3.1.1 Introduction

Small-signal amplifiers are needed to increase the tiny signal levels found at the input of a receiver into usable levels for the receiver s detector, or into the proper levels required of the final power amplifier of a transmitter. These amplifiers are Class A or AB for linear operation, high sensitivity, and low distortion in digital, AM, and SSB systems.

A receiver s first RF amplifier will be of the small-signal, high-gain type and must not produce excessive noise, since any noise generated within this first stage will be highly amplified by later stages, decreasing the SNR.

Because of the high operating frequencies, RF amplifiers may sometimes be neutralized in order to counteract any possible positive feedback and its resultant self-oscillations. However, designing with a transistor that has unconditional stability at the frequency and impedance of operation has now become much more prevalent.

The voltage gain of the small signal amplifier can be calculated as V _OUT/ V _IN, and when two or more are cascaded, their voltage gain is multiplied. However, the decibel is more frequently used, with these values simply added, or dB + dB, when stages are cascaded.

There are four vital considerations in any discrete RF amplifier design: the choice of the active device, the input and output impedance-matching network, the bias circuit, and the physical layout. Each of these will be discussed in detail.

3.1.2 Amplifier design with S parameters

S parameters characterize any RF device s behavior at...