reduces the likelihood of receiver overloading by a strong signal far removed from the operating frequency. The preselector provides image rejection in a superheterodyne circuit. Most preselectors have a 3-dB bandwidth that is a few percent of the received frequency.
A preselector can be tuned by means of tracking with the tuning dial, but this requires careful design and alignment. Some receivers incorporate preselectors that must be adjusted independently of the receiver tuning.
A high IF (several megahertz) is preferable to a low IF (less than 1 MHz) for image rejection. But a low IF is better for obtaining good selectivity. Double-conversion receivers have a comparatively high first IF and a low second IF to get the best of both worlds.
Intermediate-frequency amplifiers can be cascaded with tuned-transformer coupling. The amplifiers follow the mixer and precede the detector. Double-conversion receivers have two chains of IF amplifiers. The first IF chain follows the first mixer and precedes the second mixer, and the second IF chain follows the second mixer and precedes the detector.
The selectivity of the IF chain in a superheterodyne receiver can be expressed mathematically. The bandwidths are compared for two power-attenuation values, usually 3 dB and 30 dB. This gives an indication of the shape of the bandpass response. The ratio of the 30-dB selectivity to the 3-dB selectivity is called the shape factor. A rectangular response is desirable in most applications. The smaller the shape factor, the more rectangular the response.